Hello,
I have a st-70 that has been pretty much completely rebuilt. I originally got it off of ebay, but have replaced the power supply stage (curcio kit), the transformers (triode - both opt and power), tube sockets, and of course the driver board - I am using the one from here.
The amp was always kind of intermittant and it essentially has sat for about a year because it kept blowing fuses. Finally I've dug into it. I've determined I've had a number of bad tubes - EL-34s and KT-77s and the rectifier. Since I am waiting for a new rectifier tube in the mail, I've wired things up to use solid state rectification.
My problem appears to be isolated to one channel and I think the driver board. On the right channel, using KT-77 power tubes (these are the only 100% sure brand new tubes I have on hand that I know are good), I am able to bias things just fine. With full ccw on the wire wound, I get 46.5 mA, and I can set to 50mA without issue and it will stay there. This is without any power tubes on the left side.
The left side, the bias current just climbs up and up, slowly, but in less than a minute, I'm turning things off as it creeps past 100mA. This is without the right channel power tubes. If I pull the coupling caps on this (bad) channel, I can bias that side and life is good - full ccw is around 46.5 mA. Note, all of this testing is using the same 2 KT-77s, so I know the tubes are fine. I've swapped the 12AU7s from left to right and eliminated those as the issue. I've swapped out the 12ax7 and that works the same regardless.
Here's the one thing I am able to see. Without any power tubes in the amp, across J4/J5 and J15/J16, I see the 6.3 VAC; however, on J4 and J5 there is a DC component of about 1.3 VDC (measured from J4 to gnd and J5 to gnd). On J15/J16 (the working channel) I see a much smaller DC component - 0.150 VDC.
I've gone so far as to pull the transformer leads on this channel (green) and I see the 6.3VAC and a negligable DC component. I've pulled the wires at J4 & J5 and I do not see any significant DC component - only when things are all connected.
This leads me to think the issue is somewhere on the driver board.
Sorry for the book above, but ANY help on this issue would be beyond appreciated. The ONLY original items left in this amp are the wire wound pots (those appear to be identical when I compare resistances, voltages) and the 3 wire cap (this too appears fine). Everything else is new and wired as the schematics in the diytube manual.
Thoughts on where this DC component is coming from and causing me my troubles?
Thanks!!!
Aaron