stewdan wrote:Hi Plexus ---
I am also building one of Roy's VTA/MK3-Octal amps and Roy mentioned to me that you criss-crossed the VTA Board's outputs to the the 6550 tubes in order to solve a feedback problem.
Was this a high pitched squeal that occurred with all tubes in their sockets and about 20 seconds after power was applied?????
In my case, if there are no 6SN7s in sockets I don't have a squeal. Was this like what you ran into?
You say you are running a basically stock amp with the new VTA Board.
Did you have to drop the stock 6800 Ohm resistor on the Quad Cap to get the caps output in line with the 380 volt VTA Board's B+? I ask, because my Quad Cap's final output is about 530 volts and that goes directly to the circuit board.
Any response would be greatly apprecialted.
I didn't know about the criss cross issue so i followed the instructions i had. at first they weren't crossed and will all tubes in, i got feedback. with no 6SN7's there was no feedback. Roy's first suggestion was to cross them but I didn't see the basis for this and it contradicted what was in the instructions so i thought it might have been an off the cuff random suggestion. after a few more emails he asked if i crossed them. so in order to say "yes" i did and it worked! then he told me why. but not being that experienced with tube electronics i wouldn't have thought of that. now i understand why.
"Basically stock". i actually also replaced the quad-caps with modern cap boards, mounded internally. i can't remember the total capacitance but the board has 4 banks to 2 in-series 450V caps. i think they are 25uF each, so 4 banks of 50uF. i think. it might be a Curcio kit. i put them in 10 years ago. so its whatever resistors came on those boards. if you want i can dig up the instructions for the values.
Update: I remember checking the cap board I put in and it had a 6.8K resistor already on it going out as B+ so that was taken care of with the cap board
After having a few hundred hours on my amps, I will say the VTA octal driver sounds amazing. waaaaay better than the stock 6AN8. I had installed modern glass expoxy versions of the stock circuit using high quality matched parts. that was an improvement. but the VTA boards really open the amp up, resolve more level detail and a much more balanced sound. I switched out my 10 year old Svetlana 6550 flying C's for new Tung-Sol 6550 reissues. For no real reason other than to change it up. I did put the Svets back in just to see how much of a different and it wasn't crazy different. just a subtle change... maybe.
And the VTA boards looks cool because, well you know, fashion is a part of the whole tube thing.