Peavey Classic 20 - need a "diagnosis"

[Snowdog]

So, I was playing my Classic 20 last night and stopped for a few seconds to change a cd. Picked up the guitar again and had no sound. The power light was sill on. Checked my cord first, that was fine. Then I check the back of the amp and my 2 power tubes were out, but the 2 pre-amp tubes were still glowing. So, I turn off the amp, let it sit for a few, and I replace the power tubes (have had a spare set for about a year.).

When I turned the amp back on the power tubes lit up, started glowing orange as usual, then they just died out. Turned the amp off, then back on, and the same thing happened. I let the amp sit for a few minutes, and switched AC outlets. Turn the amp back on, tubes glow orange again, I am thinking all is fine, and then I watch them "sag" and go back out again.

Now, all along the time this was happening, the 2 12AX7's were still glowing, though not as bright as the should.

As of this moment, the amp is "dead" as in the power tubes will not even light

So - what do you guys think might be the cause of this? Transformer? There is a separate transformer for the pre and power amp sections.

Any help would be appreciated.

[zenitB]

I think that amp has EL84 tubes in the output. If so, they require 6.3v on the filaments.
Your power supply is probably a transformer with several taps, one for the 12v tubes and another for the 6.3v ones. So I would suspect the transformer though there is a chance that your amp is running DC on the filaments through some sort of SS diodes and they are failing. Just a guess.

[CA_Dan]

I would measure your voltages starting at the power supply. Could be the PT, a dropping resistor, a shorting electrolytic, or as mentioned above a rectifier shorting partially.

[LowellH]

That amp uses AC for the EL84 heaters and DC for the preamp heaters, both from the same winding. Most likely there is a cold solder joint on one of the solid leads connecting the 3 boards together. Follow the heater winding (yellow wires) to the board and trace them all the way to the EL84s. Most likely the bad joint will be visible.

[Prattacaster]

+1 on the heater connections/solder joints. Without the heaters the tubes will not work or light up. Also if the preamp has DC heaters you might want to reflow the solder joints on the bridge rectifer as well.

[Snowdog]

Thanks very much for the insight. I have to be honest, I have a very limited knowledge of electronic repairs, but enough to understand what you guys are talking about. No way I could repair this myself, though, but at least I know what to expect from an amp tech.