I rechecked the bias voltages on all 4 tubes and they are all right on at 0.40 volts after 15 minutes. So I assume the circuits are running fine. Now I power down the amp. and install the 3 output driver tubes and power up the amp again and recheck the bias setting on both channels. The bias voltages on the left channel are fine, but the bias voltages on the right channel begin exceed 0.6 volts and raise quick after 30 seconds or so.
Any thoughts on what I might check next?