Bias jumping and amp rumbling- help needed

[studiodunn]

So, this amp has been going strong since first cobbled it together. Build thread here - https://tubeamparchive.com/viewtopic.php?f=6&t=29497

I have done tons of mods and experiments on it and have essentially cut my teeth on this amp.
And, I’ve played the hell out of it.

Now today when playing it, I go to flip the standby and can hear it let out a rumble/farting noise. I flip the standby on and it’s definitely not happy.

It plays fine cold but once warmed up it does the same thing. In the cradle I can hear something tick/pop right before it craps itself. I checked the tubes and used three sets and all did the same thing.
With my bias meter on, the bias jumps about 15mA once the farting starts.

Where to start?

[Malcolm Irving]

I would start by running the amp with one or more tubes removed. This should help to narrow down where the rumble is coming from. For example, if it still rumbles with all the pre-amp tubes removed then the problem is in the power amp. After narrowing it down I would start to investigate power supply electrolytics (maybe one of them is breaking down intermittently).

[tubeswell]

Have you tried a fresh new set of output tubes?

[studiodunn]

Have you tried a fresh new set of output tubes?

I have tried 3 quads of known good tubes.

[Stevem]

One your poop out the PI tube if the issue is still there then there's only a hand full of things it could be, but having the bias voltage jumping around rules out any of the preamp tubes from the PI tube back up stream!

Unsolder the bias feed wire from the bias board, does the minus voltage still jump around?

If that checks then put in place new PI coupling caps as one or both could be leaking plate voltage into the grids ( pin 5 ) of the output tubes.

If these two things check I would be looking at bad output tubes and or a shorted output tube socket.

[Malcolm Irving]

I think any big impulsive noise signal coming through (even from the preamp) would make the bias voltage at the power tube grids jump around (same as any other signal does).

[studiodunn]

One your poop out the PI tube if the issue is still there then there's only a hand full of things it could be, but having the bias voltage jumping around rules out any of the preamp tubes from the PI tube back up stream!

Unsolder the bias feed wire from the bias board, does the minus voltage still jump around?

If that checks then put in place new PI coupling caps as one or both could be leaking plate voltage into the grids ( pin 5 ) of the output tubes.

If these two things check I would be looking at bad output tubes and or a shorted output tube socket.

Popped the PI and no more issues. I replaced it and all voltages are stable, no noise and I'm relieved. I would have sworn it was power stage related, but that's what I know.



Thanks you sir!