Peavey Classic 30, power tubes shut off, HELP!

[sixstringer]

I have a Peavey Classic 30, after it's on for a few hours, the power tubes shut off as if the heater circuit if off. This started a couple of days ago.
The volume slowly fades as the tubes cool. The tubes aren't making any funny noises and there is clicks, pops or rattles when moving the tubes in the sockets. The tubes are pretty new JJ EL-84s (maybe 30-40 hours). After gently moving the tubes around in the sockets, the power eventually comes back to the power tubes and the amps works fine again, until the next time it happens all over again. The pre-amp tubes stay on the whole time. It's as if 6.3v circuit is disconnected from the power tubes while leaving the pre-amp portion of the circuit on. Weird.
As the Classic 30 is not very repair friendly, I'm reluctant to open it up if it's going to turn out to be a bad tube or something like that.
Anyone have a clue as to why this is happening?
Thanks in advance for your help.

[xtian]

This could be a heat related issue: tube socket pins or traces expand until they fail, then work again when they cool.

[tubeswell]

The heaters in a PV C30 are DC and in series. So if one tube goes they all go. Could be a filter cap in the heater's DC supply is crapping out. You really need to take some DC voltages and that means pulling the beastie apart. You have my sympathy. Take care with all those damn fiddly jumper leads. There should be a C30 schematic and layout on-line @ Steve Ahola's website here: http://www.blueguitar.org/

[sixstringer]

Thanks for your replies.
I pulled the chassis and checked out the schematic. Looks like the power tube heater circuit is called ACFIL1 and ACFIL2, those tap off the PT before the SS rectifier, go through a 3 ohm/5 watt resistor on each side and then right to the # 4 pins of the EL-84s. The resistors look ok. The other side of the circuit goes through a 1.1pf cap to ground. The circuit is so direct and simple I don't see where it could fail unless one of the sockets has a cracked solder joint or one of the resistors has a bad solder joint on the PC board (similar to what you see on the Hot Rod series Fender amps sometimes). What else could it be?
I'll use a chop stick to check the joints.

[tubeswell]

Looks like the power tube heater circuit is called ACFIL1 and ACFIL2,

Duh you're right. Its been a couple of years since I looked up that schematic! Its the pre-amp tubes that have the DC heaters in that amp. The output tubes are AC series. What was I thinking?

The rating of that C30 PT heater winding is about 27 or 28VAC@1A IIRC (allowing for the load required for the 3 preamp tubes which are DC series and the voltage drops of the various resistors). I guess the High Tension winding is about 300-0 @ 180mA for good measure

[Gibsonman63]

I fixed a Classic 50 for a buddy earlier this year. Same issue, the series heater circuit was opening up. It took a bit of time to pull the circuit board out and carefully unfold it. Nothing obvious, so I reflowed all of the solder joints on the tube sockets and on all of the jumpers between the three boards. Total PITA, but it got the job done.

Good luck.

[Firestorm]

Similar symptoms to a Chinese Epiphone I heard about. I think the consensus was that the PT was thermally protected and when it got warm, the internal circuit breaker would open up.

[sixstringer]

A cracked solder joint makes the most sense to me, if there was at thermal switch, woundn't the whole PT shut down, not just that one tap? I'll check it out today. Thanks again for your help.

[Deric]

I worked on 2 different Classic 30's recently. Both had the same problem: pilot light was on but no sound. In both cases it was the heater circuit. One had a bad solder joint on one of the jumpers between the boards, the other had a broken jumper between the boards.

It seemed odd (maybe not) that it was the same jumper on both amps.

Good luck!

[Tonegeek]

whats a fair charge (hours and/or $) to dissassemble and reassemble one of these? I am thinking 1.5 hours which factors in some broken jumpers when you refold the chassis. I am starting up an amp repair biz and the C-30 seems like it would be hard to get paid what you need to make it worthwhile to the customer to have any serious work done. They are what, $350 used?

[Gibsonman63]

I would say an hour and a half at a minimum, but after doing one of them I think I could get the next one apart a little faster than the last one. Once you get it back together, I would leave it on for several hours, then play it hard for a while.

[Zippy]

After gently moving the tubes around in the sockets, the power eventually comes back to the power tubes and the amps works fine again, until the next time it happens all over again.

Need to sort out whether it is time/temperature based or whether moving the tubes is the clue. If it's time dependent, you may be letting it cool enough to reset a thermal breaker. If it really is moving the tubes around, look for trace issues, solder joints, or weak tension in the sockets.

[sixstringer]

Turns out it was a cracked solder joint on one of the jumpers between the boards. I went through the rest of the power side heater circuit and re-melted the joints for good measure. So... looks likes it good to go, I forgot how good this amp sounds, I did Steve's mods a few years ago on it and then sold the amp. Removing the chassis was easier then I remembered. I'm going play out with it this weekend before returning it to the owner (just to be sure it all good ) Thanks everyone for your help.

[Gibsonman63]

Those amps are a lot of bang for the buck. I didn't want to like the one I fixed, but it sounded pretty good to me.