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I helped a friend get these amps sounding and working prior to his selling them back in 2010

The little I heard, through worn out tubes, they did project a very full image and lush soundstage.

Would I pay to own a pair, No.

Are they worth the money we see them selling for today, No.

Will people buy these at ridiculous prices today, Yes.

For every hobby, there may be Cults and Beliefs

Marantz is just one example of an audio cult, alongside McIntosh, Fisher and maybe classic Scott's

Oh well some folks sell, some folks pay and others look.

Here is my logbook of working on a few classic tube amplifiers, some photos and notes. Just above I had the rare chance to work on a pair of Marantz 9 Monoblocks. Yes they are very classic, but I think a bit overrated due to rarity.

Refer to the rating system shown below as a general guide to understand the scope of work if you want to fix old amps yourself.

Level #1

For beginners starting out; Change out a few tubes, replace basic things like a frayed power cord and missing knobs, feet, screws, escutcheon labels, missing or broken parts. Learn to see Red Plates on Power Tubes. Basic soldering skills. No need for a VOM. It is a visual stage or ownership, the most basic.

Level #2

For intermediate challenges; Good soldering skills, tools and safety precautions required. Know how to measure Voltage and Current with a Meter. Good general knowledge of Electronics and Schematics interpretation, identifying and ordering of new parts, fuses, etc. Understanding resistance, capacitance, inductance, Power Supplies, and how the equipment operates. Ability to understand the meaning and difference between AC Balance and DC Bias. Understand what regulation is, and what regulators do in a power supply circuit. Learn about Filament Voltages and know how to weed out bad parts. Will know when and how to replace power tubes and match them as needed.

Level #3

For the experienced hobbyist; Able to disassemble and keep track of parts, label steps and measure/troubleshoot issues such as Signal Tracing and Voltage Measurements. Learn how to identify and isolate problems with Voltmeters, Signal Generators and Oscilloscopes. Learn to upgrade parts, and what ratings mean to a circuit. Understand Ohms Law and be able to calculate Resistance, Voltage, Current and Power. Know how to order a replacement Power and Output Transformer by specifications. Also know how to compensate for DC distribution by determining the series resistance of the Filtering Circuit of the Power supply. Be able to empirically determine a target specific DC voltage by using a temporary variable resistor or decade Box. This is the empirical method and requires minimal math.

Level #4

Tedious to restore and troubleshoot; This is where experience plays a significant role in isolating faults, noises and no volume, no sound, channel imbalances, isolating and identifying faulty components, ground loops, be able to listen for an differentiate 60 Hz vs. 120 Hz Hum, visually able to inspect for overheated components, capacitor leakage, blistered parts and other cues that hint of deeper issues like dark resistors and burn marks. Also strange smells and overheated Transformers, Chokes. Measure open primaries, be able to put a signal into an unknown Output Transformer and measure to determine the impedance of the primary, and other advanced techniques. Respect the original schematics but be able to identify mistakes and typos.

Level #5

Better left for experts; A complete tear down and rebuild with fresh new parts, replace drifted Carbon Composition resistors, test and replace all leaky Signal Capacitors and special replacement parts that need to be retrofitted, carefully drilling and adapting the equipment to accept modern replacement parts without ruining it. Updating Power Supplies to compensate for high mains voltages. Restoring the AC Balance and optimizing parts of the schematic to improve safety and performance with upgraded components. Replacing old Plate and Filament Transformers and knowing how to calculate the changes in the circuit to achieve equivalent or better performance and Sonics than the original design.

Mind clearing for troubleshooting issues & problems

- Harman Kardon Award Series A300 Stereo 15 W/ch >

Restoration difficulty #3

Budget $300, has one unobtainable Metal FP Capacitor that has to be created and installed and also has a CR Module that if bad, will need a custom created circuit with the values and topology in the schematic.

A300 Schematic

This is the year 2004, a time when I was frying EL34's and not noticing the Red Plates, until someone turned off the lights by mistake and I was shocked at the Red Glow.... but I diverge.

The A300 model has 4 x Cathode Biased 7408 (6V6), 1 x  12AU7 and 4 x 12AX7's. But the unique Cathode Biasing scheme merits mention and specific attention. If you examine the Schematic you will see that there is no Cathode Bias resistor. You may ask then, how can we Cathode Bias the output tubes without a Cathode Resistor ? Well by a trick of the trade, using the input tube filaments as 'surrogate' Cathode Bias resistors. The HK designers realized that the proper bias current for the four 7408/6V6GT output tubes was about 37 milliamperes DC for each tube. If you multiply this by four, you arrive at 150 milliamperes total  cathode current. The proper Cathode Bias Voltage just happens to be 24 Volts DC, which is two 12AX7 filaments in series wired as 12.6 Volts (Pins #4 and #5) , not 6.3 Volts (Pins #4&5 tied together and Pin #9). Ohms law states that the Series Current through a loop is equal though each discrete series element. Coincidentally, this biasing scheme makes the perfect "Cathode Bias" solution with the added benefit of using the DC Bias Current to feed the 12AX7 filaments DC. A symbiotic relationship that was also used in the A500 amplifier as well, neat. Running DC current through input tubes is a great way to lower the noise floor. The A300 Phono stage is quite revealing and enjoyable, lacking only in it's ultimate resolving ability and noise floor. The A300 was also available in a kit version as well with the model number A30K. There was a Montgomery Ward model the MKIV that sported a Brown/White look. The kit version and production sported Black Plastic control knobs w/ silver front inserts. As mentioned before a prior version of the A300 was marketed by a retail chain store as the MK-IV and sometimes comes up on EBay for sale. This particular A300 Montgomery Ward model had a different appearance sporting a beige and brown front plate with Black and Gold knobs. This amp is relatively easy to find for sale on EBay and could be considered a great starter amp for those inclined to fix these things. One word of advice, if you have an A300, upgrade the existing Power Cord to at least 16 AWG 2-wire. Also the coupling caps and power supply electrolytic can use a replacement. Some restorers like to install a modern 3-wire type power cord for added safety. But be aware that the 3rd safety ground wire, if bolted directly to the metal chassis may cause 60 Hz hum, so test accordingly. It's true that where that third PE (Protective Earth) wire goes is a subject of much debate. No doubt that using it makes the amp safer, but the grounding scheme of these old amps was not designed for 3 wire power cord operation. The connection of the 3rd (Green) safety ground wire to the chassis may cause a serious ground loop to be induced in some applications as it effectively connects the DC Ground and Chassis Ground together. Modern equipment will usually isolate the Chassis ground form the DC ground, and at most, they will connect only at one physical point on the Chassis, where there is a DC Single Point Ground. The Safety Ground places the Chassis at Earth potential and will provide a low-ohmic path for any fault currents over the 3rd (often Green or Green/Yellow) ground wire, to the ground bus bar into the ground rod, tripping the circuit breaker before it can shock a living creature. Unless the amp was single-point DC grounded, multiple DC Chassis ground points can induce chassis currents and cause hum to appear on the speakers. I don't see what a three wire circuit would really be necessary in older equipment apart from safety, but installation will reduce the possibility of chassis shock. In either case, you can replace with a 2-wire or 3-wire cable, it's a matter of choice. A ground lift modification can also be used with the 3rd wire in the case that your AC Power Feed and the 3rd wire are not from the same Political Party ....This amp was traded away in early 2009 for an Olson AM240 Chassis, Top Cover, Bottom Cover and burned-out carcass w/ 8 Telefunken 12AT7 tubes. Not a bad deal as I now have two Olson AM240's to modify and create something better. The A300 as I also had mentioned is probably the best "beginner" amp out there. But don't under estimate the cost of restoration, you will spend several hundred dollars in parts unless you happen to lie near a parts store. I would figure between $150 and $350 in quality parts. I would install the best Paper in Oil, Audio Note, Obbligato or Mundorf style caps. Oh... and have you seen the latest price in NOS 6V6GT's... oh my G ! $120 a quad.... but they sound better than most current production. Real NOS 7408...... good luck !

2008 Update: I traded this away for an original Olson AM240 Amp with Cover..... long live the A300. It found a good home.

2017 Advice: When restoring an A300, always replace all of the original Electrolytic Capacitors. But if you really care to open up the sound, replace the old and tired Stereo Volume and Stereo Balance Controls. If you also are inclined you can replace the Bass and Treble Controls as well. As far as the quality of the sound is concerned, keeping the amplifier 100% original will never allow the user (not the collector) to be amazed at how well these old amplifiers can perform.

- Harman Kardon Award Series A500 Stereo 25 W/ch >

Restoration difficulty #4

Budget $400-500, has a CR Module that is known to make a buzzing sound if the front panel switches are resistive. You also have to deal with the 7355 Tubes that are out of production. But the 6L6 is electrically the same, you will need to completely rewire the Octal Tube Sockets that is tedious work. There used to be a Sovtek 7591XYZ that was a re-based 6L6/5881, and it was only a single wire that needed to be relocated. But the 7591XYZ are dead and gone so the 6L6 is the only simple modification. Nobody today makes a 7355, but the 6L6 is the same thing, bigger and more robust. But darn, I miss the 7355, that was a very nice tube. The same mojo as a 6L6 in a small form factor.

Version 1 - A500/A50K Schematics

A500 Schematic Part 1

A500 Schematic Part 2

Version 2 - A500/A50K Schematic

A500 Schematic Part 1

A500 Schematic Part 2

A500 Complete Photoshop Version (courtesy of Dr. Tube)



Below is a link to an Excel file I made with the list of parts that I changed. Note that I used plain old regular Mylar caps and some Vintage Sprague's in the Phono section, nothing fancy, just caps that survive the Tropical Weather and the Heat much better than fancy exotics.

Note that this amp like many other's from it's day had a "Speaker Phase" switch. I guess this was necessary when most of the speaker cables used back in the 60's had no polarity indications, plain 'ole two wire brown, white or transparent clear/bronze lamp cords.

A user would wire-up the speaker system without any attention to the + or - terminals and flip the switch to correct the resulting 180 phase difference that had a 50/50 chance of happening.

I recommend that this switch be electrically eliminated but left on the front panel for cosmetic reasons.

Also, if one rarely uses Headphones direct off the A500, I would also eliminate this extra circuitry as there are some power resistors and funky transformer secondary wiring that does nothing to help the musicality of these vintage amps. All the extra wires, contacts and cheap slider switches on the front panel are responsible 90% of that pesky buzzing channel that one finds impossible to troubleshoot and end up changing parts, blaming tubes and all kinds of nasty expletives to chase the tail of the buzzing speaker. Also when hooking up these amps to a Dual-Trace scope, these types of circuits can wreak havoc on the display as the Speaker grounds are not electrically common. Therefore the Oscilloscope will make the negative speaker terminal electrically the same and also cause one channel to display more signal than the other. The only way is to measure each channel separately, but this is not good as one needs to drive both channels simultaneously to look for Power Supply issues or channel imbalances due to internal problems, mistracking controls, etc.


I help many vintage A500 owners to get their amps going. Sometimes their questions lead me to look at the schematic with different eyes. One such Web buddy wanted to simplify his design and remove as many of the controls as possible. This was meant to simplify the signal path and lead the source into as direct a connection as possible to the Preamp and Driver stages.

Looking at the schematic I saw a chance to swap-out two capacitors I had changed years ago. The two caps are C31 and C32. What led me here were two reasons. One, the Tape Monitor input always sounded better than the Aux, Tape Amp or Tuner inputs. These three send the signal through C31 and C32, which is a 0.2uF/10V cap.

Yesterday I changed this for a pair of 1uF Solen's I had in my parts bin. My amp sounds better now through the Aux input, almost as clean as the Tape Mon, and maybe even a bit more musical ?


The A500 uses four Cathode Biased 7355 output tubes now out of production since the late 70's early 80's.


1x 12AU7

4x 12AX7


In addition it feeds the three first 12AX7 Preamp Tubes with DC derived from the Cathode Bias resistor, another neat trick to learn from vintage designers. 

The A500 was available in a kit version with the model number A50K. To the best of my knowledge, the only difference was in the knobs, the one's in the kit were black and silver. A closer look and you can see significant new parts in my A500. I can tell you it's not a beginner's amp at all to fix.


One option to the now expensive and out of production 7355 tube is to try out the Sovtek 7591XYZ tube in the A500/A50K amplifier.

What is important for A500/A50K owners, the Sovtek 7591XYZ is not a 7591 tube but a re-pinned Sovtek "Coin" based 5881/6L6 type with a 8KQ - 7591 tube basing.

The 7355 is pretty much a compact 6L6 as far as specifications are concerned, but shares an almost identical basing as the 7591 tube, but not exact.

Sovtek back in the days when 7591 amp owners were whining, decided a fast fix for 7591 amp owners was to take a 6L6/5881 tube and to re base this as a 7591, hence the 7591XYZ was born.

The 7591XYZ could be used in 7591 amps but it required the installation of a Cathode resistor to increase the bias voltage on typical 7591 amps (-15VDC to -20 VDC) to (-35 to -37 VDC) for a AB1 Push Pull 6L6 tube pair.

As the 7591XYZ is still a electrically a 5881/6L6 internally, care is required when using in a 7591 amp, requiring a modification I am not going to explain here.

This 7591XYZ tube was Sovtek's first foray into "tube-relief" for owners of 7591 amps that were getting shell-shocked by the prices of real 7591 and 7591A tubes.

I am repeating what I have read, that supposedly some person spent the best part of the early 1980's traveling the USA and hoarding all existing and available 7591 and 7591A NOS. Hence this was the first great tube depletion to hit HI Fi and Guitar amp owners hard. Prices for unused, good and tested NOS 7591A quads can fetch over $100 dollars each tube ! How's that for tube collectors...

The problem with the 7591 tube is that there was no current production alternative available some 10 years back so Sovtek came to the rescue with the 7591XYZ.

Yet the subject of this writing here is the 7355 and the HK A500, so some background on the 7591 was necessary to understand why the 7591XYZ was ever invented.

What happened to the 7591 tube is a testament to what has happened to the tube industry in general, older technology displaced by newer technology, not necessarily better sounding.

Overall the 7591 tube was in itself a masterpiece of design and tube size reduction. This tube allowed manufacturers to scrape 35 Watts per channel from a pair in class AB1 Push Pull and about 40 Watts in AB2 Push Pull if pushed hard.

As a Pug Dog is often called "Multum-in-Parvo", the 7591/7591A tube was "Multum-in-Valvo."

Not only are real NOS 7591 tubes rare, they have two closely related relatives, the 7868 Noval based and the weird 6GM5 which has a miniature 9-Pin base.

All three of these tubes are almost (and I mean almost) electrically Identical except for a few Picofarads of inter-electrode capacitance.

The only difference electrically are slight variations of their inter-electrode capacitance. Physical the basing's are radically different having all 3 tubes different socket requirements.

