The Music Room
- Harman Kardon Award Series A300 Stereo 15 W/ch
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 Cathoe 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 electrolytics 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 busbar 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, Obbligatto 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
Version 1 - A500/A50K Schematic
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)
A500 ORIGINAL UPGRADES;
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 180º 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 ?
A500 RECOMMENDED PARTS LIST.xls
The A500 uses four Cathode Biased 7355 output tubes now out of production since the late 70's early 80's.
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.
TUBE MODIFICATION TO USE THE SOVTEK 7591XYZ IN THE A500 AND A50K AMP, AND MOST 7355 AMPS.
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. Physicall the basings 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 the same base as the 12A_7 family of small dual triodes, 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.
GETTING TO THE POINT !!
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 Sovteks) 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.
MODIFICATION CONNECTIONS ON PIN #4 MOVED TO HIGH QUALITY INSULATED POST (BLUE WIRE)
(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 sold this A500 on Ebay to a very lucky and wise bidder.
He got this upgraded and special amp for less than $300 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!!!
Sold the A500 on Ebay..... (sigh) it 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.
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
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......
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
These smallish mono tube amplifiers produce some very unique 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 in this amp are:
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.
I sold the pair on Ebay to a lucky buyer from Michigan. These were a bargain ar $150 each and they sounded Wow... I actually have a YouTube Video of them playing at 120VAC powered by a Balanced Power Transformer.
- Stromberg-Carlson ASP-422 Stereo 20 W/ch
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:
Sold to my good friend Brian. 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
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 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 no longer own the original ST-35 but I have made a new one... and this one has Tube Rectification and a 115/125V Switch to go between EL84's (125V) and 7189's (115V). The 115V primary setting boosts the Power Supply to take advantage of the 7189 tubes. I prefer the 125VAC setting, it makes the amp so musical
Link to a very good Dynaco web site with lots of information:
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!
My friends on Audio Asylum have concluded after several posts that the caps I am using are only rated to 55ºC. 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 85ºC 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 85ºC rated caps, not the 55ºC one's.
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.
Sold the ST35 on Ebay. Another lucky buyer to enjoy this remarkable amplifier.....
ST-35 Adjustable Cathode Bias
This is the corrected version of the ST-35 Bias Control Circuit
April 6, 2021
I have the ST-35 playing with all Tubes.... sounds great.
- Dynaco PAS2/3 & 3x Stereo Preamplifier (and PAS-4 schematics)
My old PAS2 from a few years ago, sold to a good friend who enjoys it very much. 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.
DYNACO PAS3X Pristine Example built by me from a NOS Kit from 1975.
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 stunned, but who would sell old cardboard? People do.
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
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....
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 Hoboes on a Freight Train.
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 HIGHLY RECOMMENDED BUDGET HI-FI COMPONENT THAT OFFERS LONG TUBE LIFE AND PREMIUM PERFORMANCE, A REAL VINTAGE GEM !!
PAS-4 by PANOR CORPORATION
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.
I sold mine to a guy in Europe and I really don't miss this PAS-4 much now that I have my Grounded Grid. 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 sold this 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
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.
- Pioneer SX-110 Stereo Receiver 30 Watts/ch
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 http://www.angela.com, 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 www.angela.com 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 :)
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 - 2006 (Sold to a local audiophile 2008)
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 www.transcendentsound.com 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!
GROUNDED GRID RATING:
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 www.welbornelabs.com 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.
WHAT A FREAKIN' DIFFERENCE !
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)
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 is up to these days.
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.
Sansui 1000A Schematic Diagram
- Atmasphere M60 MK 2.3 upgraded w/ autobias & power supply boost
The ultimate OTL experience delivered purely by 6AS7 and 6SN7 Triodes. This patented design is based on the original Wiggington Circuit, completely balanced from input to output, no bull.
Amazing air and texture, cool "retro" design shows it's place in time, somewhere between yesterday's heft and tomorrow's insights.
Life with OTL is like living with your In-Laws, they make music in a different way...
I also love the controversy surrounding the Output stage.
People are addicted to calling these amps Class A, and in specific ways yes. They operate Class A through most of the Power Range.
But fool nobody. the Circlotronic is a Push-Pull Output Stage, it is just configured just like a Wheatstone Bridge, balanced and pushing one side of the Sine Wave and Pulling the Other Side. When this circuit gets out of Balance, the differential generates DC. This is why OTL amps require very specific voltage controls to prevent the imbalances between Triodes in Parallel to create DC on the speaker terminals.
I love it when my Friend tells me that he Hates Push Pull amps but loves his Class A OTL's.
Oh well, some electrical tube amp theory could clarify the confusion, but I don't have time for those who don't make time to learn Electronics.
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