Blue Plates...

[Jbutler9]

Hi, first time poster here...

I just pulled my 100w Plexi clone/build out of storage and noticed the power tube plates on just one side of the "push/pull" division (both sockets) have a slight blue glow which steadily gets brighter as I turn up the volumes. Swapping tubes doesn't have any effect so I'm thinking it's almost definitely a post PI/biasing issue... I realize a blue glow itself usually isn't a bad thing, but it seems a little strange that it's only happening on one side so I thought I'd ask some of the experts here what they thought it might be before I open up the amp. Hopefully it might be something obvious to one of the pros here.

Has anyone else here ever come across something like this?

[Leo_Gnardo]

Some blue glow is absolutely normal. "Lost" electrons that aren't in the action area between cathode & plate. The more current, the more glow. And it will blink on & off as you thwack big power chords thru your amp.

What is notable is one pair glows and the other not, and it stays on the same side when you swap the tubes around. That would indicate more bias current being drawn on one pair of the tubes compared to the other. Suspecting a possible leaky capacitor to the output grids, measure the bias voltage at the drive grids of the output tubes. If the voltage is higher one side & lower on the other, that's pointing toward leaky signal cap. And of course you can measure the bias current in any of the conventional ways.

[tubeswell]

Yup - blue glow on the plates is situation normal. All plates do this, but sometimes you see it more than other times (because of the ambient lighting in the room etc)

[Jbutler9]

Thanks, guys. I'll check that cap when I open it up later this week.

[Jbutler9]

Yup, it was a bad cap - replaced and no more blue... Of course now I have ground hum(s) to trace!