Relocating a hum balance point?

theehman · Post by **theehman** » 17 Oct 2008, 19:07

I'm fixing up a friend's old Stromberg-Carlson amp for his use w/ guitar. I want to set up the output tubes (2x6L6) to have separate cathode resistors and capacitors. The one problem I'm having is that the filament winding has a 100 ohm hum balance pot with the wiper going to the (presently) tied together 6L6 cathodes (which has 25v according to the schematic). Should I just take the wire and connect it to ground or is there some place better to put it so it's getting a similar voltage?

soulsonic · Post by **soulsonic** » 19 Oct 2008, 00:38

You could probably attach it to the cathode of just one of the output tubes and get the same result. There is also the possibility of using a largish resistor (like 220K or something like that) to tap off from the B+ line to give the bias voltage for the hum balance.
Or, you could just attach it to ground... that might increase the hum... or it might not make any difference. I can't predict whether biasing the filaments in that particular amp would make a big difference or not.

analogguru · Post by **analogguru** » 19 Oct 2008, 06:11

Personally I would not connect it to the cathode of one tube, instead I would connect it to ground. Some audiophiles indeed connected the center tap to the V+ line, the reason is tube architecture and electron flow - a too long story for now. Since this should become (only) a guitar amp......

analogguru