GZ34 occasional runaway voltage

[rayma]

All this could be measurement artifacts.
Hook up two DVMs both to the B+ and see if they both indicate such high surges.

 

[jazreader]

You can compare the F2a tubes by measuring voltages on cathode resistors R9, R10 to ground.

Thanks, yes I was using that to set 50mA on each, rather than for the bias voltage. But still I had the difference. The quiet amp has less glow from the valves, I’ll check again and check heaters. The quiet ones were also older, slightly different construction. Maybe first I’ll try swapping tubes between amps to confirm its tubes not amplifier.

 

[jazreader]

Balance problem sorted, it wasn’t the amps, but a tube seating must have been knocked in my dac. All sounding great now.