If you are looking to test guitar amp tubes. PRR gave me advice on a Champ based tube tester. I could not find it in the search section, so here is the info from PRR:
Do you want to test "all" tubes?
Or just guitar amp tubes?
Build a small Champ. Open on a board so you can stick a voltmeter on any point. Keep supply voltage down: 250V is ample unless you really-really work only at absurd voltages.
Base it in the AA764 Champ, with cathode caps. Omit the tone stack. Change the volume control to 100K Audio with a 500K series resistor, with a switch to short the resistor. Omit the 2,700 feedback resistor and replace the 47 with a short. Load it with known-good tubes and speaker and get to know it. Note the plate, screen, grid, and cathode voltages on each tube. (Would be neat to label these points with name and normal voltage.)
Now any 12AX7 can be dropped in. Check voltages. If they are within 20% of "normal", the tube isn't grossly sick. Use the 500K resistor in series with Volume pot. Play. If it sounds right, it IS right. If a particular 12AX7 has odd voltages or needs a very different Volume setting, it's sick.
12AT7 will drop in, be a bit less loud and slightly different voltages. Note the Volume setting needed. Try several. If most 12AT7 come to about the same electrode voltages and Volume setting, and one doesn't, the oddball is sick.
12AU7 will drop in and work, somewhat different electrode voltages, but far lower audio gain. Short-out the 500K resistor, it should play OK. Try several. Oddballs are sick.
Tap the tubes and listen to the speaker. "dink" is normal, "CLANGGK!!! is microphonic. A microphonic tube may get-by as a PI or trem-oscillator, but will be trouble in preamp or reverb recovery amp.
6L6, 6V6, 6550 will all work in the Champ output, with same cathode resistor and nearly same gain and output. Add a 1Meg resistor pin 1 to pin 8 so you can test EL34 and 7027. Watch cathode voltage when you turn-on: a slow rise to 20V is normal, a rapid rise or going much over 20V is a shorted or gassy tube.
Add a 9-pin socket for 6BQ5/EL84. You can use the same cathode resistor as for the 6V6-6L6 series. It will be a bit underbiased (cool); you could give the 9-pin socket its own lower cathode resistor.
Make the power tube grid resistor 1Meg. Probe this voltage soon after start-up and again after some playing. It should stay within a volt of zero. Gassy tubes may show several volts and possibly increasing over time. These could get in real trouble in fix-bias amps. Hot tube releases gas into the vacuum, which can cause the grid to rise positive, which makes the tube run hotter, which releases more gas.... run-away. And in "fix" bias,there's no self-bias stabilization. Fix-bias amps favor 100K grid resistors for this reason. But if it is marginally unstable at part-power and 1Meg, it should not be trusted at full power even with a smaller grid resistor.
You can test 6AV6 6C4 and other single-triode small tubes with another socket jumpered to the 12AX7 socket. Break two leads to the section of 12AX7 that you are replacing with a single-triode.
It seems like a lot of work, but it tells you how a tube PLAYS, which is what you really want to know. And most real-sick tubes will be obvious from the initial poke: if most 12AX7 sit with 150V-170V on the plate, and you find one that sits 100V or 240V, it isn't a happy 12AX7 (it might be a happy 12AU7...). The first strum will tell you if Rp and Gm and all that crap is about-right. "Tube Testers" won't usually tell you if a tube hisses, or CLANGGs, or rattles, or has dirty-crackly pin-tarnish, or just "sounds boring".
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