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It hums with only the phase inverter and power tubes (and rectifier)? So pull the phase inverter. Hum remain or not? The essence of troubleshooting is to isolate the problem. The elevated cathode voltage of a phase inverter can make some tubes leak heater current, and that makes hum. Power tubes usually have a bias supply, and any ripple in that supply will get into the tubes. So we cannot concentrate only on B+ supplies. Of course those alternatives do not exclude the PI as having a ground issue, but we need to find out.
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