Homemade Tube Tester

[patlaw]

Has anyone here built their own tube tester (and cooker) for a 6550? It would be nice to be able to power them up individually out of the amp to see if there are arcing or sparking. An octal socket, a few resistors, a few capacitors, and a 350 volt supply ought to do it. I'd love to have an Eico 667, but they're not cheap at the moment.

[J M Fahey]

I use the amp itself as a tester, matcher and cooker.

1) to begin with, voltages are "realistic" by definition which never are in a tube tester.

2) load is more realistic than what any tester can give you.

a) it's inductive (doubt any tester supplies that)

b) it's a push pull transformer, which means test voltage reaches 2X +V .
As in: a 450V +V subjects test tube to 900V peaks, courtesy of its "partner".

3) to see the ugly truth about current emission, I feed a sinewave to the power amp, drive into heavy clipping, and do NOT compensate for impedance , so if I am using, say, a 4 x EL34/6L6/6550 amp I do not change output taps to compensate for using just 2 tubes, I use nominal value so amp clips because of current starvation.

Using a known good tube and comparing squarewave symmetry under lower than expected load impedance clearly separates men from boys.

Of course, momentarily lift NFB which tends to hide such things.

It's good practice to switch tubes position atb the beginning, to check whether asymmetry is due to differing tube characteristics or PI assymmetry.

Once tubes are measured and matched, load the full complement , connect a beefy load, drive amp to clipping and let it cook.

Weaklings won't stand it, but better kill them at the bench than on stage.

[Silvertone Jockey]

Once tubes are measured and matched, load the full complement , connect a beefy load, drive amp to clipping and let it cook.

Weaklings won't stand it, but better kill them at the bench than on stage.

Brutal but effective.....that's the only way to know for sure if a tube is going to hang

[Leo_Gnardo]

What Juan said ^^^. He goes a couple steps beyond me, I'm gonna have to catch up. I use an old Heath W4 power amp to test all pin-compatible output tubes, 6V6, 6L6, EL34, the KT series, and 6550. It puts way higher plate and screen grid voltage on the output tubes than the usual run of tube testers. Mostly it's good for culling out tubes with weak emission.

There's only one US tube dealer I know of who will test/match tubes he sells at 700V if you ask: Jim McShane in Chicago.

If you're going to test a lot of 6550/KT88 for SVT and similar amps, it would make sense to rig up a tester-amp that puts 700V on plate and 350V on screen grids. Maybe find a junker amp that you could convert, an old Univox or something like that.