Wild Oscillation

[Valvehead]

I have a Splawn Street Rod here thats going into crazy oscillation at higher gain settings. All tubes have been swapped. One thing I found is that if i move my finger close to the Gain pot , it acts exactly like a Theramin . The noise will change frequencies .

Ive tried a different pot and the shielded wire from the pot to V2 pin 7 . I havent yet replaced the V2 pin 2 wire--both of these go from Gain pot to v2 . Both shields test good to ground and so does the pot.

Another thing- i can press down on the OT and or choke with some force , enough to flex the chassis, and the noise will actually go away !

I will attach a video of this -- any initial guesses ? Loose ground somewhere ?

http://vidmg.photobucket.com/albums/...psq94auwxt.mp4

[Enzo]

Make sure any chassis connections are solid, particularly any using screws. I include things like chassis mount bolts and jack and pot nuts. The chassis in the photo looks unpainted, but it is important that pot and jack bushings make good contact with the inside of the panel electrically. A lost ground to a filter cap could cause a loss of decoupling. Scope each filter cap for the oscillation. Could be a failed solder to ground for the cap.

The back covers on pots usually are grounded to the panel by the bushing, but not always. Ground a clip wire and touch it to the rear cover of any pot that remotely could be involved, any change?

The theramin effect verifies that there is a feedback going on, and your body capacitance alters the loop.

[Valvehead]

Found it !

I checked every ground , checked every screw . When I first checked the 4 shielded cables , they tested good to chassis ground. But when i stuck a couple clip wires from them to ground all the noise stopped .
So i made a new ground wire thats bolted to the trans ground and ran it to all 4 shields. Amp is perfect now !

Thanks enzo !