50W Plexi Build - Crazy Loud Noise Issue

[cigartime830]

So I've got this Plexi build that's doing something I've never seen before. All voltages look good, it used to play just fine, but I recently added a master volume to it and moved some grounds around and this issue surfaced. Naturally, somewhere in the changes had to be the culprit and I've checked over everything I did and even reverted it back to the way it was before, and still it persists.

1. Amp powers up fine, voltages look fine. Can play very quietly, but as soon as any real volume comes the amp starts howling the most god-awful sound, somewhere between a buzz and a howl at the loudest possible volume.
2. Probing for voltages also triggers it - seems like anything that would induce a pop or a signal will cause it.
3. When flipped off the loud howl turns into a rhythmic chirp and gets quieter until powered down: chirp - chirp - chirp.
4. Power it right back on and everything's fine, rinse and repeat.
5. Turning the bias down as cool as possible and you can probe just fine without triggering the noise, and the noise is not as intense when triggered.
6. Touching V3 Pin 3 or the right leg of the phase inverter that its connected to will trigger it no matter what. No change in voltage, but touching that pin will always generate the noise.

I'm at a loss here since there seem to be so many conflicting possibilities here. Anyone come across something like this before?

[sluckey]

Swap the OT primary plate leads. Any better?

[cigartime830]

Thanks for the reply. I was going to try that next just to do it, but since it was working fine before in the same configuration I can't imagine how that would fix it. Thoughts?

[sluckey]

I assume your added MV is a double pot on the grids of the output tubes? If so, You could have easily got the grid connections reversed when you wired the pot. Reversing the grid wires will cause the same symptoms as reversing the plate wires.

If you would rather reverse at the pots, just swap the two wires connected to the center lugs of the dual pot. No need to swap the shields if you are using shielded wire.

If you don't have this type MV just ignore everything I've said.

[cigartime830]

You're absolutely right, that's the type of MV I installed and could definitely be the culprit. Keep you posted. Cheers.