Littlewyan wrote: ↑Tue Jun 14, 2022 10:25 pm
In that case, I would just ground the speaker jacks for now and see if that solves the issue. If it does then look at rerouting some wires to tidy things up a bit, but you may be ok. Do the test of running every control on 10 (without a guitar plugged in), the amp should be stable. Run it into an attenuator if you can in case it does feedback.
I finished making all the recommended changes before I saw these last two posts.
Shortened PI anode and grid wires.
Removed "Resonance" and "Variable NFB" controls. Put these circuits back on turret board (just like stock amp).
Grounded speaker output jacks.
Re-routed grid wires away from speaker wires.
All controls set to "0" and speaker jack connected to dummy load I biased the output tubes to 36mV.
When I connect a 4x12 cab, as soon as I flip the standby switch I get a high-pitched squeal.
With PI tube removed, no squeal.
Pi tube back in, 4x12 cab, poked around with chopstick. Nothing made the squeal worse or better or any change that I could tell.