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.

 

[lcsaba]

For reference, the schematic is derived from this one ?
http://www.moxtone.com/siemox.htm

It looks straightforward. Oscillation may happen. Do not
expect more than say 500V unloaded secondary dc, it
should be even less. High voltage seems to be a measuring
artifact if your supply ist the same as the drawing.

Where did you find the F2a sockets ?

Hi as_audio, I've bought original NOS Siemens sockets from askjanfirst (https://www.die-wuestens.de/): FAS063, those are very very good quality.

 

[baudouin0]

Do you have a bleed resistor for the HT?

 

[jazreader]

Hi as_audio, I've bought original NOS Siemens sockets from askjanfirst (https://www.die-wuestens.de/): FAS063, those are very very good quality.

Thanks, I asked Jan back in April but no stock available then.

 

[jazreader]

Do you have a bleed resistor for the HT?

Thanks, yes, 1M. Although it surprised me how quickly the voltage drops when I turn off, given the size.

 

[lcsaba]

Strange. GZ34 is indirectly heated and should start up very slowly.