Question about bypass/filter caps? (I think)

[Guiary]

Hello all!

I'm still doing more mods to my VTM120 and now I've hit a zone of oscillation... I think that's what it's called when you don't get that nice gradual build up of feedback but when you get that high pitched squealing feedback.

Anyway I was wondering if there were some recommendations on how to combat this terrible terrible noise? I was thinking of bridging some capacitor value (.22uf-2.2uf somewhere inbetween there) on the plate resistors of V1 and V2. I tried a very low value (.022uf) just to see what it was like on one of the plate resistors on V1 and man did it ever come out bassy! but it fixed my oscillation problems.

So now I'm wondering if someone might suggest a value I may wanna try to still maintain the gain but maybe down it a little, and also should I be trying it on the resistors from V1 or V2 or both?

I guess it's all trial and error right?

Thanks,

Gary

[Enzo]

Turn the volume knob on the guitar itself down to zero. Does this oscillation go away? If so, it is not the amps fault.

[stokes]

From your description,I assume by "bridging",you mean you are bypassing the plate resistor with that .022 cap.That is way too high a value for that purpose.Try a 500pf,or smaller.You want the smallest value that will cure the oscillation.Oscillations can be a pain to find and cure,but it is better,tonewise,to cure it without using the bypass cap method.Sometimes,after you determine which stage is causing the oscillation,connecting a wire to the plate and then winding it a couple times around the preceeding grid wire will cure the oscillation without sucking as much of your tone as the cap method.Be sure the end of the wire not connected to the plate is well insulated for safety reasons.Of course,this is assuming Enzo's suggestion doesnt work,and it is an oscillation and not just accoustic feedback.

[nooneknows]

Sometimes,after you determine which stage is causing the oscillation,connecting a wire to the plate and then winding it a couple times around the preceeding grid wire will cure the oscillation without sucking as much of your tone as the cap method.

That's what drove me mad when I modded my Jcm800 years ago, I didn't notice there was a trick like this on the first tube, when I remounted the in circuit the amp started squealing...
When someone on ampage pointed it out (thanks again) I solved the problem using a well insulated shielded wire for the grid input, with the shield connected on the plate

[Enzo]

Well now that is just odd. Of all the unusual places for spam to show up, tacked onto the end of a bypass cap thread is not one I'd expect. And considering the content, I can't imagine how they felt it would be productive here.