Each of these tubes, the 7591/7591A, 7868 and 6GM5 were conceived from the DNA of the same father, but borne from different mother's (and factories.)

The 7591/7591A came in Octal Base

The 7868 was made with a Noval base

The 6GM5 was made with a 9-Pin base.

Here's the family Portrait for your enjoyment courtesy of Vacuum Tubes Inc.

More 7591 trivia....

Antique Electronic Supply used to sell a 6GM5 9-pin to 7591A Octal base adapters, but sadly I don't see that they have this any longer.

I managed to order two sets of these, one for myself and the other for my friends who own a McIntosh MC225. These adapters were created with some type of glue that smells to high-heaven when

the heated 6GM5 tubes are in operation. Eventually the smell subsides, but the build quality was not very good. On top of that, the socket pins were so tight, I had to use a broken 12AU7 repeatedly to coax the

socket pins open. Attempting to plug the 6GM5 tubes into the adapter had me at the point of breaking the tube glass.

The Sovtek 7591XYZ Octal Base tube is provided with a resistor that is required when the tube is used in an amplifier designed for 7591 tubes.

As the 7591XYZ is NOT a real 7591 tube, but is a re-based 5881/6L6, if you drop this into a socket that is expecting a 7591 and don't use the resistor provided, damage will occur.

The tube will be operating wide open as if you put a 5881/6L6 into a 7591 amp. The correct bias voltage for a 5881/6L6 is in the -28 to -35 Volt range, but a real 7591/7591A tube needs

only -17 to -20 volts for proper bias.

If you try to operate a 7591XYZ (5881/6L6) with AB1 Push Pull with -17 to -21 volt bias, it will only be less than a minute after power-up when the 7591XYZ tube runs away, destroys itself and/or the one of a kind

vintage A500 "Freed" output transformer in your precious Harman Tube amp.


If you are in the cheapskate mode as I often am, and plan to use the 7591XYZ in a Harman Kardon A500 or A50K, here's what you need to do.

The 7355 tube's basing and the 7591XYZ basing are different, not by much, but in electronics the difference between the word 'Exact' and 'Close' is often a very expensive mistake

followed by a puff of smoke and some colorful language.

Now that we have your curiosity, look at the basing diagram of a 7591 or 7591A tube.

You will clearly see that Pin #4 and Pin #8 are electrically connected inside the 7591 tube, in other words they are internally jumpered.

This fact clarifies some Forum posts that describe the 7355's and 7591's as having identical basing, they DO NOT.

A 7355 has an 8KN Octal Base and the 7591/7591A has an 8KQ Octal base.

Logically .... 8KN 8KQ even though they are very close, but not identical.

Electrically speaking, the 7355 tube is very similar to a 6L6/6L6GC/5881 family of tubes, but with a radically different basing diagram and a smaller size.

These types of tubes were made in the peak days of tube design, and were in fact a super high performance electron device made as small as possible.

The small size allowed amplifier designers to make smaller and more compact equipment.

Now the 7355 and the 5881/6L6/KT-66 all share similar biasing and operating point requirements.

They even have equivalently close transformer primary impedance centers when you calculate the optimal range of Plate to Plate impedances in a Push-Pull amplifier output stage.

What does all of this mean....

Well it means that a Sovtek 7591XYZ tube is the perfect "cheap" substitute tube for the rare 7355 given the proper modifications are performed on the amplifier. It is so as the basing

is very similar to a 7355 tube, and this means minimal wiring changes. The basing and electrical characteristics are close enough to be worth a try, don't you agree ?

This makes me wonder why Sovtek never marketed the 7591XYZ as nothing but a replacement tube for the 7591/7591A that requires a resistor modification to work instead of mentioning that

it's the perfect sub for a 7355 tube !!

Eventually Electro-Harmonix and JJ Electronik created their own versions of the 7591/7591A tube. Electro-Harmonix (a.k.a. re-branded Sovtek) used the same physical design as the

5881 Coin Base tube which was a Russian Military design. This made this a good candidate 7591/7591A design with a much large physical footprint. As they all say that size matters, the

EH 7591A tube will not allow one to operate most 7591 amps with their covers in place, they are just too tall.

But I can probably predict that because of the real 7591 tubes made today, the 7591XYZ may find it no longer has a place in the market, so if you plan to use 7591XYZ's stock-up.

7355 to 7591XYZ Modification Instructions

(1) Unplug the A500 amp from AC power please, safety first.

(2) Place your A500/A50K amp Belly-up and remove the bottom cover.

(3) Examine the Octal power tube sockets from below. A 7355 tube does not use Pin #1 and Pin #4, but wait, there are wires and resistors connected to these points in my amp.


(4) The modification requires you to install Tag Boards or Insulated Lugs next to the Octal Sockets (one single lug per socket ideal.)

(5) Remove the wires and resistor end that is on Pin #4 on the A500 amp Octal Sockets and relocate these connections to a separate insulated physically close Tag board or isolated Solder post.



(6) Now plug in a new quad of Sovtek 7591XYZ's and enjoy the music, no further mods are required. And you can use 7355 with no problems. But the 7591XYS are now long gone :(


You can keep or sell the used 7355's for the purists at a "primo price" and buy more spare 7591XYZ for future use before they are discontinued.

Or.... you can keep them for future use, in any case your A500 amp is a tad more versatile now than before. I am sure Sovtek never intended to make the 7591XYZ for amps using 7355...

The good part is that once the mod is done, you can go from 7355 to 7591XYZ without any extra work, the amp will be OK with both tubes.

Granted this is probably a non-optimized solution as there are some additional tweaks that can be made to extract the most out of this mod with the 7591XYZ, but it's close enough.

You can experiment with changing the two resistors that tie to the Input Grid to find a more optimal value combination, The amp has

1K and 270K to ground, you may want to tweak the 270K grid resistors a bit up or down. Note that some tubes have a maximum Grid Resistor value allowed

for a particular bias scheme. Also you can trick out the Cathode Bias circuit by changing some resistors to see

where it sounds best. I believe that this amp will settle at about 30 to 32 Volts at the Cathode. One unique design feature of this amp is that the

Cathode bias current also light's the first three 12AX7 tube filaments with a 400 uF Electrolytic and a Voltage/Current divider.

This makes it a bit tricky to mod in this respect. If you eliminate this circuit, then you need to install a separate Filament supply for 3x 12AX7 tubes

and it's probably not worth the hassle. Best of luck and write me with any questions, concerns or criticisms. We love em' all.

Using the 5881/6L6/KT-66 Tubes instead

You can, if adventurous, completely re-wire each socket and use 5881/6L6/KT-66 tube as well, but a 7355 will no longer drop into the socket.

You will not need to drill any holes with this approach as you can creatively loosen the Octal socket to slip a Tag Board through.

Or you may need to pop rivets here and update the old and tired sockets to some fresh new one's. Now that you are completely re-wiring the bases why not ?

Yes the 6L6/5881/KT-66 mod is more work and once you go 6L6/5881, you can't plug a 7355 any more into the amp.

Make sure your re-label the amp if you modify for 6L6/5881 as a future user may FRY a precious quad of 7355's when this amp passes into new hands.

Always label non-standard modification for future use. The 7591XYZ mod requires  no special labeling, 7355/7591XYZ are plug-in compatible after the modification.

When you eventually join J.Gordon and the "Shaman" of Triode Heaven Dr. H. Rosenberg, make sure that SHIT does not happen, it won't be cool !

In any case, the Harman Kardon A500 and A50K amps are by far the best "Tweaker" bargains out there. What gets me is how people spend money for

used 'un-molested' A500's and then fork our another $400 dollars to get it fixed. I recently traded this A500 to a very lucky and wise Audiophile.

He got this upgraded and special amp for less than a song and a spare set of 7355 tubes to use on special occasions.

These are not a beginner amps to restore, and you will earn your Tech Wings on them as I did. The A500 was my first grown-up tube amp I restored.

I spent about 50 frustrating hours chasing Hum, a buzz in one channel and all kinds of noisy controls and leaky boutique capacitors.

These vintage amps taught me that spending $50 dollars on expensive boutique caps is money better spent on other parts.

Note: If you are the kind of Audio Lover who cannot enjoy music from old, ugly and smelly tube amps with beat-up faceplates, buy new, forget the old.

Best get a Prima Luna amp and save the therapy money for your Audio Shrink.

You may also decide to spend on more "exotic" parts that make you want to believe you do hear a major improvement over those $1.80 Poly caps that

never fail. Believe me, these Orange Drop and Polyester and Polystyrene caps will last a lifetime and absolutely refuse to conduct D.C. current.

Or at least in my 15 years never found a bad one. But I have had my share of bad boutique caps that leak after a few month's of use.

If you are going to spend, buy the caps from Chris VenHaus, these are an upgrade and well worth the money.

"Pragmatism is a philosophical movement that includes those who claim that an ideology or proposition is true if it works satisfactorily, that the meaning of a proposition is to be found in the practical consequences of accepting it, and that unpractical ideas are to be rejected"


I have had many queries concerning the modification of the A500 amplifier so I have updated the text above to eliminate any confusion and added some images. Hope this helps..........

On my way to Virginia so I may be off the air for month's until I settle into a new house..... Until then. Enjoy!!!

October 2011

The A500 is on it's way to a new home. Sad to see it go, but happy that it will provide enjoyment to a new owner who actually saw through the hype and understood

the meaning of a good deal.

October 2017

Be advised that Sovtek discountinued the 7591XYZ tubes. This is a sad moment for any A500 Owner. The 7591XYX was a re-based Sovtek 5881 tube that only required one wiring change on the A500 Octal Power Tube Socket and not a complete socket re-wire that the 6L6/5881 Tubes call for. But I want to make it clear that there is no real or substantial electrical differences between a 7355 Tube and a 6L6/5881/KT66 Tube, they are electrically compatible. Not so for the 7591 Tube that is completely different than the 7355. If I had known about the 7591XYZ when they were available, I would have purchased at least 400 pieces to be able to retube One Hundred (100) A500 Amps. Maybe I can get New Sensor to make me 1000 pieces of the 7591XYZ. What that would cost and the market I can find to replace any amp that uses 7355 tubes is probably worth it. Apart from the bigger size of the 7591XYZ, owners of tired 7355 will be totally awestruck by the sound of the 7591XYX. It is in reality the performance you get from brand new tubes. 

- Harman Kardon Award Series A700 Stereo 35 W/ch >

Restoration difficulty #3

Budget $400-500, uses the 7199 Tube that are out of production and finicky to find the one's that do not buzzzz. No CR Modules here (whewww), all discrete, no super special parts. Lots of Ceramic Disk capacitors.

A700 Schematic Diagram

A700 Schematic 1/2 (Better resolution)

A700 Schematic 2/2 (Better resolution)

A700 Bias Modification (Power Point File)

The A700 amp sports 4 x 7591, 2 x 7199's and 3 x 12AX7. Not sure if this amp was ever offered in kit form. I have read that this amp was contracted by Harman Kardon to a 3rd party to design and develop. Using fixed-bias, this amplifier is not a beginner's amp. I would only recommend this amp for experienced tweakers as myself, not for your first tube amp. As you can see I have had to swap out a significant amount of components to make this amp useable. One word of advice when repairing vintage amps that use the 7199 tube, don't use the Sovtek substitutes until the amp is fixed. Sovtek 7199's are NOT direct substitutes and often hum like bumblebees, making any troubleshooter crazy changing parts when it was the tubes all along, and yes this happened to me on this amp! You may have to buy 10 pieces of Sovtek 7199 to get two that work. I know this tube is rare and expensive, but they are a real pain. Dynaco ST-70 users beware! Replace the old power cable (see A300 section.)


Here I am again, from the work-bench. I cooked-up a nice modification for the A700 I think you will like very much. The amp now has adjustable biasing for the 7591 output tubes. Originally the A700 was designed to run the output tubes very hot, at the physical limit for a 7591 tube's plate voltage and current consumption, hence dissipated power. The -17 VDC that is derived from the input tube filament and power supply circuit is not adjustable. If the AC mains voltage is at 120Volts or higher, the amplifier's 7591's will run at their limit.

What I decided to do was eliminate the -DC bias feed from the filament supply and install a separate, variable bias supply.

(1) RADIO SHACK 120 VAC TO 24 VAC, 0.45A TRANSFORMER (Can be smaller and also higher secondary such as: 36VAC/0.1A, 48 VAC, 0.1A, 24 VAC/0.1A)

(1) BRIDGE RECTIFIER 100PIV, 1.4A (Can also use 2x1N4007 diodes and make a voltage doubler for the 24VAC on the secondary)

(1) 1000 OHM, 1 WATT CARBON RESISTOR (Can vary depending on design)

(2) 47 uF/100 Volt ELECTROLYTIC CAPACITORS (Can use larger values, this is what I had in the parts bin)

(1) 40K WIRE-WOUND POTENTIOMETER, 5 WATT (Can use 50K to 100K potentiometer, 1 watt minimum)

The small transformer is now under the chassis using one of the transformer mounting screws. The new bias supply allows me to vary the voltage from -20 VDC down to zero. I should probably eliminate the Bridge rectifier and replace this with a Voltage Doubler to get a wider range. As far as safety is concerned, I also should install an extra resistor in series with the 40Kohm Potentiometer. This would clamp the minimum bias voltage at -15 Volts to dummy-proof the bias supply. I was in a rush to get the mod going and I skipped some steps to get the amp working. The amp can potentially have the bias set to zero volts if someone turns the pot. But the pot has a locking screw that I have tightened down.

Was the modification worth the trouble. You bet! Now I can play around with any 12AX7 Tubes without fear of moving the bias point around (previous design used 3x12AX7 filaments in series for a voltage of -32 Volts, which almost always was more like -29 Volts.

Now I have a full -20 VDC on pin 6 of the 7591 tubes, not the measly (-15) to (-16) VDC from before. The amp runs well at 115 VAC on the primary via my Variac. Direct to the wall at 123 VAC the amp is too stressed to play well with modern 7591 tubes. Fresh NOS 7591's can hold the abuse of -17 VDC bias at over 400 Volts DC!

I dropped-in a fresh quad of 7591S tubes from JJ Electronic. These tubes sound 95% the same as a real 7591 or 7591A. Can't say the same about the Electro Harmonix 7591A tubes. These appear to be a variant 7591, not the exact physical analogue of the original tube. I find the JJ 7591S to have a very deep and at the same time delicate sound stage. The Electro Harmonix 7591 sound a bit rough and constricted but it is robust and bulletproof. It just may be that I need to re-test at the new bias setting. Hand's down I recommend the 7591S from JJ unless they have the same reliability issues other JJ Power Tubes have had. Just make sure that you don't have an amp that runs them close to the limit as the A700 Harman Hardon Award Series amp or most any 7591 Vintage amp.

