6G2 Princeton - Weird Oscillation Only When Trem is Off?

[AJ2000]

Hi All - Just finished putting together a brownface Princeton - pretty much stock layout except I used individual filter caps instead of a can and my PT is a bit beefier - 325-0-325 - voltages measure about where a Blackface Princeton would be. Seem ok on first glance, though I'll probably look to drop the B+ a bit.

So - here's the problem - plays fine and sounds good when the tremolo is on - no funny noises or parasitics, tremolo works seemingly perfectly - but, as soon as I ground the tremolo to stop it, I get this funny Atari/theremin sounding noise - loud - varies sort of randomly with the volume knob - starts kind of high then drops in pitch - it really sounds like playing Pole Position - sort of "biiiii-ewwwwww"/fart.

Read somewhere that V2 can be demanding on tubes being both part of the PI and the oscillator - tried 4 or 5 different 12ax7s in there (Svetlana, Groove Tubes, and a Tung Sol - all known good) - same with all of them.

Disconnected the NFB loop - didn't make a difference.

Anyone else ever run into this? Any thought?

AJ2000

[acorkos]

I also built a 6g2 amp...I used individual filter caps, too, in a compact chassis, with the filter cap board alongside the main circuitboard. this caused some lead dress problems, including oscillations similar to yours. I was able to solve it by repositioning some of the wires while the amp was running.

[AJ2000]

Thanks acorkos, will chopstick it tonight - just funny that it's perfectly fine when the trem is on - tried grounding it to a couple of different places and the result was the same.....otherwise this amp is nearly dead quiet.

Just curious - did you use 30uf caps, or go with 20s for filtering? I've got 30s in there, but find it a bit stiff and lacking something - might go 30-20-20.....

AJ2000

[MWJB]

You shouldn't need to drop the B+ voltage, late 6G2s ran over 400vdc...you can't be far off that now, PR have more gain & higher voltage and don't typically display your symptoms.

Some pics may be helpful?

Don't change the filter caps until you have sorted the oscilation, you can't evaluate the tone as it is. Princetons with 40-100uf as the main filter still sound fine.