Testing & Measuring


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.

But most Power Tubes have more than 250 VDC on the plates, often 300 to 450 VDC, when operating in Push-Pull and somewhat less in Single Ended depending on the actual Plate Dissipation.

Small Signal Tubes will need 70 Volts DC (6DJ8/6922) to 150 Volts DC (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). Then calculate the Plate Dissipation and in the Datasheet make sure that the value in Watts, you just Calculated is less than the Maximum Plate Dissipation.

The Calculation consists of converting the Plate Current from miliamps to amperes and then multiply this value by the Plate Voltage. Units are important in Calculations, and Engineers are taught this well. In High School Physics students are also taught Units, and some retain this, while others just pass tests.

The result You just calculated is the DC Steady State Plate Dissipation value; Plate Dissipation (Watts).

The law of superposition states that there can be two mutually orthogonal or exclusive parameters simultaneously present in a Circuit. This means that when the Technician/Engineer measures, they must always take two measurements, AC and DC. Ingnoring this fact can sometimes result in a component failing due to over-dissipation.

A burned resistor, a cooked Transistor, etc. You have to calculate the AC RMS Dissipation and add this to the DC Dissipation to determine the total power dissipation.

P(ac-rms) + P(dc) = P(total)

Now look at your 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. A design rukle of thumb is to never operate any Tube at more than 75-80% dissipation, this gives the Tube headroom to breathe. I liken this to Running your Automobile with the Accellerator fully depressed for hours. Of course the police will eventually stop you if you are not killed, but your Automobile Engine will not last long at 100% throttle.

I like to use analogues with practical examples for You to better visualize what I am stating.

Often, Tube Amp Hypertension from modern AC Mains (125VAC) can push Classic Tube Amps and their Tubes into over dissipation limit, if the design was already close to Max Pd (Plate Dissipation) in Watts. And many old Classic Tube amps were deliberately designed with little headroom in order to publish more Power and Less Distortion. But once You realize that Tube Amps sound remarkably better at 70% of Max, and deliver more Bass, your learning will progress into getting the feeling by listening to a Classic Tube Amp, or your DIY Amp and realizing that the Music sounds strained. Operating Tubes close or beyond their specifications creates less than ideal Sound Performance.

Solid State avoids this altogether as they use Regulated DC Power Supplies, so no matter how High your Home Electrical service goes, the Transistor is protected by the DC Regulators. There are exceptions to this and I did come across a Wurlitzer Church Organ that was 100% Solid State and lacked any DC Power Supply regulation. Rare but in the realm of possibilities if you work on 1970's and newer Solid State. Even 1960's Solid State might have a Gas Regulator Tube powering the Transistors.

Some small signal tubes can withstand more than 250VDC, but rarely more than 300VDC a typical maximum value, you do need a tube manual to validate.

These are all DC measurements (and Filament AC or DC), 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 current sending resistor in the Cathode Circuit provides a cheap and effective way to Measure DC and calculate the Current.

The 1 Ohm series resistor creates a small voltage drop, and from this voltage drop across the 1 ohm resistor, we use Ohms Law to calculate the current.

I = V/R; so R = 1 Ohm, and if you measure 0.050 Volts DC, the current is .050 amps, or 50 milliamps. I cannot stress the importance of MEMORIZING OHMS LAW. You need to have the equation IN YOUR HEAD, NO EXCUSES.


But please do not drill-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 and low value current sensing resistors. You are better off buying a good VARIAC and setting that to 115/117 VAC for Classic Amps. These are the normal center voltages that Classic Amplifiers love.

Putting a Classic Tube Amp on 125 VAC is like getting a Sun Tan on Planet Mercury. You will Tan, but you will need Sunblock 1 Million, something I doubt will ever be real.

Unless you have your own DIY home made Amp where modifications are typically seen as enhancements, instead of permanent value affecting damages to equipment, try to keep your Classic Amplifier as true to the original. Some discoloration, fading, stains are normal.

One pet peeve of mine is ruining equipment originality, by installing new 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 nonsense. Just build adapters and use them.

Audiophiles love to chase shadows, and often are victims of audible delusions. They claim they can hear subtle improvements by butchering old gear, something I have never done, and will never partake of.

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. I have not yet restored themm but they are on my list.

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 slightly lower DC voltage reading than the actual value. Usually negligible but present.


The Impedance of a modern Digital Voltmeter is much lower than that of a classic VTVM (Vacuum Tube Voltmeter).

So when you bridge connect a modern VOM 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 1 to 10 Megs.

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 on Tubes.

This is why people still use VTVM (Vacuum Tube Voltmeters) as Tubes relate to Tubes. I want to build a Digital VTVM, this is on my short list to market to Tube Repair Shops.

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. You will NEVER find this type of real, factual truth on any Audio Forum, and when You do please send Me the link so that I can congratulate the author(s).

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. Eddie Van Halen knew how to use a VARIAC and understood this stuff. Not only was Eddie a Master Electric Guitarist, He was technical and knew stuff people were not aware of, hence the Brown Sound.

The rule of thumb is when tubes run HOT (i.e. higher standing DC bias currents, they sound brighter and thinner with less Bass).

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 bloated low frequency content, and have the "stereotypical" Tubby the Tuba sound, and may be pleasing for long term listening, you decide.

But the best place to park a Classic tube amp is at published specifications, or just 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.

Biasing Notes

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?

 think FarmersOnly.com

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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