The A700 sound is best described as 'wet' and 'syrupy' while maintaining a very broad and deep low end, slightly under-damped. A good match for any high efficiency speaker system that has at least 8 inch woofers with 94 SPL or better. I know that this amp works well with my JBL LX-500 speakers. Hopefully the rest of the speaker family will get a chance to work with this great vintage sleeper amplifier series.


I continue to be amazed by the evident and present improvement this amplifier has undergone. I am not 100% sure as to exactly what made such a dramatic difference in the sound quality. But I am suspect that the combination of the JJ 7591S tubes, 2xJJ 250uF/500V Power Supply Voltage Doubler capacirors and the separate Bias power supply upgrade combined launched the sound of this classic 40 year old amp to levels unexpected. If you need help on the details of these modifications, I would be glad to send you digital photos of the work and also schematic diagrams of each modification.

One thing is evident, in that the sound quality of the rebuilt circuitry paid off. I have completely replaced every ceramic disk tone capacitor with silver mica types. The signal caps are NTE style 0.25uF/600 Volt, not the ultimate but sounds acceptable to me. The fast recovery diodes in the power supply added to the completely new circuitry surrounding the 7199 tubes completes this re-build. The original Phono Section electronics are still there. This is one piece of equipment that could probably benefit from a matched quad of Jensen 0.25uF/600V Paper-in-Oil Copper Foil caps.

But now the JJ 7591S tubes apparently are not holding-up. They have developed red-plates and are now put away until I can sort out what is going on with my A700. JJ's out EH in.... stay tuned....


After a long hibernation period (about 5 years) my A700 went on the bench today for some ambulatory surgery.

Recently ordered (10) 200K Miniature Linear potentiometers. The original Bias Balance pot's were cooked beyond recognition.

These Bias Balance I.M.D. pots actually sit dead center, in between the bases of the 7591A tubes.

Like sitting right in between two campfires on a warm evening, hot hot hot.

So out came the old and in came the new. The original 200K Linear carbon comp pot's were, in effect cooked out.

Even the color indicated many years of heat and chemical transformation. These parts measured 240K and 260K each.

The pots had a weird waxy residue probably from the heat. With the small currents that fixed bias circuits draw, it's probably the old and

resistive nature of these pot's that made my A700 a problematic amp to settle.

With the new 200K potentiometer and the McShane 7591 Grid Resistor mod, modern 7591A will not be an issue.

I first used a quad of NOS 6GM5 with the adapters and the sound is glorious, almost to the point of wanting to hear the Mormon Tabernacle Choir

at full tilt, or Beethoven's 9th Symphony.

With my new found sense of optimism, I reached for the Quad of JJ 7591S tubes. These JJ 7591S tubes had always given me red plates.

The old saying "I told you so" came back with a bang ! With an almost predictable sense of doom, sure enough after 15 minutes of play one of

the JJ 7591S blew-up, it lost it's vacuum.

This tube took out the 3 amp fuse on the A700.

But in 5 minutes I had her up and working again.

A 3A replacement AGC fuse and the quad of 6GM5's, music............

So far this quad of JJ 7591S have earned a bad reputation.

I would love to be able to use JJ 7591S, they are smaller and allow the amp cover to be used.

Unfortunately the current breed of 7591A replacements don't quite replicate the original design.

One thing is important when purchasing the current production 7591A tubes, buy them from a reputable

dealer who is testing them in real 7591A amplifier, not a tube matching unit that uses 300 Volts DC on the plates.

I was able to see by testing these tubes, at 300 Volts they have characteristic curves that look identical to a real 7591A.

But then apply 400 Volts DC to the plates and things start to change. All in all I would modify any 7591A amp to accept 6L6 tubes

and avoid the problems that current production 7591A tubes have. In closing, and this is my personal opinion, the Electro-Harmonix 7591A tubes

seem to be the more robust of the current lineup of tubes......

October 2017:

The A700 is one of those amps that can be totally fixed as it does not use any special CR type modules that are today, unobtainable. My only comment on the A700, I have found that the original Output Transformers are lacking in Core Size, and I think that they are a bit under-dimensioned. The 7591 Tube is a design that runs currents similar to a EL84 but develops the additional 8 Watts of Power by the much higher Plate and Grid Voltages. It is completely imperative for A700 Owners to make sure that the Bias Circuitry is refreshed with new parts. The lifeline of any Fixed Bias Amplifier is the presence of the Bias Voltage. On 7591 this Bias Voltage is typically between -17VDC and -22 VDC.

- Stromberg-Carlson AR-411 Mono (pair) 8 W/ch >

Restoration difficulty #5

Budget $400. Very dense construction, brittle cloth covered wires. This is not a beginners amp and 6CM6 tubes, they sound great, but are out of production. The 6CM6 is electrically equivalent to the 6V6, but a different physical design. So I basically had to replace every single part except the Controls and the Transformers. At the last moment I also replaced the final Metal FP capacitor that is not shown in the photos for the new owner. I also ran new wires as the existing wires would crack apart due to age and oxidation. Lots of these wires were cloth covered.


The Metal Can caps were both swapped out as the failed due to age, but lasted a few years...

These smallish mono tube amplifiers produce some very unique Jazzy like sound, but are limited by the fact that the 6CM6 pentode power output tubes don't 'speak' with much authority. The circuit topology makes up for part of the limiting aspects in it's sound quality with the non-standard use of a 6AV6 tube as phase splitter. One positive aspect of the AR-411 lies in the fact that they are tube rectified adding longevity to the life of it's tubes and providing that special 'clarity' unique to tube rectification. 

The only original parts left after my restoration in these amp were:

Case, Transformers, Pots, 1 metal can cap, tube sockets and bias power resistor. The rest of the components were swapped out in sheer frustration when nothing I did would make these amps settle down. Now they are every day amps with smooth sound, yet needing efficient speakers. As with any old tube amplifier, replacement of the old existing power cable will improve the sound as well as the safety.

A lucky new owner in Michigan has these, and they are daily players. I put dozens of hours into these, and learned a tone of new tricks.

These were a bargain at $150 each and they sounded Wow...

I actually have a YouTube Video of them playing at 120VAC powered by a Balanced Power Transformer.

Anything more than 120VAC would send the 6CM6 tubes into red-plate territory.

- Stromberg-Carlson ASP-422 Stereo 20 W/ch >

Restoration difficulty #3

Budget $300-350, the 6DY7 Tubes are quite rare and unique. These amps have a very nice sound, and look super retro. They come up for sale less often now. The big caveat are the 6DY7 tubes. These are a super interesting Dual-Pentode in a single tube so with two power tubes you can make a Stereo Push Pull amplifier as we see here. Just the looks alone make this a museum piece. The existing signal caps were all leaky, so I replaced with Russian PIO "Greenies". Lovely sound indeed.


This General Dynamics design was built and sold by Stromberg Carlson in the heyday of tube audio. This amp is quite rare, and even rarer to find in original condition. The tube lineup is 2 x 6DY7 and 2 x 6U8. The 6DY7 is unique tube that has dual pentodes inside one glass bulb in with an octal socket base. Apart from it's unique design, the biggest sonic improvement to this particular amp was the replacement of the NOS GE 6U8 with Amperex Holland Gold Pin 6U8's. The sound is now rich and full bodied with enough power to drive the average speaker to very pleasant levels. All main Signal caps were replaced by K40 Russian Military types and the power supply section was completely rebuilt by me. Once again another $250 dollars worth of parts and time in a $50 dollar basement find. Please replace the existing power cable with a fresh minimum 16 AWG type.


I put the ASP-422 up on stage and the amp had caught a cold. The Right Channel was significantly lower than the Left Channel. The sound was almost lifeless and getting weaker. I popped the bottom cover and with a good 100 watt lamp and my trusty Bi-Focals I saw a crispy 56K/2W Carbon Resistor. In addition the circuit breaker that this amp uses was also intermittent. I upgraded these parts and replaced the old and tired circuit breaker with a standard fuse. The last tweak was to replace the bias supply half wave rectifier diode with a 1N4007 UFR (Ultra Fast recovery) diode. I also said 'what the heck' and replaced the four 100K/1/2w grid resistors that feed the -30VDC to the output tubes. Measuring these old Carbon Comp resistors yielded 122K/118K/112K and 107K. Now they all have 100K 2% resistors that measure each 99.98Kohms, a much better balance was attained in feeding the signals from the Phase Splitter towards the 6DY7 Output Tubes.

Next the amp was re-tubed, brushed off with care and powered-up at 80% on my Variac. She literally sang like a canary. The fuse I used was a 1/2 amp type which gave-out last night watching the new Al DiMeola DVD. The AC Current Meter read 0.38 amps AC, so a 1/2 amp I though close enough, wrong!

It ended up to be too close to the limit so I promptly replaced this fuse with a 3/4 amp fast acting and turned the amp on again. The soundstage improved with the thicker fuse and the fuse did not fail again for the evening. We then played an older DiMeola DVD called "Live at Montreaux 1986/1993". Man can Al play some serious acoustic guitar while sweating up the instrument.

This Stromberg Carlson is ready for another several years of music and sound. I really wonder how these vintage amps will be perceived by people +20 years from now, when they reach their 65th birthday and the Rolling Stones are touring the USA in their 70's doing a benefit concert sponsored by AARP.

Nov 2011 update:

My good friend Brian owns this amp now. I thought about putting this on ASP-422 on Ebay. It was such a special amplifier to me, so I wanted it to find a known home, not end up on a shelf somewhere.

Yup, I sometimes get sentimental about these old tube amps and this Boston special was no exception.

- Stromberg-Carlson APH-50 Mono (pair) 50 W/ch >

Restoration difficulty #3

Budget $450-500. The Power Supply is fundamental to the proper operation of these classics. Also, these come with a Input Transformer that is often missing (for Balanced Audio Connections). Uses a 6N7 tube out of production. These amps use LOTS of tubes, and are not particularly good in the musical detail department. But for bright speakers they match well. Also these make phenomenal Bass Amplfication heads with a 15" Driver matched. If you are a Bass player and want an exotic amp, these may be what you need to look for. Rare and awesome, these amps have the goods, and are a tonal festival using dual push-pull 6L6 tubes. I used them on my High Efficiency Zu Druid speakers, and the match was obvious. Synergy and tone, hand in hand.

I cannot say enough about these 50's Movie Theater amplifiers. I replaced all power supply and internal electrolytic caps. In addition some leaky 0.1 uf capacitors were upgraded with some Russian K-40 Paper-in-Oil 1000V Green Axial caps.

These amps may not be Hi-Fi, but they sure have plenty of percussive 'slam' from a quad of Svetlana 6L6GC tubes. The Output Transformers have plenty of taps to which I can only find use for 8-16-32 Ohm. The rest of the taps are 70 Volt and above, owing to the professional sound application nature of these classic mono-blocks.

Interesting to note that these mono-blocks don't offer the standard RCA Input Jacks and Screw Terminal speaker outputs. I feel that whenever these classic amps are 'live' that I am actually hearing the past in the present.

These are relatively rare in availability, and they are also harder to find in an un-altered condition. Due to the fact that previous owners tend to drill, modify and 'upgrade' them to accept today's style Banana Plugs and John-Holmes rated RCA connectors, I feel lucky that the one's I landed were in pristine condition (see the photos.)

The  most unusual thing about the biasing scheme on these amplifiers is how it is derived from the center tap of the power transformer. These amps come with a special one-of-a-kind multiple tap 60 watt power resistor. It is via this ceramic monster that the center tap of the Power Transformer is connected to ground. An electrolytic capacitor is connected to the point where these two meet and connects to the grids. I will draw-up this schematic or scan the original as best as I can. A picture speaks a thousand words.

These amp are not considered HI-FI, yet the can deliver some very interesting music with the proper loudspeaker system. By proper I mean one that can deliver full range sound, not the bookshelf types. Last night I wiped the dust off these amp and connected them up. The sonic difference from no-Variac operation to having the Variac installed was akin to lifting a veil from the sound. Without the Variac the transformers has a small vibration, one a bit more than the other. Yet the woofers exhibited a low level hum which was almost imperceptible when I lowered the mains voltage to 115 VAC down from 123 VAC. In addition the power tubes and the complete amp relaxed.

Once again I face the indisputable fact that un-regulated power supplies depend on a reasonable value of mains voltage, which today is now actually 120 VAC instead of 115 or 117 VAC. Replace the existing power cable with a 16 AWG minimum AWG, preferrably a 14 AWG type or even a 12 AWG. The sound will improve dramatically over the older types which have long lost their ability to transfer power without a significant amount of series resistance effectively 'choking' the sound at the source (the AC wall outlet.)

- Dynaco ST-35 Stereo 17.5 W/ch (I made my own SA-35 custom build w/ Tube Rectifier) >

Restoration difficulty #2

Budget $250-300. The original Dynaco Power Transformer runs super Hot on 122 VAC (as do most Dynaco Plate and Filament Transformers) so a Variac is necessary not to drive the EL84 tubes past their published operating points. You will know when your ST-35 is stressing, the Bass performance suffers and the amp will sound bright lacking presence. See my suggestion to build one from scratch with a Hammond Transformer and a 5AR4 Tube. Now this is Hot Rodding!! like putting Headers on a 240Z.

So I no longer own my original ST-35 Ebay amp, but I made a new one.

Despite the fact that this is a "clone", I have placed this in the restoration section, and it is much better than the original ST-35.

My version of the ST-35 has a far more robust Plate and Filament Hammond Transformer, sports a dual primary 115/125 VAC.

The Dual Transformer Primary allows Hi and Lo DC voltage distribution, and this is an awesome feature.

I use Lo for EL-84/6BQ5 so they settle at the best possible operating point, no more dead 6BQ5's, from being run into the ground.

I use Hi for 7189/EL84M for that extra plate voltage needed to squeeze the extra few watts out of a pair of 7189 power tubes, the amplifier sounds a bit tighter in the Bass region with 7189.

Also my version has 5AR4 Tube Rectification, and it runs circles around any stock ST-35, and I mean ANY ST-35... I say bring it on buddy, You have never heard what an ST-35 can do.

April 6, 2021

My DIY ST-35 is now playing with all Tubes, no diodes....

The amp sounds simply so much better than stock.

Using a 5AR4 Tube Rectifier "musicalized" this amp. This is by far the most significant sonic upgrade to this amp I have done.

No matter how many fancy parts, expensive caps, tantalum resistors you swap out, nothing beats a better Power Transformer and a Tube Rectifier, this amp sooo musical now.

I am tempted to say that this amplifier, in some near future will become a significant DIY project for people looking for better sound.

One word of advice, if you can buy original Z565 replacement OPT's, do so now. I can unequivocably state that the original Z565 OPT's are amazing.

Current production replacements are good, but not as good as the Dynaco Z565's.

