Recently I went through the amp as it had some long term low end hum in the R channel. Tested all tubes, replaced any out of spec resistors, replaced caps on the board with Solens, Micas, reflowed all solder, replaced selenium with a Diode, replaced bias pots with parts from Dynakit, cleaned all tube sockets, etc.
The issue now is when I attempt to bias the amp. Without any input connected, the amp bias sets fine and is stable on both sides. When I connect an RCA cable with a source, the bias on the R swings dramatically up to 3.5v or so, L swings down a bit. If I attempt to correct the bias back down to 1.3v when I get close, the bias goes wild and the amp starts to oscillate with a loud low end thumping. I cannot get the right side then to adjust past .5v without the swing and oscillation. L side can get somewhat stable but also swings.
I’ve tried swapping tubes from side to side, problem stays in R channel. I’ve tried other known good GZ34, EL34 and 7199 and the problem is still present. I’ve been through the amp several times, rebuilt the RCA inputs and nothing changes the problem. Transformers, read proper ohms.
I’ve also tried a direct RCA to iPad and a PAS 3, alternate cables and all create the issue. Hopefully someone else has had the same issue. I would love to put what is a great amp back into rotation.