Standby hum - this one is a first for me...

[EFK]

This one has me contemplating the slow pulling out of my hair. Not quite there yet, but I'm thinking about it. I don't like standby switches, but the chassis I just used had a slot for one so I had to put something there. This amp sounds great and is dead quiet when running. However, as soon as I switch to standby, there is a very loud BBBBBBBBBBBRRRRRRRRRRRRRRRrrrrrrrrrrrrrrrrrr r r r r r .......... that immediately comes through the speaker, quite loudly, then fades into the background (but is still noticeable) after around 5-10 seconds (as the caps drain down). Definitely sounds like 120 hz hum, not deeper 60 hz. The funny thing here is that even when it fades, the hum on standby is considerably louder than when the amp is actually off standby. I have checked and double checked grounds and I do not have this problem with any other amp being plugged into the same outlet. Have already checked the wall outlet. Rectifier is a 5R4, power tubes are 7581A, all tubes seem to be good as the amp sounds wonderful and works perfectly otherwise. Power section/cathodes all grounded to a single point near the PT, PI and preamp all grounded at the inputs. Power cord and PT copper shield are grounded to their own bolt.

I can't figure this out at all, but if I can't get it straightened out I think I'm going to move the standby switch into the line between the screens and the preamp.

[Enzo]

Well, it would be a lot easier to comment if we knew what you were working on and posted or linked to a schematic.

You talk of moving the standby switch, but unless I am blind, I don't see where you have it now.

Your wall outlets have no way of knowing if you are in standby or not, they are innocent.

[Bruce / Mission Amps]

Is the "standby switch" wired into the center tap of the power tranny hi-V secondary and also taking the 6.3v filament center tap out at the same time because of a goof... I dunno just thinking out loud.

[EFK]

Sorry! The circuit is my own, loosely based off late 1950s Supro 1690T. I generally do not use a standby switch, however I'm trying out different chassis and this chassis (from Weber) had a dedicated, marked "Standby" hole so I had to put some kind of standby in there. Its right between the rectifier and first cap, typical location. I have to say I already decided to avoid the whole headache and rewired it to lift the power tube cathodes, which of course instantly fixed the problem. A perfect mute switch and probably electrically better than where it was.

I do have the filament CT attached to the cathodes to elevate it @24 or so volts, don't know if this was affecting things since - as I mentioned - I've never used a standby before with this circuit.

[Steve Conner]

How much voltage appears across the switch in standby? Could be overstressing the heater-cathode insulation of your preamp tubes.

[Bruce / Mission Amps]

...
I do have the filament CT attached to the cathodes to elevate it @24 or so volts, don't know if this was affecting things since - as I mentioned - I've never used a standby before with this circuit.

That's why I asked.
Biasing the filament string is the way I do it all the time too, even if it means I have to build a little +vdc supply at the end of the B+ rail.
By lifting the power tube's cathode from ground, it means your preamp tubes are still running hot with filament voltage but now their filaments (and the power tube's filaments) have ZERO reference to ground and there is no bias voltage applied to the filament string.
Off the top of my head, I can't see how that would hurt anything.