I have a Sunn T50c 50w 1x12 combo. On the high gain channel, I have an uncontrollable feedback problem. I can only get so much gain and so much volume until it starts to have a steady constant feedback, even with the guitar volume knob on zero, or even when no patch cord is plugged into the input. It only goes away when the volume and/or gain is lowered, or when put on standby.

This has never too much of a problem for me, because I never really needed to go into the level of loudness that produces the feedback, but now I'm going to sell the amp. I know that if someone wanted to use this channel in a metal or punk band, this feedback would be a constant problem.

Thanks in advance for any advice, and please feel free to ask any questions of clarification about the issue. Since I'm clueless about this problem, I don't quite know which info is relevant for diagnosis.

Try a new preamp tube in V1 or V2.

Thanks, but that didn't work. I put two 5751s in both V1 and v2.