SVT Bias changes with signal/redplating

[rf7]

I'm trying to resurrect an old blue ink SVT. I've got all the preamp issues straightened out, but whenever I input signal to the power tubes, they redplate almost immediately. Also, the output transformer sounds like it's going crazy when this happens. I've noticed that the bias voltage climbs (goes less negative) with signal input. Also, the B+ voltages seem to drop when the bias voltage changes. I have this problem with both old and new tubes, and I just recapped the amp with Sprague Atoms. What could be causing this?

[rf7]

I figured it out, the amp originally had it's output connector on a pigtail wire hanging out of the chassis, which I replaced with a 1/4 inch jack. I should have known to isolate it from the chassis, but I did not do that. Now that the output is isolated from the chassis, the tubes are no longer redplating and the bias voltage is steady. Aack!