Making a DIY tube tester - have some questions

Joe Garfield · 2217

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Offline Joe Garfield

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on: March 31, 2023, 03:27:29 PM
I'm going to build a tube tester for the select few tubes I use most: 12A_7, EL84, and 6080. This is partly for me to learn to better understand the circuits, and partly to check old tubes to prevent damage to my amps. For reference I'm using the schematics in the RCA Radiotron Designer's Handbook, and the Valve Heaven DIY circuit:'s-Handbook-3rd-Edition.pdf (pp.244-245)

I'm starting with a static tester (fixed DC grid voltage) and will eventually implement a signal source for dynamic testing.

The biggest question I have right now is about power supply and cathode resistors. The tube specs call out a plate voltage and cathode resistor value, but the test schematics omit cathode resistor (as I understand it) in favor of a regulated (or 'stout') power supply. If I use a 230v transformer (322 VP-P) and a pot to adjust the plate voltage, do I still need the cathode resistor to get the right current? Or, should I leave the raw voltage at the plate and make the cathode resistor variable to control current flow? Is it a bad idea not to use a regulated power supply?

I have a nice Mesa fixed bias amp. I'd like to measure it's stock 'Mesa' tube so I can shop for tubes with similar performance, and also be able to relatively match sets of tubes for that amp. I'd also like to be able to provide some very rough reassurance if I decide to sell a 6080 tube.

Thanks for reading and any suggestions! I've built a few Bottlehead kits and am excited to be finally understanding and designing/modifying circuits.

« Last Edit: March 31, 2023, 03:50:54 PM by Joe Garfield »

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Offline Paul Birkeland

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Reply #1 on: March 31, 2023, 07:16:18 PM
It's soooo much easier to provide negative grid bias in a tube tester rather than a cathode resistor.  This takes the guess work out of power dissipation across that resistor entirely, as you just plunk the correct negative grid bias voltage on G1 of whatever you're testing and call it good.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man