I cannot say the same for the PA-774 Dynaco Power Transformer, I dislike the old and new versions, they run HOT, and have no headroom.

My clone sounds so awesome, I have to eventally make more of these, for the world to enjoy.

If and whenever I take this hobby to a business and register it, we shall see. Maybe in 2023 or 2024. For now this remains my Hobby and pastime.

View of my upgraded ST-35 with the awesome Power Supply & 5AR4 Rectifier, also a Bias Board.

I can dial-in the current through each power tube, yet I always recommend matched sets on each side.

A mached quad is the ideal, but not necessary, matched duets are fine.

My ShermanAudio SA-35 built on a chassis that I got on Ebay

Bottom view of my hot-rodded SA-35, you can see a C354 Choke added, another upgrade over just a resistor.

Costs are always a factor but when one makes equipment from scratch, we can put an extra $25 in the amp and not

have to worry about a List Price and Lower Margin....

My review and views of having owned a real vintage Dynaco ST-35

The ST-35 uses two EL84 Power Tubes and Two 12DW7/7247 Dual Triode Input Driver tubes. The 7247 Military designation is today hard to source so Ei in Yugoslavia is making a really nice 12DW7 version that sounds every bit as good as the NOS originals. I recommend that if you happen upon an unadulterated example of the ST-35, that you don't alter the physical characteristics of the amplifier, please accommodate it's design. By this I mean that I have seen too many ST-35 amps 'ruined' by people's so called  'upgrading' the speaker connectors and literally butchering the chassis to install new power supply capacitors that just don't fit. My recommendation is to make a good pair of speaker cables with spades that fit properly, with enough heat-shrink tubing to prevent chassis shorts. 

The sound of the ST-35, legendary! It's just as good or even better than as all the web sites raves put together. The Z565 output transformers were wound to exacting specification and have an amazing frequency response and musicality. The amplifier design (Ed Laurent- Dynaco) is also responsible for the amazing stability and sound of this classic amplifier from the golden age of Dynaco amplifiers. It helps to have a pair of efficient speakers as you only have 17.5 watts per channel and that's with a fresh tube set and good FP PSU Cap, ST-35 owners..............................Enjoy!!!

BTW, remember to replace and upgrade the power cable.


I am sorry to report that the CE Manufacturing Capacitor that you see below failed. This really surprised me as the maximum DC voltage in the ST-35 does not even come close to the 525 VDC rating of these recent manufacture caps. The Music Room cannot recommend these caps unless someone informs me that they have used them with success. The only survivor from my Mullard Mono-Blocks was given to my friend. Let's see what he says about using this cap. CE Manufacturing claims to have the original Mallory machinery. But I think that they lack the Mallory know-how. The ST-35 is not working now. I will replace this FP Metal Cap with some JJ 100/100 uF and install a Spague Atom 20 uF/600V under the chassis. This should take the ST-35 to where it needs to be!


The inmates on Audio Asylum have concluded after several posts that the caps I am using are only rated to 55C. The conclusion is that this temp rating in not sufficient to withstand the rigors of Tube Amp operation. Therefore I have to suggest that one order the 85C rated CE caps and this should solve the problems with the failures. I was very concerned that out of the three CE caps I have used, 2 have failed in the exact same manner, despite the fact that they were installed in two very different amplifiers. The failures were not immediate, but took several month's to occur. I remain skeptical of these caps, but in all fairness, the Asylum friends have used these with zero defects so far. So my advice, order only the 85C rated caps, not the 55C one's.

You can fetch this diagram for your efforts on the ST-35, lovely and color coded.

Courtesy of Mr. Bob Latino (Bob's e-mail is in the diagram).

There are lots of Dynaco manuals here:

Bob is an internaitonally acclaimed Dynaco SME - Subject Matter Expert.

Bob also sells amazing kits and has a forum where new Tube Hobby Audio enthisiasts can ask questions.

Thank you Bob for sharing your Dynaco knowledge, you make this Hobby cool again.

Check out Bob's 120Watt Monoblocks, those amps do not fear Thiel Speakers...


The fried CE manufacturing cap will either go into the garbage, or returned to the factory for analysis. Anyone out there want to see what a CE cap will do with more than 100 volts under maximum is welcome. I ordered a 50uF/50uF @ 500 Volt JJ Cap with clamp. Plus I also ordered a Sprague ATOM in 10uF, 20uF and 40uF. Whatever fits under the chassis will win the contest! The new power supply caps should bring my ST-35 back on-line, just in time for my Zu-Cable DRUID MKIV speakers!


Replaced the CE cap with the dual JJ 50/50uF @ 500V. I was able to squeeze under the chassis one Sprague Atom 10uF/500V. But I plan to replace this with a smaller diameter electrolytic so that the bottom cover fits snugly. The ST-35 is not officially working. I just want to change the original Cathode Bias resistor, and the two main Power Supply resistors so that the unit is 100% stock values brand new. At the moment this classic is awaiting the last upgrades before being put into full service again. Into the amplifier pool to get rotated at least once every 2-3 months. I usually play each of my amps for 5 days before I rotate it.


The ST-35 continues to amaze me. I mostly use it to test Preamplifiers on my workbench. The tiny mighty puts out enough power to move my DIY 2 way's and keep a grin on my face. I recommend to anyone attempting to restore this amplifier to consider replacing the PC board and re-stuffing it with your favorite parts. Install a fresh JJ 50/50uF or even a 100/100 JJ Cap and a nice 470 uF Electrolytic (ELNA or Black Gate ) under the chassis with a new fresh new Cathode Resistor to finish it off.

Oct 2011

ST35 is gone to a new home. Another Tube Lover to enjoy this remarkable stock amplifier.....

ST-35 Adjustable Cathode Bias

This the ST-35 Bias Control Circuit

I highly recommend this as an upgrade to your ST-35

Allows tweaking of the bias current a few milliamps

And the design metering points

are calibrated to map DC Volts to Milliamps (very nice circiut design).

You can look at this upgrade here:


- Dynaco PAS2/3 & 3x Stereo Preamplifier (and PAS-4 schematics) >

Restoration difficulty is a solid #4

Some design critical components are out of production like the Balance Control. If the Balance and Volume controls are less that working, and you have never heard a properly restored PAS, you will think that this preamp sounds "tube-like", nothing can be farther from the sonic truth. If you want to seriously ugrade and pay tribute to the real schematic, budget around $300-500 in parts for a properly restored unit. These are densely wired and often, come from very poor Kit build skills. I would look first for a Factory Wired unit if you can find one. Lots of the performance is lost through amateur solder connections and a weak Power Supply. These PAS are best sent to an expert, as they will only perform their best when completely checked-out on an oscilloscope. The Balance Control is impossible to find today and plays a major role in the performance of this marvelous design. I have a PAS3X that sounds like a dream, but only after many dozens of hours of careful change/listen sessions. I have a pair of 0.22uF TFTP Vcaps and this absolutely rocks the house down. Vocals and Tympani sound so real. I love to listen to Vangelis "Mythodea" on DVD through my PAS3X. And the Tone Controls are not always centered :)

My old PAS2 from a few years ago, given to a a good friend for $50, the price of the nee FP cap and new 12AX7's. He enjoys it very much to this day. On this PAS I learned about how mushy these old preamps sound when operated in their full old original state.

My NOS PAS-3X purchased as a partially assembled and botched kit. It was like fixing an old brand new Time Machine.

Even though the parts were all NOS from the Kit, both the Volume and Balance Controls were horribly out of tolerance.

So be warned, when you buy an brand new NOS part such as these, they come aged and these parts don't age well, and neither to Electrolytics and Carbon Comp resistors.

The Bass and Treble PAS3X Controls were so stiff, I said a prayer before using CaiLUBE.

These PAS3X Tone Pots are impossible to find.

The PAS Balance Control that have unequal wiper resistances, work with the resistors when they are correct in value, else you will have noticeable L/R volume differences with tha Balance Control in the center position.

All old Dyna PAS2/3 users, you will NEVER hear what these preamps are capable of when you use them in stock condition.

These age poorly and the music suffers.

I can state that after I refurbished my PAS3X, it is a KEEPER and wil be on my last Will and Testament.

My DYNACO PAS3X is a pristine example built by me from a NOS Kit from 1975, with the necessary parts to break away from the boring, sleepy vintage Dynaco PAS.

Note that I am not one that would sell the original box on Ebay, and some may cringe that I discarded it OMG!!!!!

The cheapskates are shell shocked, but who would sell old cardboard?

Some people do...

My lucky Dynaco PAS3x NOS Kit built in 2012

This is my amazing Dynaco PAS3X all tricked-out and sounding simply musical and crisp.

The signal flows clean through the wiring and I traced ever single solder node to make sure that nothing was lost or subtracted.

Even the old cheap and rusty 1uF Electrolytics on the Tone Controls were painstakingly sourced by me in the form of real ASTRON Paper in Oil 1uF Caps, sweet.

These were not leaky (yessss) so I ordered a few pieces for future PAS3x restorations.

That SDS Labs Power Supply Board offers great filtering and a very quiet performance, simple.

I kept the Tube Rectifier as I prefer how smooth they sound and how soft they are on the tubes.

So here is a photo tour of my PAS3X below, the result of parts searching and exact value components, & lots of bench time

I did find that some of the original parts still measured good so these were left in place, but other NOS parts were an embarrassment.

None of the original Capacitors passed my bench test, some had leakage (and they were NOS pieces) and some with significant drifts in center value by more than 20%.

On the main Blend Channel Wafer the 68K Carbon Comp was NOS and it measured exactly 68K so I chose to use it.

The Made in China ALPS Volume Control runs circles around the stock PAS Volume Control, there is no comparison, the low noise and perfect tracking is meant to be heard.

I conclude that some of the magic of the PAS2 and 3 Preamps is all lost in old Volume and Balance Controls, and also old solder joints.

The Volume control is OK to replace as plenty of options abound, but the PAS Balance Control is another story.

I actually puchased a NOS new in the Box original and it really made the Preamp sound alive.

Also both L and R channels are balanced on my Oscilloscope, something impossible with the stock and drifted Balance Control.


Every switch, contact, connector, component, etc. was measured, and tested for optimal operation.

I even left the Metal Can cap in place, it had never powered up, but leaked DC like a garden hose.

The Volume Control is an EBay ALPS and I have wired the Loudness Tap so the Loudness sounds just awesome, musical and great for late night use.

A good Stereo Volume control is an ABSOLUTE MUST. You will be amazed at how much better an old Tube Preamp will sound with a fresh

new Volume and Balance control that uses much better materials than the old legacy Carbon types.

April 23, 2021

I love DYNACO stuff. it is how I really learned about Tubes, Amps and Fixing/Upgrading and even building new examples for optimal performance.

So here I am back again, and have a little bit of interesting information about the PAS Preamps to share.

About 1 month ago I updated on my PAS3X the Balance Control (yes I bought a brad new NOS piece, impossible to find these days).

And noticed that the resistors on the Mode Selector wafer, specifically the 68K and 33K on the PAS3X measured wayyyyy off.

So this morning, I found an old photocopy of a PAS-2 Schematic when going through my old ARRL Radio Amateur Handbook 42nd Edition 1965 that I had folded inside of this book some 20 years ago.

There was an interesting side-note on the bottom of the PAS2 schematic that provides an explanation and insight into the PAS Preamps.

Just read below The Venerable Dynaco PAS-2, and now that I understand the importance of the proper values of these specific resistors, I was floored at how much of an audible improvement I achieved with the new 750K Balance Control and exact value resistors on the BC


The PAS3X has special Tone Potentiometers and these are impossible to source.

The\y actually were designed to "disappear" when in the 12'Oclock position.

And 1uF Electrolytic Capacitors in the Tone Circuits lessen the POP sound these controls make.

Also different value resistors on the BS Wafer make the PAS3X different than the PAS2/3, otherwise they are identical.

The most difficult parts to substitute are the Balance and Tone Controls.

Now the Balance Control has a strange pair of Resistive Elements and has been out of production for decades.

This Balance Control works with the BS Resistors to allow the Channels to be selected

Stereo, L, R and also to Blend the Stereo Image with discrete components.

SCHEMATIC and Notes courtesy of an old 1990's PartsConnexion Catalog

These resistors are key to getting the PAS Balance Control to work well, else one gets way different volumes across each channel.

Now the Hafler3 Speaker System? Sounds interesting, don't know what that is all about yet, gotta do some research.

So if you own a PAS2 you need to make sure that the 560K and 270K are within 2%, same as the PAS3X where my 68K was measiring 38K and the 33K was measuring 18K.

Carbon Comp resistors are super nostalgic, but drift like Hobos on Freight Trains. If you can get them to measure less than 10% from value, what the heck, go ahead!

July 3, 2017

Here I am checking in yet again on my venerable Dynaco PAS3x series classic Preamplifier. A few years ago I purchased a partially built, yet mostly un-built, PAS3x. A lovely Ebay 1971 Dynaco Preamp, in the original Box with the 5 original tubes; 4x Telefunken Smooth Plate ECC83 and a USA 12X4 Rectifier, sweet stuff if you ask anyone in the know.

Of course, I built the unit from the original parts, hoping.... Yes hope, that all those old NOS parts would hold-up. Well, after 1 day of use, I heard a loud POP  and the music went, and so did the Selenium rectifier.

I decided that for the best performance, I had to upgrade the Power Supply, so off I went into upgrading.

My first choice was to order and install the SDS Labs Power Supply Module. I also replaced the old two 1uF DC Blocking Electrolytics on the special PAS3x Tone Controls with very nice NOS PIO 1uF caps/50V caps, metal encapsulated and ordered a pair of 0.22uF/400V Vcaps TFTP for the output sections.

The rest of the Preamp got fresh parts of all kinds, I like to mix & match, like a real good Salad. But my criteria is to always install matched parts between the Left and Right channels.

From there I used this PAS3x as my main Preamp for many month's and it became a few years....... until one day I finally put in on the Bench, on a hunch that the Right Channel was weaker than the Left Channel.

Yes it was very much weaker, as I traced a 1KHz signal from input to output my troubleshooting isolated the drop in level directly to the Volume Control. When I finally removed it one Carbon Wafer measured 283K and the other 221K, here again the source of the dramatic channel imbalance. Yes one can correct this with the Balance Control, but it never makes the sound as good as when the electronics are all truly balanced.

The challenge with any old Dynaco PAS Preamp is that the oroginal Centralab Volume Controls uses a "unique" taper on the resistive element.

With a unique Bass Tap resistance value.

So when an original Centralab Volume Control is replaced, the Dynaco PAS Preamps lose the orginal Volume range at lower levels.

The original Volume Control for the Dynaco PAS were designed to slowly increase in resistance until a specific point is reached. This allows for a nice low volume low-level adjustment.

