Yeah, I agree with you. Like I said, I wanted a project to work on. Guess its a repair project. The problem is, the soldering job is really hackish and I am worried that the board itself may be corrupted. I'll find out when I get into it. Joy! I'm hoping bob will chime in and let me know if the z-caps in the middle of the board were installed by him or were installed by the previous owner. The soldering looks really good, so maybe the z-caps were from the original build. Thanks for the input.
Sorry about your problems with the amp ...
That amp was "worked on" by someone and I do not know what was done to it. Those large "Z" caps are not part of anything that I would ever use and were not installed by me. The "Z" caps had to have been installed by a previous owner. The photo shows a mishmash of caps on the bottom of the driver board - none of which were installed by me ...
The thing that really tics me off is that with the two cap upgrades that I supply as an option, that amp is taken about as far as you can go with "upgrades". The only thing that you can really do to "improve the sound" is to roll different tubes in there and see which tubes you like.
What I advise is return the board back to it's stock upgraded form. In fact you might be better off just installing a new board and parts set with cap upgrades rather than take the time to pull all those other capacitors off there and then reinstall what should be in there.
In an ST-70 (with an upgraded power transformer as you have in there) bias KT88's at .500 volts DC per output tube. If you had an ST-120 you would bias them at .550 VDC.
The adjustment pots on the underside of the board are the 10K AC balance controls. These two pots adjust the phase splitter/phase inverter section of the driver circuit. The kit builder should set them at their center position which is always very close to their optimum setting. To set them more accurately you need a signal or function generator which most kit builders do not have. They are put on the bottom of the board so you won't "readjust them" accidentally when setting the four main bias pots on the top of the board.
Last edited by Bob Latino on Thu Feb 18, 2010 7:38 pm; edited 1 time in total