Something is wrong. (Solved!)
Posted: Sun Jun 12, 2016 5:16 pmI recently decided to rebuild my ST35 into a smaller enclosure. It uses Shannon's driver circuit with EFB and just one bias point per channel. Unfortunately, I got overconfident and screwed up some wiring. Upon powering up the amp I just got a really loud tone. I had missed a wire connecting the high voltage at cap 2 to the 360K resistor in the EFB circuit. As far as I can tell I didn't blow up any resistors or caps though I suspect the LM337 may have been damaged. I fixed the miswire and quadruple checked everything else. The most obvious problem I'm now encountering is the high voltage seems higher than it should be.
With 117vac going into the amp I get the following measurements. This is with no tubes as I didn't want to damage them.
PT secondary - 641vac
1st cap - 418vdc
2nd cap - 418vdc
3rd cap - 418vdc
V1 plates - 405vdc
V2 plates - 418vdc
LM337 - out - 2.08vdc, adj - 8.92vdc
It seems weird that the B+ doesn't really drop after the 6.8K resistor. I replaced it to make sure and got the same results. I'm suspicious of the LM337 so I went ahead and put in a new one. I haven't powered it up yet but I thought I would post this for any possible help.
With 117vac going into the amp I get the following measurements. This is with no tubes as I didn't want to damage them.
PT secondary - 641vac
1st cap - 418vdc
2nd cap - 418vdc
3rd cap - 418vdc
V1 plates - 405vdc
V2 plates - 418vdc
LM337 - out - 2.08vdc, adj - 8.92vdc
It seems weird that the B+ doesn't really drop after the 6.8K resistor. I replaced it to make sure and got the same results. I'm suspicious of the LM337 so I went ahead and put in a new one. I haven't powered it up yet but I thought I would post this for any possible help.
