output phase? disconnect nfb and recheck.
Yeah, I just pulled the NFB and the oscillations were completely gone even with the speaker. I did not consider this to be the issue since the amp worked nicely without oscillation with a resistive load.BTW, have you checked the OT wires to the tubes? They can squeal if the OT is wound backwards. Swapping them over is a simple fix
I really hope this could be the source issue so fingers crossed.
I had exactly the same issues with parasitic oscillation, which mainly was lead dress on V2 pins 1 and 2. I also added the diode to the trem intensity pot, and a snubber cap to the 220k resistor before the 0.1uF cap that feeds the intensity pot.
The thing that really threw me though was the copper tape I'd lined the inside of the cab. It all worked fine with the chassis on the bench, but once it was bolted up the noise would start again.
That heater voltage looks to be within the 10% margin. The Fender schematic specifies 6.3v.
I've never had an amp this quiet before- I'm thinking of calling it the Red Oktober! View attachment 964759
Nice and clean build mate, well done!
I will try to move the the wires a bit but the V1a grid is shielded, so I'm hoping for the output phase as the source of the issue!