Ampeg SVT classic bias problem

[Helmholtz]

From working on these amps for years, I don't see any change in current thru the power tube's cathodes switching in and out of Standby.

How that if the standby switch kills B+?

[nevetslab]

I guess the bias settings preset where the bias voltage will be when the HT Xfmr is turned back on each time. I don't hear any major thump or noise switching in and out of Standby. And, monitoring the cathode resistors, where I always monitor the bias from for each half of the output stage, it settles quickly to where it was previously set when switched from Operate back to Standby. That Standby switch DOES turn off the HT, Screens, and Preamp plate voltages, so interesting that it is so repeatable. I never stopped to ponder that.

[g1]

But the power tube cathode current does stop when in standby. nevetslab I think your wording in post #15 was a bit confusing and you meant there wasn't much drift in the cathode current through repeated cycles.

[Helmholtz]

If the driver tube supply is switched off during standby, power tube grid bias should drop to around -180V.
Reason is that the -45V bias is produced by a voltage drop of +135V across the 47k cathode resistors. Without supply no cathode current and no voltage drop, so voltages at both ends of the 47k resistors should be the same.

[Voske_1980]

Alright, thanks for all the replies so far!
I've been on vacation since my last post and working on the amp again this evening.
With all the power tubes removed the bias voltage still changes from -45V to -25V when the amp in switched out of standby.
I've measured the ripple voltage on C23 (which creates the -180V supply voltage for the bias circuit), which measures 60Vac. I guess the capacitor is my problem...
After replacement the ripple was gone (the original cap measured 1uF instead of 100uF) and the bias supply voltage was -190V. That looks more like it!
I cannot test any further right now, but tomorrow I will continue.
Thanks guys!



<< Update >>

Yep, found some extra time tonight to check everything out and the amp is running sweet a can be!
Thanks again for the headsup!