Once I got the amp working with a cheap chinese 6v6 I had lying around, I tried other output tubes. I’m having problems with several tubes. Note I have not hooked up any impedance taps save the 8 ohm one, so I know I am running the amp with some mismatches, but that doesn’t explain the problems I’ve had.

Chinese 6V6 new production: sounds great. very quiet, no squealing.

JJ 6V6 new production: squeals at low and high volume settings. Sounds thin relative to the Chinese 6V6.

NOS 6K6 tubes (National and RCA): sounds great. very quiet, no squealing. A little less bright than the chinese 6V6, and a little less volume/earlier breakup, but it’s a bright amp and it can be made to sound like the chinese 6V6 with a tone knob increase.

Tung Sol Reissue 5881 new production: HORRIBLE. squeals terribly and is totally unuseable. the brief moment I tried playing I thought it sounded like a promising setup but the squealing is prohibitive.

Valve Art Chinese EL84 new production: Somewhat like the JJ 6V6. Squeals at the volume extremes and doesn’t seem to sound good.

I’d attribute the problems to impedance mismatches except that this makes no sense, because the single 6V6 setup should be a perfect match and the JJ sounds awful and squeals. And the 6K6s, which are mismatched impedance, sound great with no squealing.

Any ideas?