OK, I have a weird situation. I am modifying a guitar preamp. The preamp is a module with 4 tube stages: 3 gain, 1 cathode follower, and there is also a common stage (not on module) to make 4 gain stages in total. I've connected a gain boost on the stage 1 cathode by adding another resistor, along with a bypass cap in parallel, to the original cathode resistor. I've installed toggles to bypass any number of stages, at a time. I've successfully done this before, many times, but something is different here and I can't figure it out. Here's how the toggles work: clean channel-bypass stages 2 and 3. rhythm channel-bypass stage 3. lead channel-nothing bypassed. Here's the problem. When I engage the gain boost, the rhythm channel gets a nasty squeal and harsh distortion noise. The squeal will change frequencies when I adjust the gain up/down and it will also go away when I turn the mid range pot down far enough. The squeal can also be heard on the clean channel, but not as badly. The lead channel doesn't have any squeal. I've already tried 6 different tubes, so no microphonic tubes. I've also changed the cathode resistor on stage 3 and plate resistor of 4 (and 5). I've rewired it with new wire, and I've resoldered all ground connections. It seems like a ground problem to me and maybe a bad component that can't handle the gain boost. But, I hate shotgunning individual components, so I was hoping for some insight from here. Thanks a lot!