But it these old original parts comes at a hefty sonic penalty, the original controls just don't do the PAS justice, and many a reviewers accuse this PAS Preamp design of sounding soft and dull, not detailed.

But once the cheap Volume Control is upgraded, it opens up the Preamp to a higher level of performance, without the need to alter the original design with after market re-designed boards.

My plans now include the measurement of the original Centalab Part and hopefully get a quote for a manufacturer to duplicate the original, but with top notch quality.

As I state, not all Volume Controls have the same standard Logarithmic Taper and the Dynaco is one of those Preamplifiers whose Volume Control was specially designed and manufactured with a unique Taper.

From what I have heard from the dozens of PAS2, PAS3 and PAS3x, the resistance starts rising slowly until the wiper reaches the Loudness Tap level. It then starts to rise faster in impedance, the slope changes. This design offers the Dynaco user the ability to enjoy a very wide low range, and then kick in the gain once the 11 o'clock position is reached.

I have used the ALPS 250K and 100K, both work well. The ALPS clones available on Ebay also offer a Loudness Tap and this allows the Loudness switch to remain operational and is often something that is lost with a standard replacement volume control.

The Balance Control is another key performance debilitator, but not as much as the Volume Control, and in my PAS, it is fine as it is.

Another channel balance optimization tweak is to measure the 3.3 Meg resistors on the Hi Cut switch. If these differ by more than 10%, it will cause a channel imbalance so it is a cheap upgrade to replace both of these resistors.

So, am I enjoying my PAS3x.... heck yeah.

Was it worth all of the new parts, of course. But some purists would argue that keeping the PAS2-3-3x original also has some value.

I agree in general, but to see what this Preamp can really do, new, matched, balanced parts make both channels perform at a level a 50 year old piece just cannot achieve, no matter the kicking and screaming of the Dynaco pursists out there. Mind you, I am not into butchering, I love to make small changes at a time and listen for the improvements as I listen and measure.

My Signal Generator and Oscilloscope allow me to check for channel balance and proper operation. My distortion meter helps me check the IM and THD.

It is almost imperative that listening and measurements go hand in hand to validate any improvements. Just changing parts and not measuring is like upgrading your Car and leaving it idling in the Garage.

I cannot say enough about this most classic of tube era pieces of classic 'Americana.'

Many of these PAS Preamplifiers are available on Ebay today, and if I was you, I would seriously consider making one part of my audio stash for future reference. If stock, the PAS-3 can provide a level of quality that is often associated with preamplifiers of their vintage, but with a bench test and parts refresh, this classic can sing with the best.

And there are many Power Supply upgrade kits in existence that can elevate the performance of the classic schematic or completely substitute the design with improved sonics.

In my particular case I had once back in the lat 90's completely restored my older and prior PAS-3 with the Vacuum Tube Valley upgrade kit (no longer available) and hunted down some special value resistors and Silver Mica caps for a stunning sonic result.

With this PAS3x I have been a real happy owner since I upgraded, and it makes an excellent partner to my Dynaco ST-35 and also my MK3 Dynaco Monoblocks.

Most PAS preamps are prone to noisy selector switches, so when going from AUX to Phono, please lower the volume to ZERO or risk a loud POP. This is all part of being 'Vintage Tube Qualified' the small idiosyncrasies of tube stuff we learn to live with (like the crackles in LP's.). Many PAS3 Preamps suffer from inter-input bleedover and this is mostly due to interactions in the internal wiring.

The PAS preamplifier underwent many incarnations around the basic design. The first incarnation I believe was the PAS-1. From here evolved the PAS-2 with 0.02uF output caps. The PAS-3 had upgraded exterior cosmetics and 0.22uf output caps. There were also some other versions PAS-2X and PAS-3X with special tone control's that actually 'disappeared' from the circuit when in the dead center position. Some of these special potentiometers make a small 'click' when placed into the center or 'bypass' position. There are two small 1 uF electrolytic capacitors on the PAS-3X that could be replaced by film or Oil types for improved overall sonic performance. If real picky then order ELNA, Black Gate or Nichicon MUSE 1uF electrolytic caps for that special extra performance! Sprague ATOMs will also do fine.

The PAS-3 wa- conceived by the genius of Mr. David Hafler (may he rest in peace) and Mr. Ed Laurent.  Back when tube audio was in it's heyday, Dynaco was at the helm. There are many special features which distinguish this preamplifier from any design past or present. The tone controls are very unique and provide a level of musical control uncommon in typical tone controls. Replace the existing power cable for much better sound. Any good 2-wire 14-16 Gauge type will do perfectly.

I had once modified my PAS2 with an original 250K Black Beauty Alps Volume Control. The original Centralab Stereo potentiometer was intermittent and worn-out. Unfortunately the sourcing of an identical replacement is imposible. This performance improvement was the single most startling benefit to this little Preamp I had ever imagined.

The Sonics are rich and palpable, the Mids and Highs densely packed with musical enjoyment, Bass is tight and controlled. I highly recommend that if you own a stock PAS2/3 that you upgrade along with a complete set of tested and matched 12AX7's and a good 12X4 rectifier.

With so many modifications available for this classic preamp, about the only one that makes sense to me is the selector switch mod to replace the NARTB and Special equalizations for line-level inputs. This way one gains an extra set of inputs allowing the PAS preamp to switch among more sources.

One can also source the volume control from DACT or CTS. The CTS control can provide the tone-tap required to keep the Loudness Control operational. In my case I never use the Loudness and will probably convert to a Low Cut filter for when I play LP's that wiggle the woofers a bit beyond my comfort level.



A pair of dedicated designers sat down and produced a humble but heavy killer, the PAS-4. At it's price point one would argue that this piece was built to last. The PC board HW such as the RCA's, Volume and Controls would have been better to be chassis mounted instead of PC the cost saving PC board mount types.

An italian HiFi purchased this PAS-4 from me using Audiogon. Despite this the PAS-4 is capable of remarkable performance on a fresh set of 6DJ8/6922/E88CC/ECC88. Some people put 6N1P and pull the Phono 12AX7 Tubes. If one uses 4x6N1P and 2x12AX7 the filament regulators will stress out. One needs to modify and put a higher capacity regulator but risk burning out the transformer secondary.

Here are the scanned schematics for your perusal and interest....

PAS4-1    PAS4-2    PAS4-3    PAS4-4

The PAS4 is similar to a Audible Illusions in that these run the 6922/6DJ8 at higher than average currents, cutting their life by half. But the sound is amazing. 

Enter the PAS3X

Gosh I missed the old trusty PAS3 Preamplifier. I had given it away to a buddy from my office who also bought from me the Stromberg ASP-422, the one with the odd and out of production 6DY7 Tubes.

One evening while surfing the Ebay site, the results came back, an original and partially assembled, N.O.S. PAS3X Preamplifier.

Spending some real good money, I had to wait over 7 weeks for the Preamplifier to arrive. I was, as typical for my current job, sent unexpectedly on what was to become a 5 week extended Business Trip.

Just as I had sent the Paypal payment, I had to have the seller hold the item for over a month as UPS was his only method to ship.

Funny that I had to fetch the Preamp personally at the UPS office as of course, my Paypal address is my home, and of course no-one was home.

After 5+ weeks in exile, I was ready to open the new toy.

So.... here she is, a PAS3X out of the original box in all of her 1960's glory.

Picture was from the evening I got back from UPS and after I put the knobs on, just for looks. The nest day I was on my bench, checking out the unit and warming up the soldering iron.

Of course any 1967 kit that had been previously opened and partly assembled will have to be missing some pieces I expected, the Pilot Light Jewel was nowhere to be found.

Someone tried to start the Kit  and installed the middle switch upside down, so that person, that day, got it wrong.

I had to unsolder about 16 wires, removed the mode selector switch, rotated it 180 degrees (red dot up), correct some jumpers and spent another hour soldering it back into place.

The first thing POP was the Selenium rectifier as expected.

After bringing her up slowly on my Variac, over the course of 12 hours to 117 VAC, an audible POP was heard and the sound went away.

Immediately suspected that the Power Supply was shot as I did not see any lit filaments. No 12 Volts from the Selenium Rectifier voltage-doubler circuit.

I remembered that back in my old days I had ordered an "SDS Labs Power Supply" PAS board.

Off I went hunting for it among all of my stuff. A few expletives later, and over 11 boxes opened, I rummaged across the board and the instructions.

In a matter of 2 hours I had the PAS3x up and running with the Diodes, I chose to take out the 12X4 and use the stock tube for a higher purpose.

Why waste a 12X4 when there are diodes standing by ready to power the unit.

As you can see I have no Tube religion or belief system. I am a practical soul who wants things to work.

A Blue LED and a few out of tolerance resistors later she was singing.

May I remind my readers that when restoring Vintage Equipment I hate when people use the term "un-molested" in audio, I HATE IT WITH A PASSION.

Imagine driving a 67 Chevy with the original un-molested Radiator Hoses and Belts...... (get my drift).

All four Line Stage plate resistors were 30-40% away from their values and also differences between channels.

The Preamp had a squewed sound stage, almost like listening to music at 45 degrees. It was due to a channel un-balance due to the

out of spec Carbon Silver Banded resistors. The Cathode 4.7K and 1K Line Stage Resistors were also measuring quite differently as well.

I happened to have some Tantalum 4.7K, 1/2 Watters and some precision Metal Film 1K's @ 1/2 Watt.

Swapped out the 8 x Plate and Cathode resistors and the voltages are now only 2-3% different, now it's the tube sections not the Resistors.

Precision components go a long way to make things proper.

How does she sound...... Vintage and very, very smooth.

I have not changed any capacitors and will have to let her break in for 100 hours to see if her character changes. If it ain't broke, don't fix.

My current temptation is to replace the Signal Caps with Jupiter caps, just to stay Vintage, we shall see. I dont spend money like I used to, ending up with

parts in a bin after the so called "new parts" made the sound worse.

In my humble opinion, and based on past PAS3 restorations, Illinois Signal Caps get the job done with great sound, but impart a brightness I sometimes wish was not there

in my previous PAS3.

My Zu Druid MK4 speakers are very efficient to begin with and already on the bright side..... So this PAS3X mellows the stuff out well.

Here she is right out of the box, brand spanking NOS from 1967. Oh... and I include the box as well.

OHHHH forgot to mention... yes the obligatory Dynaco Branded Telefunken ECC83 tubes, yup they were included in the Kit all 4 of them !!

And they are all NOS as can be, I have to remember to be careful as the writing rubs off the tubes on a flash, it's like powder.

I put all 4x Telefunkens into the Dyna as it deserves the best, yet when I brought her up slowly on a Variac I had used a quad of regular 12AX7's.

So far, and despite the well known Curcio Phono Stage corrections, I will enjoy her flawed beauty. Only 8 resistors needed immediate attention, and the rest is pure 1967.

The SDS Labs Power Supply board really tightens things up nicely. And the Phono Stage sounds quite good if not a bit noisy when the volume is taken-up.

I suspect a few Carbon Composition resistors in the Phono Stage PC board need to be checked, but I will resist the temptation and let these components

run in for awhile before I plan my next move.... Oh that old Pioneer Receiver keeps calling my name.

Update April 7th (Great day, Sunshine):

The Vintage 1uF/100V Astron PIO caps arrived. Removed and saved the 1uF Electrolyitcs in case a dies hard Dyna Fan wants these exact caps to make his original...

Always the possibilities in the DIY hobby are endless...... networking is key.

Smooth is the sound with these vintage 1uF/100V PIO Astron caps....

And I connected the Blue LED to the +12V DC supply instead of across the 120V AC line as I had.

All I needed was Ohms Law:

12V = Resistor  * I + 0.75 VDC Diode... so I chose 5 ma of current.

This simplifies to:

Resistor = (12V - 0.75) / 0.005 A

Resistor = 2250 Ohms.

This is the majik,,,, but I had 2700 Ohms so I used that and the current will

be a bit less than the 5 ma DC, but who cares....

The Blue LED gives this Dyna PAS3x a true futuristic spirit in the context of preserving the

vintage circuitry. I am of the ones who prefer to leave the original parts there, unless they exhibit signs

of stress, overheat, discoloration, obvious swelling or discharge of electrolyte, the list on vintage

amp repair is like a time machine emergency room with sleuths setup to look at what is needed.

Wonderful hobby..... love to make things be the best they can.


A rare all original mint condition PAS-3x. Even this one with NOS parts needed TLC. Old Dynaco Parts were not very high quality.

- Olson AM-240 Stereo Amplifier 25 W/ch >

Rsstoration dfficulty #4

No known schematic exists. Budget around $300-400. Many examples have burned out Power and Output Transformers when used with EL84/6BQ5 and not 7189 so be careful when buying. This is pure 7189 ONLY AMPLIFIER and needs a Variac set to 117 VAC. The Power Supply runs very hot on 120VAC, it needs 115-117 VAC. If someone uses 6BQ5 tubes, the amp will play for 5-10 minutes, then fry.

This Olson Integrated Tube amp uses a quad of 7189 and is fixed biased from a 44 volt tap of the power transformer and 5 x 12AX7's. Unfortunately I lack the Top Cover, Bottom Cover and the Schematic, but this did not stop me from getting this little special 25 watt per channel amp operational! This is where my experience with tube equipment allows me to venture on w/o schematic diagram. 

This Olson AM-240 is diode rectified and I believe that many people upon the first power tube change substituted with EL84 instead of the 7189 (yes some manuals list this as a cross reference.) If one uses EL84 (6BQ5), the AM-240 may have lasted about 2 hours maximum before melting the cherry-red plates off the EL-84's or suffer other damage to the Power or Output Transformers. Another possibility is roasted resistors around the output power stage! The cause of this trouble is simple. The 7189 and 7189-A tubes have maximum plate voltages of 450 Volts DC. But, EL84's can only take a maximum of 300 Volts. I actually measured the plate voltage from plate to cathode with my Olson on after 5 minutes warm up and some 16 Ohm load resistors on the output. Measured Plate Voltage on the Russian 6p14p-EB equaled 435 Volts DC! Bye, bye EL84's and welcome the 7189's (or 6p14p.)

The 7189 bias is set to -17 Volts at the grid so this makes for 25 watts output with the +400 volts on the plates! Absolutely imperative to use only REAL, new and tested 7189 or if you can find them the venerable Russian 6p14p-EV or Sovtek EL84M that can take the 450 Volts on the plates with absolutely NO PROBLEM, actually can be pushed beyond 450 Volts on the plate if one respects the 12 watt plate dissipation rating. Thanks goodness for Russian Military quality (that is with tubes!)

