You would adjust (increase) the negative bias voltage to lower the measured test point voltage which reflects the lower current draw on the tube.
At 400 plate volts and 33mA, the tubes are sitting at only 13.2 plate + screen watts. An EL34 has a max plate rating of 25 watts, so you are running them about 1/2 of the max. Pretty conservative. Even at 450 volts they only see 15 watts at idel.
If your amp is not stable, you could have oscillations that will trip up your tubes. You could also have bad socket/pin connections. If you have not thrown out your bad tubes, you might test them and find that they are not bad.
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Long Live Dr.Gizmo
