Cathode voltage is lower than expected, and plate voltage is higher than expected... are heaters voltages correct?
If you have almost no flow of current that would be the first place I would look at: low to no heaters, low to no current flow.
Preamp cathode bias mystery
Moderators: pompeiisneaks, Colossal
Re: Preamp cathode bias mystery
Thank you very much, Roberto, that must have been the reason initially. After poking and doublechecking all the board connections we probably improved the connectors of the heater supply because we noticed afterwards that the V1 voltages were "miraculously improved"; 1.04VDC on the cathode and 236VDC on the plate of V1A. This is still not perfect but better. The heater voltages are unusually low in this model; 5.5VAC on power tubes, 5.1VDC on the first preamp tube, which has DC supply. I had the opportunity to check another guy's amp, same SVT-2Pro model, and it had 5.6VAC on power tube heaters. So it seems that low heater voltage is normal on this model.
But apart from that I tried to follow the signal through the stages and everything was close to spec (e.g. with an input of .15 VAC there was 17 VAC on the cathode of V3A (before the tone stack).
The last good signal is on the master pot, there is about 1 VAC. Then it goes to a cathode follower on V5A before going to the preamp-out jack and the poweramp board (where it goes to another gain stage and to the inverter before the bias tubes and power tubes). So before the cathode follower the signal is 1VAC, but right after the cathode follower, on the preamp-out jack, the signal is only .04VAC... Also I get a loud bang from the speaker when I try to measure VDC on the grid or the cathode of the CF (V5A). Should I suspect C30 on the grid of the CF? Or C31 on its cathode? Or maybe I should measure VDC relative to local circuit ground, not chassis, to avoid the bang?
But apart from that I tried to follow the signal through the stages and everything was close to spec (e.g. with an input of .15 VAC there was 17 VAC on the cathode of V3A (before the tone stack).
The last good signal is on the master pot, there is about 1 VAC. Then it goes to a cathode follower on V5A before going to the preamp-out jack and the poweramp board (where it goes to another gain stage and to the inverter before the bias tubes and power tubes). So before the cathode follower the signal is 1VAC, but right after the cathode follower, on the preamp-out jack, the signal is only .04VAC... Also I get a loud bang from the speaker when I try to measure VDC on the grid or the cathode of the CF (V5A). Should I suspect C30 on the grid of the CF? Or C31 on its cathode? Or maybe I should measure VDC relative to local circuit ground, not chassis, to avoid the bang?
Re: Preamp cathode bias mystery
Problem gone. Not sure why. After putting the guts back together to move elsewhere the amp works normally. I think it could have been a loose connection somewhere. The amp has a few suspect-looking ribbon cables, even for heater current for three tubes. There was also one capacitor (100pF C1 on AC-term.board, not critical) with one leg not even soldered in from the manufacturer.
Thanks to everyone for valuable help.
Thanks to everyone for valuable help.