Peavey C30 - Quiet then no sound.

[Shawnobi]

Hi guys,
Girlfriend had an issue with her Classic 30 last night. Apparently whilst playing it went quiet and stayed that way. Then no sound at all.
I fired it up today and the middle 12AX7 (V2) had no glow. I replaced it and replacement glowed but still no sound and all 4 power tubes won't glow. Before I grab the multimeter and tools later this week has anybody got any thoughts that may aid diagnosis? Will the blown tube have caused one of the fuses to blow? Or perhaps a bad power resistor?
Thanks.

[Jazz P Bass]

Yes a bad output tube could blow a fuse.
It is hard to see the heater on the 12AX7.
The output tube heaters are in series, so if one is bad, none will light.
Bad OP tubes have been known to take out the big power resistor.
Open it up.

[Enzo]

And the heater fuse is probably ceramic, so you cannot tell if it is blown without pulling it and testing with a meter.

[Shawnobi]

Hi Guys,
Just opened her up.
Pulled both the internal fuses and they check out ok. I also checked the R58 power resistor and that looks good. Bearing in mind I put a meter across it in situ and didn't lift a leg. Just because these are such a pig to work on. But I'm getting 388 Ohms so I'm fairly confident it is ok.
From what you have said Jazz, it's possible I could have a bad EL84 causing them all to not work? The 12AX7 could have also gone? Or maybe just a red herring but it appeared to have no light compared to the 2 either side.
Sadly I don't have a spare EL84 to swap around.

[Jazz P Bass]

You could try & measure the resistance of the heater pins of the EL84 tubes.
If one is open, there you go.
This won't tell if a plate is shorting to the heater (one failure mode)

[Shawnobi]

Thank you Jazz. I have a mate with a valve tester so paying him a visit tomorrow.

[Enzo]

Before that, look lower left in schematic, See R71, R72? 3 ohm 5w.

They are on the end of the middle board, near to the fuses. If one of those opens or breaks free, you lose power tube heaters.

Heaters do fail in tubes, but really not very often. I vote on those resistors.

[Shawnobi]

Before that, look lower left in schematic, See R71, R72? 3 ohm 5w.

They are on the end of the middle board, near to the fuses. If one of those opens or breaks free, you lose power tube heaters.

Heaters do fail in tubes, but really not very often. I vote on those resistors.

I see them. Will inspect tonight. Just tried ordering some from Peavey, but they are on back order until january. And can't seem to find an alternative stockist in the UK as yet

[Jazz P Bass]

If the resistors are at fault, try Farnell.
Your Search Results | Farnell United Kingdom | Results

[Shawnobi]

Update. Output tubes all checked out ok.
I've pulled R71 & R72. Both are good. I can also probe between them and their respective pins on V5 & V6 and see the 3OHM. So looks like the ACFil should be able to get to the tubes...

Had a poke around the circuit board and dabbed a few solder joints. Popped it back together with view to checking all the tube voltages. Fired it up on the off chance and it works! I can't say that fills Me with confidence. But hopefully I got a dry joint by luck.
Thanks for the help guys. I hope this thread won't get ressurected any time soon!

[Shawnobi]

I'm ressurecting this thread as there have been some other issues that may all have a single root cause. The initial problem in this thread I put down to a bad solder joint. My Girlfriend took it to rehearsal following this repair - at the end of the night it blew a fuse (F2). I fixed this and it sounded great. This week at rehearsal it had another problem right at the end of the night where by it started to make some horrid noises. Like crackling over the notes (Her description). She says it wasn't hum or distortion though. I was unable to replicate the fault at home. Sadly I can't use it at rehearsal volumes in the house. But I left it on for 4 hours and then it played fine. So without being able to replicate the fault I am struggling. It seems all 3 of these things have happened after she has been playing at volume for around 3 hours. (She uses channel 1 only at between 5-6, no louder, with only a RAT pedal plugged in the input). She tried different guitars/leads etc. And didn't have the problem when she switched to our Fender Silverface. Anybody have any thoughts on this, maybe it rings some bells with people? I'm tempted to just start with a full retube, though the existing ones haven't had a hard life and are only about 2 years old. The 12AX7's checked out fine when they were tested 2 weeks ago. But I guess F2 blowing could point to issues with the EL84's.
Cheers folks,
Shawn.

[Enzo]

The amp lights up? The tube heaters all lit? The large tubes all work as a group, and the small tubes all work as a group. If one tube in the group loses the heater, they all do. SO if one tube in a group lights, chances are they are all lit, even if they don;t lok it. SO, all tubes lit?

Sitting there does not replicate use. Use vibrates. Your problem is it lights up but loses sound, yet sometimes works. That tells me you have a bad connection or a failing tube. Pops and crackles and blown fuses are common signs of a failing power tube. DIm the lights in the room and watch the tubes. If you see any flashes inside the tubes when a noise occurs, that is one sign of failure.

Moving on, turn the reverb down, ball up your fist, and whack the top of the amp. Does that cause a noise? Does that wake the amp up? Does that put the amp to sleep? ANY reaction to whacking means a loose connection.

FLick each tube with your fingernail, like you were flicking a bug off your sleeve. Do any of them react? Particularly the power tubes? They should ignore flicking.

The amp is made with a three-section circuit board. The sections are joined by rows of short, bare-wire jumper wires. Power down, unplug from the mains, and discharge the power supplies. Now go down the rows of jumper wires and gently tug on each to see if any come free. If they do come free, they were already broken, you didn't just then break them. And that could well be the problem.

How long it sits there on, or how you play it really rarely matters, but 2 years is a pretty good life from EL84s for an amp that gets played regularly. I figure a gigging amp needs new ones once a year or even more often.

[Shawnobi]

Great tips Enzo. Thank you, I'll give them a go.

[Shawnobi]

OK, think I may have something. No noise from the punch test, but flicking power tube 2 gave a popping sound when flicked. None of the others did. I switched tubes 1 & 2 and the problem moved with it. So I guess that points at a failing tube? Something bad inside it.

[Jazz P Bass]

Tech Tip #44: The old 'flicking the tube' trick.