Peavey Classic B Series - Plate Current

[tdlunsfo]

I have a customer's Peavey Classic here that blew a mains fuse. I fired the amp up and everything is good till you start to push it and then something appears to short. I traced this down to an oscillation that starts when the amp enters distortion, and the tubes begin to red plate popping the fuse. So, knowing the amp has sat for years prior to being placed back in service, I completely recapped it. Idle current is now much lower, but I saw something curious while doing the burn in test as the plate current in one of the tubes is taking a dive while it's being pushed. When pushed hard, one 6L6GC is around 48mA and the other I can get as low as 12mA. Bias is set to around 60% dissipation. It dawned on me that I have never tracked plate current on both tubes while dimming an amp and I'm unclear if this is normal. Bias voltage and screen voltage match on each tube while being pushed. The amp makes ~25W clean and 50W dirty. Thoughts?

Link to Schematic: Classic-B series.pdf (adrsoundsense.co.uk)

[Helmholtz]

Bias is set to around 60% dissipation. It dawned on me that I have never tracked plate current on both tubes while dimming an amp and I'm unclear if this is normal. Bias voltage and screen voltage match on each tube while being pushed. The amp makes ~25W clean and 50W dirty. Thoughts?

It's not normal.
Are both grid signals equal when pushed hard?

[tdlunsfo]

Yes, about 50V peak to peak.

[Helmholtz]

Is output clipping symmetrical?

What are the idle currents?

[Helmholtz]

Did you check the flyback diodes?

[tdlunsfo]

Idle plate current is 35mA and 36mA @488V. Bottom half of the wave clips first and then the top half is right behind it at ~26W into a 16ohm load

[Helmholtz]

Any difference when lifting the flyback diodes?

[g1]

25W output is not normal. Have you not tried other power tubes yet?

[tdlunsfo]

Did you check the flyback diodes?

They measure ~1v one way and OL the other. I'll lift them in a bit and report back.

[tdlunsfo]

25W output is not normal. Have you not tried other power tubes yet?

I've tried some spare used tubes. I have a pair of new JJ's but was trying to solve the issue before potentially ruining a set of new tubes. I feel comfortable enough now that I can catch an issue if needed and can throw them in. Give me 30 mins and I'll report back.

[Helmholtz]

Yes, about 50V peak to peak.

Grid signals should be clean up to 110V peak to peak.
Best check with NFB disconnected.

[g1]

25W output is not normal.

I'll edit this and say that the A and B version classics are 4 ohm load, so 25W may be normal for 16 ohms.
Is the cab set up with 16 ohm arrangement?

[Helmholtz]

Nevertheless both tube DC and AC currents should be the same.

[tdlunsfo]

I'll edit this and say that the A and B version classics are 4 ohm load, so 25W may be normal for 16 ohms.
Is the cab set up with 16 ohm arrangement?

It appears to be a 4 ohm load and a meter lead issue fooled me into a 16 ohm load earlier. These are Peavey speakers but the impedance is not specified. Nevertheless, when swapping my dummy load to a 4 ohm load the amp now breaks up around 18W. New tubes make no difference, same symptoms. Removing the flyback diodes now.

[tdlunsfo]

Flyback diodes made no difference but the new tubes are giving me different results. Now when I really crank the amp, I am getting more like 100mA and 54mA but I think this is because I am using a 4 ohm dummy load now.