Basic Vacuum Tube Measurements and Safety Tips
ShermanAudio created these slides for a person who asked some questions about how to go about measuring vacuum tubes and interpreting the results.
As I haven't come across a textbook that presents, in clear graphical form these methods, these slides I made in PPT will show how a particular Vacuum Tube is behaving in the DC realm, and how the voltages are settling. AC Signal tube testing is beyond this forum, but I promise to develop a section that explains Tube AC behavior. The AC I present here is related to the Tube Filament supply, not the audio signal.
It is always best to have a Tube Manual handy, to relate the measurements to published specifications. By taking some static AC and DC Voltage measurements one gains an insight into how a tube is operating in quiescent conditions (static).
Most Power Tubes manuals publish their specification with 250VDC on the Plates an informal standard. Yet most Power Tubes have 300 to 450 VDC on the Plates, when operating in Push-Pull and somewhat less in Single Ended.
Small Signal Tubes will use anywhere 70 Volts (6DJ8/6922) to 150 Volts (12AX7, 12AU7, 12AT7, 6SN7, etc.) you need to look up the datasheet. The best way to check a tube is to measure the steady state DC Plate Voltage and also measure the Cathode Current (not straightforward but can be done). The convert the Cathode Current from miliamps to amperes and multiply this value by the Plate Voltage. The result is the DC Steady State Plate Dissipation value. Look at the Tube Manual to find the Max DC Plate Dissipation in Watts. Your measured and calculated values must be less than the maximum for safe operation. Often Tube Amp Hypertension will push the Tube over the DC dissipation limit if the design was already clsoe to max Pd (Plate Dissipation) in Watts.
Some small signal tubes can withstand more than 250V, but rarely more than 300V a typical maximum value, you need a tube manual to validate.
These are all DC measurements, and of course, current measurements are possible but they almost always require a disruptive circuit intrusion, de-soldering, etc. and not recommended for the average person. One can choose to install permanent "Tip-Jacks" with an SPST switch soldered across to allow for simple current measurements on DIY amps. Installing a 1 ohm resistor in series and calculating the current from the voltage drop across the 1 ohm resistor is one approach. But please do not dril-out Classic Tube Amps unless absolutely sure that you will not ruin the amplifier.
But due to the intrusive nature of current measurement, I would stay away from drilling perfectly good production amplifiers to install tip jacks. Unless you have your own DIY home made amp where modifications are typically seen as enhancements, instead of permanent value affecting damages to equipment.
One such peeve of mine are ruining equipment originality by installing plastic speaker connectors where once were perfectly suitable and time period correct screws or pressure contacts. The idea that modifications to speaker interfaces provide audible improvements is a low probability thing.
Now updating ancient balanced audio jacks with modern XLR is one thing that for me may be necessary to allow for the regular use of modern XLR cables and interfaces. I have a pair of ancient Stromberg-Carlson Mixers and these have the old Switchcraft threaded Balanced Receptacles.
Also understand that using a Modern Digital Voltmeter on Classic DC High Voltage Tube Circuits will most likely "load down" the circuit, giving the Technician a lower voltage reading than the actual value.
The Impedance of a modern Digital Voltmeter is much lower than that of a classic VTVM (Vacuum Tube Voltmeter). So when you connect the meter to let's say the Plate Pin #3 of a 6L6 tube, the low impedance (as comparing Solid State to Tubes) Digital meter is "bridging" the Tube circuit (a.k.a. parallel connection) with less than 20 Megohms, maybe 10 Meg. This can have the effect of 'loading" or lowering the DC Voltage on that tube circuit node by providing a non-trivial DC leakage path through the Digital Voltmeter. It can and does happen so just be aware that the Digital Meters can show a DC voltage value slightly below what is actually present. This is why people still use VTVM (Vacuum Tube Voltmeters) as Tubes relate to Tubes.
This is what I love about having my own Website, I am free to explain facts & truth, and not simply give my visitors only part of a bigger Picture.
So if you only have a Digital VOM, just add 2% of the measured voltage to the value displayed on the meter and this will give you a closer to actual reading, if not 2% is not that significant at 250V (5 Volts) but at 450V (9V) it can be a bit significant. Nevertheless a 10 Volt Difference can confuse when looking at old schematics that used VTVM to publish their measured voltages.
You might say "2-3% what difference does that make?"
Lets use an EL34 as an example.
The VOM reads 450 Volts DC. Now take 3% of this value; 450Volts * 0.03 = 13.5 Volts DC.
So the actual DC Voltage is 450 Volts DC + 13.5 Volts DC = 463.5 Volts DC
If you use the wrong value of DC voltage to set the Bias, you will inevitably cause the Tube to draw more or less DC current, and might take the tube too close to the "Maximum Plate Dissipation", causing premature Tube wear, and also affect the Tone of the amplifier.
The rule of thumb is when tubes run HOT (i.e. higher standing DC bias currents, they sound brighter and thinner).
When Tubes are run at spec, they do what the designer intended.
When Tubes are run under voltage, they will appear to deliver a bit more low frequency content, and have the "stereotypical" tubby the Tuba sound and may be pleasing for long term listening. But the best place to park a classic tube amo is at spec... or slightly below spec.
Just make sure that you are not 30 to 50 Volts over what tyhe circuit is supposed to measure.
Just as well don't start to increase the value of a Cathode Resistor to compensate for high Plate Voltages and standing DC Currents, this inevitable cuts off the Tube and makes the Plate Voltage Rise and the DC Current drop and shifts the operating point of gthe Tube into a place that usually does not sound good, but muddy. Lowering the value of the Cathode Resistor is also not recommended. Always use the specified value of Cathode Resistor and make your DC voltage changes in the Power Supply. This is where the compensation happens when we have too low or too high voltages. One can play around with the values of the power supply resistors and dial-in the correct voltages on the amplifier circuit.
Usually with a 2Watt o 5Watt potentiometer in the range of the stock value resistor always keeping in mind that you start at the reference value of resistance and increase the resistance. Don't lower resistance unless you can see that the voltages are not going in the proper direction when adjusting the potentiometer.
So WTF is Bias?
This is a question that Non-Technical folks struggle to understand
no kidding, and I mean to really understand Bias.
I took many hours of my time
and developed this Tube Amp Biasing Guide.
What is Bias?
Non-Technical folk just don't get it!
Note the common mistake made when measuring Plate Voltage. Plate Voltage is the "potential difference" measured between
the Plate and Cathode connections, not between Plate and Ground, of course unless the tube uses Fixed Bias and the Cathode
is connected directly to DC Ground. I make this clear in my explanation above.
The measurement above applies to Cathode Bias and Cathode Current sensing resistors.
The Cathode current is calculated from the DC Voltage measurement and the value of the cathode resistor.
It's always a good idea to remove the power tube and measure the resistance of the Cathode resistor to
verify that it has an acceptable value according to the tolerance of the resistor.
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