A quick and short update. I managed to start testing the power supply and the power amplifier. The power supply works just fine as it is. I have the following values:
V
HT= 385Vdc (no load), 360Vdc (loaded)
Vdriver= 330Vdc (no load)
Vinput= 188Vdc (no load)
Voutout_screens= 188Vdc
Then I have the lower voltages that are used for gate bias:
Voutput_bias= -20.03Vdc
Vinput_bias= -37.8Vdc
Vdriver_bias= -10.37Vdc
I then proceeded to power the power amplifier and I stared by applying the 385Vdc to the center tap of the output transformer and I adjusted the -20Vdc grid bias to get around 45mA through the output tubes. I will crank up the bias current through the output devices once everything is thoroughly tested. The rest of the amplifier (input stage and driver stage) were not powered at this stage.
I proceeded to power the input stage and the driver stage as well. Unfortunately I was running out of available time and I only got the quickly measure the bias current through the input stage (~2mA - 1mA through each half of the tube) and through the driver stage (~12mA - 6mA through each half of the tube). While powering up the complete amplifier I added an 8OHM resistor on the output winding and I ran the feedback wire back to to the input stage. I did notice some oscillation (of course there is oscillation
) after the output stage tubes warmed up. I did not have the time to dig a bit deeper into this but I did notice that the current through the output tubes went out of balance. One had around 45mA through it and the other one had around 60mA. Nothing alarming but definitely needs looking into.
Next I will power the output stage by itself and I will set the bias and the balance again. After I will power the input stage and the driver stage with the output stage disconnected (either not powered or by disconnecting the coupling capacitors). By doing this I want to precisely set the DC bias balance through the two stages and only after this is set I will connect all three stages together.
The oscillation will most probably still be there and I will start investigating with the scope. I also realized that I forgot to add the R-C zobel network that should be on the 8OHM output (feedback trace). I will add that as well.
The tests above were all done with the output configured a pentode. I will also test with UL to see how it behaves.
The first suspect for the oscillation is the output transformer. Therefore I will add a 1nF+1K between the anode and the screens of the output devices (UL tap and the anode tap of the output transformer) and see what this dose to he oscillation.
Any suggestions are welcome