problems with preamp on home brew

[jeeptechfred]

Hey Yall!
I just put together a copy of webers 6a20x2. I built the 6v6 quad power amp and the normal channel. I did not build the vibroto and reverb channel. in place of that a built a marshall type channel. The thing works and is crazy loud. the marshall channel seems ok and the fender-like channel sound great except when I turn up the treble and the volume in the right proportions I get a squeel and then it seems to clamp down. I can see one half of the power is force to draw a bunch of current while the other side drops in current. I have been over the diagrams a bunch of times and I believe all is connected correctly. I have electrically disconnected the marshall channel, no change, I have tryed swapping tubes around, no change. could this be a parasitic ossilation? Any ideas? I have included the diagram. ignore the reverb channel. I am using a dpdt switch to switch the inputs and outputs of the preamps in case you were wondering or if it is important. thanks

[dcgillespie]

This sounds to me like a layout problem -- sensitive input circuits too close to the output circuits. The symptoms you are describing are classic to this type of problem. If your wiring is neat and tidy, and well dressed close to the chassis, the some manner of physical shielding might be needed to keep the coupling from occurring.

Dave

[jeeptechfred]

While you were posting your relpy I was figuring exactly what you suggested. My spdt switch that I had sensitive input and the output going through same switch but different gangs. I took the outputs off the switch and expierimented with mixing resistors. Seems good now. I'm kinda new to this stuff, thanks for your help.