The Olson AM-240 uses 5 x 12AX7 tubes, so it helps to experiment with tube rolling for that optimal soundstage. Unfortunately, I have yet to fix the AM-240 Phono Stage, so I cannot say how this sounds yet. The 25 Watt per channel rating is possible with the 7189, and no EL84 can produce 25 Watts per channel unless operated in Class B. Typical EL84 output power designs range in the 15 to 20 watt per channel range, the 7189 gives the extra 5 watts. A new power cable will probably make for a better sounding amp. This guy sounds very nice, gotta be those mystery Japanese output transformers !

This Olson Amp had some flaws that made it self destructive. First of all this amp uses a Power Transformer that has a Primary wound for 115 VAC. So plug this amp into 125 VAC and you are killing the tubes.

Another sad reality is most people in the 1970's did not know that using EL34/6BQ5 instead of 7189 would make this amp burn out and literally melt the Output Transformers, eventually the Power Transformer as well.

I have two of these and both are dead. I plan to have the Power Transformers re-wound for a 125VAC primary. With this basic modification, these amps can use the Russian 6P14EB variants and the Sovtek EL-84M for stunning sound.


The AM240 needs a new Power Transformer. This amp is on my project list. The Output Transformers have survived the prior EL84 saga's. Again these amps are rare in working condition. I should have taken photos of what happened to the Power and Output Transformers from my donor AM240 parts amp. Like old Cars, it is good to have a donor amp on these rarities and I gutted it for parts before I threw out the carcass.


I have a plan and the perfect substitute Power Transformer. The leftover Hammond 273BX has a 700VCT secondary that is 350-0-350 VAC, the Filament Winding is 5A and the Rectifier is 5A. My approach to optimizing the AM240 is to revert to Cathode Bias, use the 115VAC Tap to get boosted B+ and throw in a 5AR4 tube on the chassis. This will allow the amp to use 7189 and deliver the goods. I will need to modify the Transformer for horizontal mounting, should not be a big issue. This work will be done in Rochester once I get my storage goods delivered. Once I have the AM240 updated I will post photos of the final version.

- Pioneer SX-110 Stereo Receiver 30 Watts/ch >

Restoration difficulty #5

Most Tube Receivers are a real bear to work on. Budget around $800 when completed. There are a dozen of dry Suzuki capacitors in the signal and the power supply ELNA Caps are leaky. Also Tuner section may or may not work well. It would really require a full alignment when parts are replaced. The sound is amazing. I want to build the amp section of this stand alone with 6AN8 and 7868. A future project.... Note that 7868 tubes are again available and have the ShermanAudio seal of approval. I think they are better than NOS myself. Electro-Harmonix branded. The output transformers are special, they have a very smooth HF response, reminds me of the Dynaco Z-565.

I lost most of the photos of the SX-110 but found a video of when I finally got the amp playing and one from my last listen.


Enjoying AJA on Reel to Reel. Grundig TK-245 restored, but gifted away to a friend.

The Pioneer amplifier arrived today 1/26/04. The SX-110 was expertly packed by the Ebay seller, a necessity for the long trip from the US mainland. I will be inspecting and testing this wonderful receiver and should have pictures and a preliminary review by the weekend (more like end of year!)

The SX-110 spent most of the year dormant awaiting my evaluation. I did hook the receiver up to a Variac in the late summer only to find that the output tubes were glowing red and the amplifier was not able to stabilize, a sign of needed component replacement.


The last two nights were spent looking and writing down notes with the bottom cover removed. The SX-110 is one of those amps that can intimidate anyone unfamiliar with Tube Receivers.

After so many attempts to run the amp using a Variac with red plate 7868's, I really sat down with my VOM and began the task of troubleshooting.

Visually there were two questionable resistors and one capacitor that had a noticeable bulge and the typical white powdery stuff that signifies the end of the capacitors life.

Replacing what was critical and obvious, I began to turn-on the amp, measure and turn off. This I repeated several time before going straight to the culprit. The bias voltage on the schematic is -23 VDC at point (K). This means that the -16 VDC I was measuring was really not enough. My immediate conclusion, the diode bridge rectifier was not working well. In fact I substituted with a known good bridge and viola, -22.6 Volts appeared at point (K), I was elated.

Now today it's a trip to Radio Shack for a 50 PIV; 4A Bridge I see advertised on their web page. I would prefer a 100 PIV; 2 Ampere bridge, but one has to adjust to availability.  Maybe I take a stroll down to my trusty TV parts store and check out their NTE and ECG stock.

The transformer section provides 24 VAC @ 0.3 amps. With the two 12AX7A tubes pulling 0.6 amps @ 6.3 Volts, the design of the SX-110 feeds the grids of the 7868 and also DC to the filaments of the two input 12AX7's (for lower noise I guess). The SX-110 Bias section has several resistors, one 60 Ohm, 20 Watter! and one small 5K linear potentiometer that was and is now differently wired from the schematic that I have. This is typical for several reasons. The Sansui 1000A also went through various incarnations with circuit modifications that were not always obvious. The Pioneer is no different. I opted to re-arrange the circuit to match the schematic and now I have the 5K wiper control in series with a 1K 1/2 watter versus before that the control did very little to provide an adjustment to the bias with the 1k in parallel to the potentiometer.

If you look at the schematic of the SX-100, visualize the 1K Resistor in Parallel with the 5K Potentiometer. The adjustment would be from a parallel combination of 1000 Ohms and a 5K variable (0 to 5K bootstrapped pot). This amounts to an adjust that goes from 0 to 833 Ohms. Mathematically 1/Rt (Pot at 5K max resistance) = 1/5000 + 1/1000 = 1/ 0.0012 = 833 Ohms, turning the pot from here takes the resulting combination to zero ohms. Wow.. drastically different from the schematic with the 1K after the potentiometer ..... with an electrolytic cap at the end of the line, mistake or mod?? If you ask me, I think that the factory made a 'boo-boo' here, but I could also be mistaken and the Bias control is to work in that fashion, but I don't think so. Now that the amp is stable and working, minus some 'spitting' capacitors and a raunchy Balance and Volume control, I need to wait for the parts order that I made from, YES I am putting 2 x VK 150 uF @ 350 WV Black Gates in the Voltage-Doubler circuit, the engine room needs the upgrade. 

Additionally... there are about 1 to 2 dozen ELNA electrolytic and Oil caps that need changing with new parts. Once these get old.., their chances of bursting quadruple. After all is said and done..., this is going to be one SX-110 for the FM/AM music lover, the tuner section sounds just amazing.

Ah yes.... what about the SX-110 schematic. Just for those of you just too curious to follow this restoration by text only! It's a big file so be patient.., why not also post the Sansui 1000A schematic as well. The 1000A schematic is not too big a file. Some of you may enjoy comparing the two designs.... 

The SX-200 is the flagship Pioneer model so I would not say my SX-100 and the Sansui 1000A are apples to apples, more like an apple to a peach. But it's fun to look at how things were done back then (yup in the early 70's.)

The SX-110 photos are coming soon..., not before but after pics. 

Darn, I seem to have misplaced the battery charger for my SONY Cybershot camera again.....!

Here are the pictures I promised.... the SX-110 is working but I need to change some basic life-insurance capacitors, most all the electrolytic types. This is the 'bum' Bridge Rectifier replacement. As usual the original was a selenium wafer type that are notorious for losing it's operational capacity after several decades. I used an NTE167, but it also could have been a NTE166. What I do notice is that a cool way of calibrating Vf (Forward Voltage Drop) in a new bridge rectifier when replacing the older Selenium types is to over dimension the bridge and the Forward Voltage drop will eventually equal the original Selenium value which is higher than standard Si (Silicon). This has to do with Energy Bands and Fermi Levels, but I won't go there now..

Another group of pictures that shows the under-belly of the SX-110 and some replaced parts already. She does play, and what a sound...... I ordered a quad of 7868 EH replacements after measuring 3/4 weak power tubes. The Bias voltage at point K with the new NTE167 Rectifier is now -32.8 which is a good sign that the new bridge rectifier is operating properly and delivering a tad more voltage than the original Selenium bridge unit. I surely hope to have this SX-110 ready by 3-Kings day. All I have left are the parts to arrive from and the Tubes from Triode Electronics. A matched quad of 7868 is unheard of today. Just want to thank our friends the Russians and the US reps at Electro Harmonix for coming through time after time, re-creating a venerable audio tube from years past. 

It's not always that a good friend can provide yesterday's technology with significantly improved performance. The 7868 is about the rarest power tube from the heyday of hot-rodded tube receivers. Sherwood and Fisher amps also share the love for the venerable 7868. I just can believe that Bass response of this tube, deeper than the 7591 from what I have heard so far.. or is it the output transformers that are installed in this amplifier.


The SX-110 has been updated but still shows signs of some noisy components. I will inevitably have to either borrow my friends signal tracer of take the receiver over for some capacitor testing and resistor evaluation. This classic piece has amazing sound, but also allot of parts that are in need of replacement. Stay tuned on the SX-110 restoration.

SX-110 Schematic Diagram


My friend brought over his Heathkit Signal tracer. After about 2-1/2 hours of intense testing, the culprit of this amp is one of the modules that is part of the Tone Control circuit. The module is a small black square that has 4 wires coming out. Unfortunately these are one-of-a-kind parts.

Last night I built the module with some parts I had in my bins and this week I plan to install them to test. If the module works well, I will make the installation permanent. This repair will require me to remove the front panel and take off the tuning string from the main tuning capacitor. I will make a good sketch of this before I return the tune string to it's original winding configuration around the pulleys.

If this proves a success, then I will measure the rest of the ELNA Oil caps to make sure that I weed out the bad one's. One limitation of the signal tracer method is for caps that have drifted completely in value, but still act like a capacitor, the tracer will not show them as being bad.

The two ELNA 0.02 Oil caps that come off the volume control were supposed to measure 0.02 uF, but actually measured much less, approximately 0.0078 and 0.0045 uF, making them swap-out candidates.

A prior Sansui 1000A restoration was saved by the Signal Tracer, let's hope that this module is the one that will return this receiver to a fully normal specification.

Yet I went ahead and changed the module, same problem! Now I am at the point where all of these old ELNA Grey Oil caps will have to go. In the past my experience with these parts has not been good. At their peak, Paper-in-oil caps were available in all shapes and sizes. Now we plainly see that the one's made today are bigger and often do not fit into the original spot.

I ordered some Xicon caps from Triode and once they arrive, it's off to a recap session on my bench. Boy do I love the smell of burning solder, too bad it causes cancer. I will also replace the existing power cable, maybe install an IEC type receptacle to test some fancy power cables :)

October 2017

Having the hands-on experience with this Pioneer and the old Sansui 1000A, I can say that these and any old receiver of that time like the Fisher 400/500, Scotts and other AM/FM Tube Integrated Amps are not beginner projects.

These old amps require not only a change of all the Electrolytic Capacitors, but also the Paper-in-Oil Coupling Caps are also in the throes of being beyond their lifespan.

Budget about 120 Hours of Bench Time to fully restore one of these old beasts. And If you are lucky, the AM/FM sections don't have any bad RF Coils.

- Transcendent Sound "Grounded Grid" Preamplifier Kit >

Build difficulty #2

In 2006 I built my Grounded Grid and it was traded to a super happy local Audiophile in 2008.

This Preamp Kit is now discontinued (:, the Amp PC Boards are still 4 sale, Hurry!!!

You can build the Fixer instead, and achieve better functionality. I plan to order a Fixer myself.

Bruce is the best!!!



I had been looking around for years to get my hands on a copy of Audio Reality. I had read Mr. Rozenblit's Tube Audio Design many times over but have still to hit the test bench to brew some new designs armed with Tube CAD.

Posting on Audio Asylum I was able to buy a new copy of this out-of-print publication. It is this book that started me on my way to understand how Mr. Rozenblit thinks as a designer. Mr. Rozenblit is a problem solver, and Engineer's are trained to solve problems over and over. One aspect of an Engineering career is that we are always asked to come up with solutions. The mind of an engineer should have at least some semblance to the mind of a designer, yet this is not always the case. I know of some peer's who after receiving their degree's in Engineering swore never to touch another scientific formula. These friends went directly into am MBA or an entry level management position at one of various high-tech companies.

What I like about Mr. Rozenblit is that he looks at problems with an open mind. Notions of Paper-in-Oil caps, Tantalum resistors and all that mumbo-jumbo are cast aside. The remaining issues are laid out and analyzed with whatever tools and concepts Mr. Rozenblit has at his disposition, and believe me there is no substitute for experience. The mind of a designer is full of ideas that accumulate over years of trial and error, technical dead ends and just pure effort and perspiration. Mr. Rozenblit is such a designer, one armed with a good mind, insight and common sense.

Enter the Grounded Grid.....

This KIT is a 'thinking-out-of-the-box" preamplifier line stage. One can hereby appreciate how Mr. Rozenblit has applied his current knowledge to an old circuit topology invented many years ago in Radio Transmitter and Receiver design. One can peruse the web site and read more about the Grounded Grid preamp and the other products being sold. I find Mr. Rozenblit to be an Engineer's Engineer, an audio iconoclast who shatters myths like aDarth Vader strangle without using hands. Bruce uses the 'force' to guide him, and I am happy to have laid down the $499 bucks for such a nice KIT.

I ordered the preamp and received it in a very short time frame. I was not 'over-charged' on shipping as do many other companies with their sneaky tricks padding and extra $10 to $20 dollars to the cost and pocketing this difference. I always look at the actual shipping costs and compare to what I was charged. Transcendent has set a new standard for clarity in a world of fools. Thanks Bruce, not only are you a good Engineer but a fair businessman as well.


Last saturday I woke-up ready to rumble. After running my usual errands and a pit stop at my local Radio Shack for a 5 Volt Blue LED and metal panel LED holder, I fired-up my 35 watt Weller soldering iron and screwed-down my Panavise to begin PC board stuffing.

The assembly of both PC boards took me about 2-1/2 hours working slowly. I measure each part before assembly with my VOM for proper resistance, as I have had parts that are DOA in the past, no DOA on any Transcendent parts was detected, another A+ for quality.

A small tech-break to get a bottle of water and stretch my back and I began to stuff the Power Supply board.

The only difference in assembly sequence was with the PSU board. I opted to solder the transformers last so that I could manage the PC board on my Panavise. If I were to have soldered the transformers first, I ran the risk of snapping the PC board suspended by the Panavise. Leave the heavy stuff for the end...

Another quick break to eat an early dinner and afterwards the final assembly of the chassis ensued plus remaining chassis wiring. I used more than just the Yellow wire as I had lots of 600 Volt hookup wire I bought on my last trip to Fry's Electronics in Pasadena.

The only major deviation in my preamp was the installation of a 100K Stepped Attenuator I acquired from Ebay. I ordered the one that is sold by the fellah in Taiwan that uses Vishay-Dale resistors. Apart from this and the Blue LED, the preamp kit is 100% stock. No fancy paper-in-oil, black gates or any audiophilia-nervosa parts.

