Hi folks — My ST35 (from a Dynakit Parts kit) with EFB had been running perfectly for months until a couple of days ago when one of the EL84s red plated. I traced that to a bad coupling cap; so I replaced the caps and the tubes (new matched quad). The amp will now hold bias. The only problem is an imbalance between the two channels on the EFB board. Before the incident, the difference between the two channels measured at the test points was about .02VDC: .26VDC, in one channel .28VDC in the other to get an average of .27. Now the difference is about .08VDC, and that can't be good. Any advice about what to check next? Could I have damaged the regulator? I don't really understand the circuit, but I can follow directions.
Register to hide this ad
Test by moving the righmost pair to the left side and move the leftmost side to the right. If the difference moves to the other side your tubes are not perfectly matched.
This is no problem you should still be able to use your amp.
if its a single adjustment, you're seeing a minor difference between two different pairs of tubes. There really isn't anything you can do about it without individual bias adjustments, but maybe you can make it a little better.
This is slightly unconventional and only works because of how EFB functions but you can remove all but 1 6bq5 tube and take bias readings on it. Go through all 4 plugging each into the same socket, noting the bias reading on each tube without making any changes to the adjustment pot. Pair them by closest reading and put one pair in each channel. It still may be slightly off channel to channel but thats really not a problem.
also maybe worth confirming the values on the resistors you measure across. If those are off, the voltages will be off too.
This is slightly unconventional and only works because of how EFB functions but you can remove all but 1 6bq5 tube and take bias readings on it. Go through all 4 plugging each into the same socket, noting the bias reading on each tube without making any changes to the adjustment pot. Pair them by closest reading and put one pair in each channel. It still may be slightly off channel to channel but thats really not a problem.
also maybe worth confirming the values on the resistors you measure across. If those are off, the voltages will be off too.
Thanks to both of you. The voltages flip when I swap the tubes, so peterh was right; it's just a poorly matched quad. I wasn't sure how much of a mismatch was tolerable, but I'm glad to hear this isn't wildly out of whack. I'll try gadget73s method to see if I can improve the match. Appreciate the tip.