Horrible Noises from my Fender Prosonic

[markbt]

[cross-posted on other forums]

Hi All,

I have some nasty white noise and pops/bangs happening on my Prosonic when running with the solid state rectifier, and I'm hoping someone here will be able to help me figure out what to replace.

It's hard to describe so I put a video up on youtube: http://youtu.be/S1oYRBgbh98?t=1m

Skip to 1:00 and enjoy!

Sometimes it's a slower buildup than this - the noise might come in low and build over the course of a minute or so before a big pop comes and it goes quiet again. Output power doesn't seem to be affected.

The power tubes are all relatively new, and it doesn't happen when the amp is running in class A mode on the valve rectifier.

Any suggestions for what's going wrong here?

Thanks in advance for looking.

Cheers,
Mark

[tubeswell]

White noise and pops/bangs usually indicates bad output tubes. They might be new, but they could still be bad. Have you got another set you could chuck in to check?

[Stevem]

In solid state recto mode the output tubes are getting hit with some 30 to 40 more volts from the power supply and if they are flacky even though they are new that added voltage may be pushing one or both into the failure area!
I would pull out one output at a time, turn on the amp in SS recto mode and see if one or the other outputs acts up.
Also the switching section of the amp that takes it from SS to tube could have bad solder connections, or the switch section itself (contacks) could be pitted.
If you get a schematic and jump around the switch you can prove out if that's the issue.
Also when the amp is in fixed bias mode the outputs are seeing more voltage on there plates.