Total assembly time - 6 hours, not rushing things. I could have built this preamp is 5 hours, but I opted to dress the wires, shrink-wrap and tie wrap for a neater look.


A quick power-up to check voltages and I was ready for the acid test. I plugged my Grid into my system, sat back and watched my jaw hit the floor. This Preamp is an order of magnitude 'quieter' than my Dynaco PAS-2/3. The unit placed the sound right there, exactly where I was expecting it to be, right between me and my ice-cold Beck's.


I can recommend this preamp to anyone needing a medium gain line stage with no frills and lots of sonic attributes. I will post some photos of my personal Grounded Grid. You may ask, is this kit worth $499 USD...... ABSOLUTELY...     In today's dollars this kit would have costed the 1960's hobbyist about $89 USD back then. The price is right, the design totally unique and the results spectacular. Now all I need are my Zu Cable Druid Speakers and the 2 meter pair of IBIS cables to feed my sonic hunger.

I could have built this preamp from scratch for $350 bucks, but I thought it was better to support Transcendent Sound than to spend 15 hours drilling and making the same thing with Point-to-point wiring. Heck, the unit is very handsome and the quality speaks for itself. The front panel is elegant and the ventilated stainless-steel case functional.

This kit is meant for those who enjoy music. Not for the one's who need lots of gizmo's. I never touch Bass and Treble controls myself, and a recording Output is only a drill and RCA pair away. My Grid is already a classic and it is only 1 day old!


Value: A

Performance: A+

Aesthetics: B+

Design: A+

Originality: Cannot truly be measured or compared, this is a one of a kind design and tube product.


We rolled some tubes on Friday evening. Starting with the stock EH 12AU7's that came with the KIT, we installed a trio of JJ ECC82 tubes. The difference was clearly audible. Much smoother sound. The EH tubes sound aggressive as compared to the JJ's in the Grid, yet I know that some break-in is probably in order. For now they are my backup trio.

Next-up was a trio of vintage Sylvania 5693's. Now this was a major difference, the sound became very dry, almost spooky in the sense that the forward gain dropped a bit, allowing me to crank-up the volume with amazing clarity.

I love my Grounded Grid preamp. Yes this unit may not compete with the likes of DeHavilland, Atmasphere and others. But I agree with Bruce in one thing. If we invested about 500KUSD in Transcendent Sound and came-up with some heavy chassis, fancy knobs and all kinds of Audiophile visual's, I am positive that Transcendent Sound could sell this Preamp for over $5000 dollars and still keep a straight face like the rest of the Hi-End audio manufacturers, or maybe not. Some color close-ups of a $100 dollar milled Volume Knob or maybe a $500 stepped attenuator may make the non-believers become advocates. How can anything so simple provide such a sonic whallop!

Me, I prefer to keep my Cash, invest in more CD's and DVD's and just maybe even the matching "Grounded Grid Phono Stage."

We spun some vinyl and I can say that my Linn setup benefited significantly from having the Grounded Grid in the line-stage. I ran my LP12 into my restored Dynaco PAS-3 and the tape outputs were connected to Input 3 on the Grounded Grid. Man! Holy Cow! We spun some "10 Years After" and I have to say that I never heard my Grado Gold cartridge pick up the stuff we were hearing.

On my Njoe Tjoeb 4000 w/ 6922 JAN ECG's I have now in the sonic wallpaper department some Dream Theater "Octavium" I picked-up at Kmart last night. I like how this preamp does Rock Music. New Age takes on a sweetness that one cannot duplicate with my ADCOM GTP-400, Dynaco PAS-4 and Dynaco PAS-3.

Chris Spheeris, Jean-Michel Jarre, Keiko Matsui and others come alive in my space.

Even Keith Jarret "Radiance" took on a totally new meaning. Now I have to dust-off my LP copy of the "Sun Bear Solo's" and fasten my seatbelts.

My continued impressions will be posted here on a regular basis until the unit is fully run-in.

I expect that my Zu Cable DRUID MKIV speakers and matching 2m IBIS cables will be here in about 1 or 2 weeks and hasten the break-in period.


The plastic selector switch that came with the Kit on my Grounded Grid preamp broke. This did not come suddenly, but the selector switch became a nuisance. My preamp started to lose volume, clarity, focus eventually becoming intermittent, right channel in and out kinda problem. When the main channels became intermittent this lead to my troubleshooting with my VOM and it's eventual replacement. Surfing around the web ...Mouser had some very nice choices; but they give me the backorder blues most of the time. So I wrote and found a CT2 selector switch by Grayhill. It arrived last week but I travel 90% so it was not until last night that I was able to sit down and LOSE one of micro sized selector locking pins that come with the CT2, despite the warning label. Nevertheless .... I stripped some AWG24 wire and used that as a second surrogate stopper, 1-2-3 and it was installed, aligned and soldered. Good that I left plenty of slack on the wires. My faith in that plastic part was not much even from the day I built the kit.


Now my preamp has all the guts and probably most of the glory of the first time .... yes the first time I fired her up. I have to say nothing beats quality parts, absolutely nothing. Forget the $100 dollar "fanto-techno" paper in Oil's first. Go straight for top class RCA's, good 600V wire and the best stepped attenuator you can buy. Get a nice power cord and some NOS 12AU7's or 12AT7's if your Grid has the high gain mod...... enjoy.... :) all smiles, now my Transcendent Phono Stage met it's destiny, a fully functional Grounded Grid stereo preamplifier. Light, yes..., a kit.... I think so,..... sound, basically "Champagne" on a Beer budget. I would suggest that the Grounded Grid become a mandatory staple in every Tube'o'phile's collection. The only preamp that smoked the Grid hands down in my area was a stock MP-3. This Atmasphere preamp drove SUNFIRE amp with 250 watts/ch. That sound still haunts me today. Yup, one can achieve 90% "Holy Grail" sound at 20% the price. But heck, that Xmas bonus money could land you a used MP-3 from Audiogon, or a hot-rodded new one with Jensen step-up phono transformers..

My buddy says that Audiogon is the site for the homeless in Hi-End audio. I prefer to say that Audiogon is the site for those who like to complain about shipping costs. A few bidder's who seem to develop "shell shock" at a standard $170 dollars DHL, Fedex or UPS rate.... shipping costs, yes. Inflation dudes!!! Life is materialistically more expensive so when one bids add on the shipping. Somehow sending a package from the Caribbean to Europe is not a trivial matter $

These guys bid as if shipping costs did not exist. OK I went on a Tangent.... but who really cares, truly....just enjoy the music.

- Sansui 1000 A Tube Stereo Receiver (full featured) >

Restoration difficulty #4

It uses a pair of rare matched Sony PNP Transistors in the Phono Amp Stage, and has many capacitors to swap out. I would budget around $500-650 for a full restoration. This is not a beginners amp, but works very well when restored. The Presence Control make the Midrange bloom. Lots of cool features on the front panel. The SX-1000 version (earlier model) uses 25E5 Sweep tubes and may be a unique alternative to this 1000A unit.

I once owned a "Sui-1000A" for a few years. I just had to mention this amplifier here, even though it was sold many years ago. I bought this amp in Portland on Sandy Blvd. It was from the original "Coffee Stop", I wonder what Ian Joel is up to these days, He was the absolute definition of humility and honesty.

The Sansui 1000A is an amp that kicks-butt. I fell in love with it and also tried to fix it, it kicked my butt. I was just getting started with Tube Receivers and they are a super dense collection of old parts and lots of wires. I was somewhat apprehensive about diving into this alone so my tech buddy repaired it for me. He changed lots of bad Japanese PIO caps, pico caps and some resistors. I put the original Toshiba 7591 tubes in and Voila, super hot rod sound. It uses some PNP transistors in the input that were there to lower noise. They are impossible to find today, as are most Solid State devices of the 60's and 70's. So the only resort were the NTE, EGC, etc.. You get close, but in my ears, not close enough.

So in a tribute to this venerable "Boat-Anchor" HiFi piece of late 60's early 70's Japanese hot-rod, full featured, super tuner with Nuvistors (6CW4's) it was a real treat. The transformers are high quality and the build is hefty. The only issue with these amps over time is the stability of the tuner components and the overall resistors and caps. Power supply wise it used state of the art ELNA electrolytics that can eventually ooze that whitish/brown powder and the game is over rover.

The Music Room recommends a total recap, resistor check, fresh power supply and a good re-bias with a pair of real 7591 tubes. Update any rectifier to new parts, believe me today's rectifies sound way better than that old stuff. You are in for a treat when you restore a Sansui 1000A, especially if you are into or are lucky to have a FM Stereo station that broadcasts "un-butchered" music. The Sansui 1000A FM and AM tuner design is superb.

For those who like to play, there are two types of loudness controls that take the sound into the likes of a good guitar pedal, you can Hot-Dog the sound in ways most control-less sterile pieces of today can't.

Just make sure that you fasten your seat belts. And oh before I leave... be warned..... the Sansui 1000A was equipped with KILLER RCA'S. And I don't mean this in a positive sense. The RCA connectors on the Sansui 1000A have a much larger Shield diameter, so if you have a $500 dollar pair of Kimber's, be careful, these guys can snap these Male RCA connectors apart, the shield metal parts can snap off.

The Sansui is part of the Tube Receivers, and that makes them dense and complex to service. There are tons of old dry capacitors to change. I would figure $350 of labor plus $300 in parts to get one to where it is safe, stable and sounding great. The Phono Stage uses Transistors, so make sure that you are prepared to order a few to see whuch one's work.

These old Tube Receivers are not beginner amps, I would stay away from these unless you have a Tech friend with lots of patience and love.

Traded this away before I moved stateside.... lovely amp, great new owner.

Sansui 1000A Schematic Diagram


- Atmasphere M60 MK 2.3 factory upgraded w/ Autobias & Power Supply boost

Restoration difficulty #1

No restoration necessary, just retubing. These amps were purchased used from a 3rd owner local audiophile. They were operational when I first turned them on, and of course they had weak tubes (expected). So I measured all of the 6AS7G and 6SN7. They measured blah...  then I went to check the Fuses and was shocked to find that they had been replaced by larger Fuses, a definite NO-NO lazy thing. So I corrected the fuses and went on enjoying them. Eventually I replaced the 6SN7 tubes with premium tested, low noise and also put in NOS Russian 6AS7G from Ukraine. One of the amps went back to AtmaSphere, it suddenly was blowing a fuse when I took them off my 120V Balanced Power and connected them to our standard 120-125VAC outlets. Lo and behold, the factory found a wiring error on a Fuse Holder and honored the repair under Warranty, resetting that amp to 3 more years of Warranty (this is now long over). These units operate flawlessly, and are a tube owners dream. No fuss, no maintenance, no hassles. They are my only #1 rated amps in the history of my owning these things as they are easy to live with. I do recommend owning a Tube Tester to check out the 6AS7G tubes every 6 to 12 month's. These tubes operate at 250V on the plates so most vintage Tube Testers are perfect to merit test these Dual Triodes. The amp operates the Power Tubes below max so they last years.





1954 Article on the Bridge Amplifier, very good reading for OTL fans who would like to hear the same topology but with an Output Transformer.

The OTL version required some "out of the box" thinking, hence we have AtmaSphere today, building amazingly clear and neutral music amplification.

1954 Bridge Amplifier

Available for trade and/or local pickup

All reasonable trades considered

No cash, No checks, No Crypto

E-mail if interested at

I can send as many photos as you need and create a new video of these amp playing marvelous music. 

Cutoff date is Saturday July 24th. After that they are crated and traveling to New York.

The next availability date for these is Winter 2021


(Originally MK2, then upgraded at AtmaSphere to MK2.3)


MK2.3 Serial Numbers 476 & 480

Originally Sold by A1 Electronics

I am the 3rd owner, all prior were Non-Smokers as myself

Well taken care of, looking for a home with some good speakers and a quality Preamp

Zu Birth Power Cables are also included as well as extra 6AS7G Tubes



This obscure part of my website was my diary of decades of intricate personal & technical journeys into restoring and owning each piece of equipment that I show here.

Some I still have, most were traded away for Parts, Components, Accessories, Tube Testers, etc. I really don't sell much other than the occasional EBay listing.

Now there is plenty of wisdom to share, all pro-bono, and none is I hope, pretentious, it's just free from my personal journey into classic amps and making new things.

This is all created in the deep interest I have for electrical phenomena, and also handing it all down to others for their benefit and reference.

I have logged close to 8,000+ hours of bench and field time in my career. Most of my early Bench Work was in College during my Physics Labs, Freshman and Senior Projects.

Then Motorola and G.E where I fixed tons of Radios, Power Supplies, and Classified equipment (shhhhh).

I consider myself a restoration expert in what I would describe as "classical electronics". I like point to point wiring and find that it stands the test or time.

PC Boards.... hummmmm. That is my Love/Hate thing going on. PC Boards are what has taken us to a better technologocal place in society.

But folks, and this is my humble opinion; PC Boards are what point to point lovers call "solder-bridges".

A PC board does not offer a secure Mechanical Connection. PC Boards use solder to bond two or more metal surfaces together.

The Solder is there to cover the connection and hold the part in place. Eventually that solder will degrade and you end up with what is known as :cold solder joints:


I have helped over 30+ people get amps working since 2002, and sometimes taking calls on rooftops and radio sites on my coffee breaks.

As I have not tallied the total actual amount of man-years I have spent restoring all of these amplifiers, it's anyone's guess.

The facts are that when I am in the hobby zone I am standing and OFF from a reclining Sofa, or fast walking my 3 mile M-F regimen to pump some fresh Oxygen and flush my Lymphatic System.

Most of today's health issues stem from lack of physical activity. If we just allow our bodies to sit still, toxins build up and our Lymphatic System cannot expel them with efficiency.


This Tube stuff really got into high gear in 1996, when I began to build my very first Tube Amps from the Mullard Book.

Then I started to acquire, repair and restore old audio and Hi-Fi Tube equipment that I describe below from 1997 until probably 2013.

After 2013 I stopped scrounging for old amps and just focused on making new equipment from scratch.

My prior Tech work was in fixing Motorola, G.E., Standard, EFJ, etc. Radios from 86' until 97' when I went to AMPS Cellular Radio Systems.

I supported Nortel DMS based D-AMPS upgrades and eventually GSM, WCDMA and LTE a.k.a. 3GPP.

Also, I was designing LMR Trunking and Simulcast Systems, with Paging and Military Communications overlays.

I have designed and also maintained and repaired for many years:

>Centracom I and II electronics

>Page Bridge and PURC Systems

>Motorola HT90, HT440, MX300, MT500, MT600, Radius and lots of Sabre Radios.

>Ericsson BTS, GSM, BSC/TRC, SMS, Prepaid Platforms and also Network Management FCAPS.

