DIY tube tester

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Hi folks,
I am currently (trying to) design a computer controlled tube tester. My first goals is to do a bunch of tests on double triode tubes like the 12A*7 family. I only recently started with this, and so far only have my transconductance test up and running. I can now read out the values for gm for both triodes.

Now I would like to implement an emission tester, but I am having trouble understanding how this test is best performed. In the case of the 12AX7 tube, I would have to connect the grid to the plate and connect the cathode to the ground to make the tube function as a diode. That I understand. So now I am wondering: 1) which voltage should be applied to the plate+grid? 2) When I then measure the plate current, which value do I use to compare it with?

Also, as I am programming the user interface in a computer program that is able to perform frequency analysis on the measured data, I was thinking of doing some sort of test for example to find relevant disturbances within the audio range, decreased gain response at certain frequencies etc… Does anyone have experience with frequency related phenomena in old/bad tubes?

Please excuse me if these are stupid questions.. I am relatively new to the vacuum tube world :)

Any help, advice, reference to a good book,… would be very much appreciated! If you would like to know more about my project, don’t hesitate to ask me!

To test emission you compare anode current with full heater voltage against anode current with slightly reduced heater voltage.

Variations of response within the audio range are extremely unlikely, unless there is severe microphonics. Almost all of what people say about various valves affecting frequency response is either imagination, attributing to frequency what is actually due to distortion, or (in guitar amps) relates to 'fixed tone controls' created by huge grid stoppers and the Miller effect. Valves usually work fine from DC to about 20MHz - above that issues like grid current and transit time begin to have an effect.
Thanks for your replies!

@ DF96: Do you know of a book/web page where this method is described in more detail, or can you tell me more about this? I can see that there will be a difference in anode current, but how do they give me a 'emission value' and how should this value be interpreted?

I now found a small table with values for voltages to be applied on the grid and anode, and a current value to compare your own current measurement with. For example for the 12AX7 they say anode voltage should be 25V, and the anode current value they provide is 50mA. They also say this measurement can only take about 3 second maximum (to not destroy the cathode). Is this a method that you are familiar with?

@kevinkr: I checked that page out! Found myself a nice read for this evening! Thanks a lot for that!
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The emission test is really of limited usefulness, IMHO, and I would certainly not run a 12AX7 at 25V, doubt you could get 50mA through one under any circumstances without destroying it.

It may be useful to remeasure the transconductance at lower filament voltages as an indication of life remaining. You would make a determination as to whether the tube had useful life remaining based on whether or not the transconductance met reasonable minimums for the application intended. All very arbitrary really..
I find it extremely hard to 'set' these minimum gm values. Is there any sort of convention about at what percentage of nominal transconductance value this minimum should be set? Also, I did this lowered heater voltage test on a bunch of old, intensively used and new tubes, and the result was a very linear curve for all of them. (I recorded gm for 8 heater voltages between 4.8V and 6.9V) So no sudden drops in gm with lowered heater voltage. Was this a coincidence? Should tubes that are at the end of their lifetime show a 'sudden' drop in gm?
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