70's SB12 OT wiring

[Steelyman]

As much as we'd all like to help, sussing out amp problems is nearly impossible without having possession of the amp. I seriously recommend bringing the amp to a repair tech. That said, check to make sure C6 in the tone circuit has one end grounded. If it is not referenced to ground in that part of the circuit, it could cause the problem that I assume I understand you described.