Champ preamp tube oscillates

[captntasty]

I built a Champ (based on the STF electronics super charged amp - I have built two of these and they work great) and when I turn the Master Volume past a certain point I get some pretty bad oscillation. As I turn the MV up the oscillation frequency goes down... here's the weird part - (I know this is a no-no but only did it for troubleshooting purposes) when I remove the speaker cable it is the 12ax7 that is mechanically oscillating - I can still hear the oscillation only it is not amplified through the speaker... I have tried a new socket, different tubes, new MV pot, moved leads around with no luck. Any ideas on where to go from here. What voltage measurements would be relevant in this situation - I will gladly post them. Help is much appreciated...

[guitician]

It sounds like your getting positive feedback through the circuit. Check your power supply caps.

[captntasty]

After further investigation I think it is actually the OT that is whining / oscillating. With this in mind I took a shot in the dark and removed the (what I believe is the negative feedback) lead from the 1.5k resistor (R10) 22k (Rf) resistor to the output jack and the whining disappeared. What does this tell me?

guitician: are we talking about the same thing? Positive feedback / negative feedback... what would I "check" with the power caps?

edit: I forgot in my first post - the schematic
http://i27.photobucket.com/albums/c1...turboChamp.jpg

[hollisdevillo]

i think it means that your ot was hook up backwards. so you were sending positive feedback back into the circuit. it'll work without the NFB resistor, but if you want it, you have to reverse your ot leads. then the NFB will work without the noise.

[captntasty]

Bingo! That did the trick... thanks both for your help trouble shooting.

[guitician]

Thats what I thought it was, but not from the NFB connection. I'm glad your problems solved.