>Harris VIDA and Codan Voted/Simulcast and Stratus Transportables

Programmed many, mane Code Plugs in my day and worked on Multisite Trunking Sites all night long.

Harris Portables 7300 and Unity, OpenSky, and also MASTR III & MASTR V, then on to Codan MT-4E & Cascade, all fun and good to enjoy.

Many times I had to drive 5 hours when I was in my late 20's round trip, to fetch a single Power Transistor at 2am, then another 2 hours back to the Tower Site

The pride I felt when putting a UHF Medical-7 channel back on the air, and shortly after, listening to a Doctor radio talk to an EMS tech and save a life.

This is known as a "Medical Control".

These efforts of mine saved a human lives over LMR radio channels, often victims of an accident or shooting.

This is what I work and live for, other people in need, and I hope to fill that need by giving away my knowledge for free here.

The Middle School Years....

But this all regresses back to my Middle School years, when the Tube Bug infected me, and I became a licensed Amateur Radio operator.

That made me order and completed the NRI Course on Advanced Amateur radio and passed my NRI exam in the 6th grade

Passed the Novice Amateur Radio License in the 9th grade and Extra Class in '87 when I was working for the US Navy.

The passed the FCC General Radio Telephone w/ Ship Radar Endorsement '89

Finally in 2003, I passed my Professional Engineer examination and currently hold a PE license.

Just a humble hard working guy, who trolls love to feed on, while my electrons are helping others.

I used to visit Audio Forums and had to stop, it was just too absurd.

Lots of comments, not enough knowledge and theory, reminded me of "Flat Earth" forums.

People with no Electrical knowledge imagining things...

Now back to Tubes:

The estimated costs I show below in my restoration diary are from 1996-2015 Dollars.

But today 2021 I have slowed down fixing stuff, and now make new equipment from scratch for fun, and also fixed stuff for friends.

But also I barter for services and pay with equipment, I think trading is the most ancient of transactions, fair and square.

What the restorations below amounted to, without any labor costs back then, was quite a big investment in my funds and my time.

But I learned, by doing, and making mistakes. This is what makes life worth living, making mistakes and getting better.

All of the work was done by me, on my time, as my Hobby. On kitchen tables, rugs, benches and also churches.

I fixed lots of Hammond's, Wurlitzer's, Leslie's, etc., pro-bono, as long as Churches ordered the parts, and paid for them.

But I had to stop, as the old organs are now really hard to work on in Churches.

Sitting on the floor for hours, with a lamp and a flashlight, I would cut my arms reaching inside to change e Vibrato Scanner on a C3 Organ....

In 2021, prices for parts and tubes has risen significantly.

Shop around... there are also lots of shortages and things that are hard to find.

I invested $1500 of my bucks on used test equipment over the years, and at least have an Audio Distortion Analyzer and also several testers like a Cardmatic and a Maxi Preamp II.

The problem with old equipment, is that the tool-up costs to properly work on equipment is quite high.

A voltmeter "VOM" will only allow you so you so much insight, you really need also an Oscilloscope, Distortion Analyzer, Frequency Counter and Signal Generator

Plus a HV Variable DC Supply and a real good Variac.

If you are lucky, you may also want an FFT analyzer and Spectrum Analyzer as well, all very nice to own (I don't).

Also over the years, I have created my own custom pieces of test equipment/jigs that allow precise control and measurements, also simulations.

I will post photos of them in my website with an explanation and also schematic when I have the time.

Working on these types of old gear is not like grabbing an iPhone or surfing on a PC.

There is plenty of effort, frustration, dedication and grit to reach the level I have, and it's all just for not getting bored and really loving every moment.

Being active is so important, you will not worry about a rut, you have to flip your reactive mind to active mode and make things happen.

Once you are in the Zone, it just gets better and better.

Please leave the Drugs behind as a learning phase. Alcohol also makes one do and say stupid things, and eventually fall asleep and wake up feeling unwell.

I now drink way less, and realize that the best "buzz" is a natural state of mind and body, where you can feel life, not just look at it.

I learn to fast walk at least 3 miles a day and get some fresh air, this clears the mind.

But ironically, I cannot enjoy music when I walk, I listen to the sounds of nature.

I find walking and listening to music distracting.

Also one must have a passion, for life, friends, family and a good job that pays the bills.

Then learning new things and new ways. Reinventing and evolving.... this is what makes a new Amp so exciting.

Making things happen, not while waiting; you have to be moving, traveling, creating, visiting, loving and paying bills.

I see so much hopelessness in society today, yet the only one who can save You from Yourself is You, then allowing your creator to take control.

We call on the Creation and the Creator by many names, symbols and images we evoke our energies that harmonize with our vibrational self.

All we need is to close our mouth's, open our hearts and our minds will sync with nature, the creation will speak to you from the earth, sky and universe.

But when the voices come from your inner self, you have connected to the source. You have to dominate your primitive brain and command your frontal cortex to RULE!

The journey and paths I have traveled:

Work and Time were needed to make each one of these Amplifiers and Preamplifiers safely operational.

You can decide for yourself if buying old amps is an economical way to get into Audio Tube hobby, without having frequent issues and problems.

Getting old amps to be daily players takes real time, expertise and $money.

After all it's a hobby, but also a lesson in reality.

Denial gets people cranky, and saying that tube equipment is unreliable.

Most people are addicted to simplicity, and this is an addiction that works against any Hobby.

Even Dynaco ST-70's that are selling today for exorbitant prices on EBay today will cost $$$ to get working at peak performance, at least what you spent on the amp pus money for parts and tools.

Why just not spend your money on a new ST70 Kit from Dynakitparts or Bob Latino? This will save you all of the hassles and allow you to build something, and is a great learning tool.

Building kits requires 80% less tooling and test equipment. Fixing old stuff is far more of a big deal, if taken seriously.

With older pieces of equipment, old Preamps will exhibit some Channel Bleed through.

There is nothing wrong with the Preamp, it was like that in 1964, the sources at that time were much lower than today's standard.

Back then sources had a much lower signal level so bleed through was not a nuisance.

So when you use a Vintage Preamp the impedance of the audio connections are different today.

This may manifest as a Volume Control that cannot be turned past 9 O'clock (typical).

As back then, sources had voltage output levels of about 0.7 VRMS, digital sources today can output up to 2.0V RMS!

If we calculate the difference we have Gain; dB = 20 Log (V1/V2) = 20 Log (2.0/0.7) = 20 Log (2.86) = +9.12 dB gain, that is almost 10x the Line Level of a 1970 Cassette Deck.

Also, some Vintage Amp Power Transformer primaries may not to perform well on today's 125 VAC Grid.

Sometimes older amps will work best at 115 VAC, and 117 VAC.

After all, Tube Amps will draw more current when the voltage is increased and the power supply being un-regulated causes this "hypertension".

Voltages will rise in proportion to the current drawn in most vintage equipment.

Very similar to what a 60 Watt Light Bulb connected to 100V, 110V, 115V, and 120V will do.

Which one will glow brightest? of course, the same bulb at 120VAC.

So when you see a "bargain" on EBay, and it has not been worked on or restored, just double (2x) what you paid to give you the final restored and fixed price.

If the unit is in good physical condition, consider that looks alone are not in play, that is just the start.

We face pure costs for needed parts, and also labor.

If you need a technician, just 3x the Vintage buy price you paid as a baseline to get it working properly, but not always optimized, but safe enough.

The safety concerns are exploding Capacitors, burning Resistors, burning Insulation, Vintage Transformers overheating, old wires with distressed insulation.

Also weak and high current Power Tubes can fry Cathode Resistors, the Plates can glow Red.

Safety is #1 my friends, and 120VAC with 450 VDC is no place for amateurs, be careful...

The way we assure safety is by evaluating the piece of equipment.

This may entail lifting some solder joints or wires to insert test measurement leads.

Power Supply Capacitors have to be checked for leakage and also Capacitors for drifted values and DC leakage.

Electrolytic Capacitors can short to ground, and give nasty DC shocks on the chassis, also when a Cathode Bypass Cap starts to fail, the chassis can become HOT with DC.

Old Capacitors can being to conduct DC current and heat-up. until they either explode, short & explode, or if lucky, open circuit and send massive 60 Hz HUM to your amp and speakers.

Maybe and old stressed power supply resistor may not burn, but the Transformer winding may melt, and open circuit (catastrophic failure, no repair, must replace) before the fuse blows.

If a Power Tube shorts, it can open circuit the Output Transformer primary, another doomsday scenario I am familiar with.

I once killed a Bartolucci OPT in a millisecond  with a lousy KT-90.

So when you insist on using old amps in 100% original mode, think again.

Ayn Rand said it well; "You can ignore reality, but you cannot ignore the consequences of ignoring reality"

In the end, if you pay $500 for an old amplifier, you will end up spending $1000 to be happy if you do the work yourself and $1500 to have a Tube Tech make the equipment safe and playable.

Buying parts, clamps, tools and stuff to make it happen is the hobby path, and it just gets better every year, like playing Guitar.

I don't want to be negative, just share my pragmatism. Remember enthusiasm can be a momentary rush, it is only when you are in the hobby that you feel the pain and the joy.

This can mean expensive new tubes, fresh Signal and Electrolytic Capacitors of correct value and ratings, and other parts needed.

Then comes the Time invested, swing lamps w/magnifiers, Good solder, 35/60/100 Watt Soldering Guns (yes you need all 3) and plenty of tools and your own dime.

If you want to build amps, you need Chassis Hole Punches, Chassis Nibbling Tools, good Drill Bits and a good Drill, goggles and all kinds of small tools like Calipers.

Bench time is when I do get my real thrill of being in concentration, honing my skills, and approaching equipment in the professional way, that is the whole point here.

Building new stuff is like creating art, writing a new song or learning a "cover" from your favorite musician or band.

Restoration is more far more tedious of a thing, and some may prefer fixing stuff, I prefer making new stuff.

>Some technician's only restore/repair, that is very cool.

>Other's build and create, that is also way cool.

>Some do both, that is super cool.

So when you browse the internet, and look at old amps, get into the habit of adding up

>New tubes, New power supply parts, New signal caps, New resistors, New Wire, New fuses, etc.

You can always calculate quickly the price you will eventually have to pay using many of the websites like; for General Repair for Guitar Amps  for Caviar parts.

Newark, Jameco, Mouser, Ebay, Amazon, all of these are good places to hunt for parts.

A simple cost example - Dynaco ST70

A old classic Dynaco ST-70 $500.00 "un-molested" on an EBay Auction, but seen sometimes up to $750-$900, ouch, with crispy components and darkened PC boards.

Lets assume, the cost is your basis at $580 (with shipping) if you got a normal EBay deal.

You will open the box, plug it in, and risk everything I have stated and proven above with regards to 'un-molested' equipment.

Now if you own a museum or want to put it on a shelf to admire 100% original, that's perfectly fine, just don't plug it into an outlet straight from the box.

Then reality sets in, the parts to restore cost about $500 (New Tubes, New Power Supply Components, New Signal Caps, a few Resistors).

The final cost is around $1000 if you do the work yourself, and are lucky if the Power Transformer is healthy plugged into 122 VAC, you may need a Variac.

Add then, you just might need that "optional" tube technician time, and you climb past $1000, to around $1200-$1500, if you have a reasonable technician available for $50/hr.

Restorations can take 6 to 10 hours....

You may also need to add $200 shipping costs if you have to have the amp shipped out and back for the work to be completed, ship costs today are super high.

Or.... just get over the whole Vintage Craze and weird notions about the magic they produce and buy an ST-70 Kit and have some real fun.

A full ST70 Kit is around $1000 and you have a brand new amp, all new parts, you will not spend any frustration fixing anything, and you will build something new.

The ST70 Driver Boards today just crush the old 7199 driver boards, and sound so much better.

This is where it becomes Fun.... not hours of fixing an old amp, and possible getting shocked and burning things up by wiring errors.

This sounds like a deal to me, so check websites and shop for some real fun and lots of good sound with a Kit or a good tested used amp...

Note that I bought an Un-Molested PAS3X Kit on EBay, and had to discard the Volume Control, The Balance Control and most of the Capacitors, they were all so bad, but NOS.

Most of the Carbon Resistors were about 20 to 40% away from their values.... and that FP Metal Can original cap, it leaked 5 milliamps at rated DC voltage....

And these were original parts in their plastic containers.

Yes, we age, but old electronics do not age gracefully, they deteriorate.

Below are a few links to very nice Tube Amp kits.

They save Time, Money and you can even order them already built for a reasonable fee.

The most advanced kits are from Transcendent Sound.

The designs are made with better technology and advancements in design.

The kits offer more bang-for-the-buck and are more on the traditional realm, and very nice sounding.

Summary of the Vintage Amp reality:

I am a realist and have seen too many "unmolested" amplifiers on the Internet, that are being sold as shock and fire hazards to amateur Tube hunters.

To date, I have not yet acquired ANY amplifier on ANY Internet sale that was safe and functional, none of them arrived working, despite the "tested and sounds great" descriptor.

Yes, there is a collector aspect to buying "un-molested" equipment and you can work on it from an original standpoint, and learn.

But some of the work being done to these old amps makes them unsafe, and also when improperly done, will require repair and restoration of bad work.

I have seen parts dangling in the air with exposed 400 volt wires just waiting to short circuit....

So, conclusion;

If you are new to this Hobby, buy a new Kit, new Amp or used and tested from a reputable dealer.

In today's dollars $1500 is an entry level price, and you will be able to enjoy the music and not worry about the hum/noise.

Or buy a good used or current production amplifier like a Prima Luna, Rogue, VTL, etc. these are amazing for $2500

Reputable dealers have great amps for sale and great prices, rummage though Upscale used equipment, you can often find a Conrad Johnson, Audible Illusions or VTL at a great price and these are tested by a reputable shop.

And I have to re-state that the Tube Amp hobby is not cheap, or for people afraid of anything over $200.00, (this is about $1382 today in 1970 dollars)

So in order to work on these amps, you will need to have invested at least $1500 in test equipment to include a good pass/fail Tube Tester from the 70's (i.e. EICO667).

> When a sale is described as "I have no means to test"

> "Un-Molested"

> Consider it broken and in need of restoration*

> Pay the least that you can, and don't get pulled into "bid-shielding" traps, there are lots of angry people out there.

Note*: Old amps are not "repaired", they are all restoration projects. The parts content alone can be a grueling internet search for similar parts that may or may not be drop-in but need to be adapted. This also goes for equipment with rare out of production components such as CR Modules (A300 and A500 Harman Kardon). Sometimes a work-around is needed and how to do these things is not found on any iPhone or Website. The school of hard knocks awaits the unsuspecting. You just cannot Google what I present here, unless you are a "Search Engine" Genius out there with a loaded Smartphone and quick keyboard reflexes to the Ego needed to comment on forums of subjects to advanced for the people who believe what is stated.

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