Peavey Classic 30 static

[A1Sensei]

Hey, I could use some help from a one of you repair tech types.
I have an older Peavey classic 30 that is usually super reliable, but it has developed an annoying static noise. Pots are all clean. The first thing I tried was re-seating all of the tubes. That did not help.
It is not very loud and turning up the volume does not effect it. It is there on clean and gain channels. The reverb control has no effect on it, but the tone controls do. If I turn the treble up or down the static gets crispy or dull. If I put a patch cord in the effects loop it makes no difference, but if I just plug into the effects return it goes away.
So, the source of the noise is after the preamp and reverb, but before the eq and effects loop.
I solder well and am aware of capacitor safety issues. I have a basic multi meter. Can anyone guide me through finding the issue and fixing it?

Thanks in advance for any help!
Sensei

[Stephen1966]

Have you tried swapping the tubes out?

Describing a hiss, rather than a hum... Swapping the tubes out is the easiest and quickest to check. Not just reseating them, you understand, but replacing them.

I notice this is your first post so be sure to check out the safety advice dotted around this site. If you are familiar with internals of tube amps, then you will have some idea of the myriad ways you can kill yourself, but don't let it be said we didn't warn you.

If the hiss is DC in origin it could be any number of things and if you want to get to the heart of it yourself, instead of taking it to a qualified tech, you will need to start ruling out all the possibilities. One by one, painstakingly checking and rechecking.

If you are handy with the multimeter, and just replacing the tubes one by one doesn't do anything, then you might go around the circuit and check voltages.

When you post questions to us, try to break down your steps so that we can follow what you did. The schematic is not essential but can save a lot of time and confusion.

Peavey_classic-30.pdf

You can use the schematic to record your tests. Check voltages.

Check your capacitors are fully discharged before touching anything inside the chassis.

[maxkracht]

Those are a real pain to work on, pcbs are in an odd 3d U shape, so testing things is no fun. If tube swapping doesn't work, I would suspect bad solder joints before anything. Tube sockets, jacks, pots, and jumpers between boards are a good place to start. Just reflow anything that looks remotely suspect. Probably needs to be recapped while you are in there and it's not a bad idea to modify the power amp and give each el84 its own screen resistor. I think there are a couple low value resistors that tend to cook the board as well but I can't remember which ones off the top of my head.

[Stephen1966]

Those are a real pain to work on, pcbs are in an odd 3d U shape, so testing things is no fun. If tube swapping doesn't work, I would suspect bad solder joints before anything. Tube sockets, jacks, pots, and jumpers between boards are a good place to start. Just reflow anything that looks remotely suspect. Probably needs to be recapped while you are in there and it's not a bad idea to modify the power amp and give each el84 its own screen resistor. I think there are a couple low value resistors that tend to cook the board as well but I can't remember which ones off the top of my head.

Do these boards become conductive?

[maxkracht]

Do these boards become conductive?

Only the charred bits, as far as I'm aware.

[A1Sensei]

Have you tried swapping the tubes out?

Describing a hiss, rather than a hum... Swapping the tubes out is the easiest and quickest to check. Not just reseating them, you understand, but replacing them.

Thanks for the reply. I would not describe the sound as a hiss or hum. It is a more of a crackling sound, irregular and sometimes goes away completely for a few moments.
I know tubes are always a possible culprit, but these are quality tubes that are only a couple of months old. Before I buy another set do you have any other ideas of where and what I could check?

[A1Sensei]

Here is something new, before I went to try swapping out the preamp tubes one at a time with an extra I have, I turned the amp on to play a little. The crackle started immediately, then it went totally silent and passed no signal. I looked in back and the power tubes were all dark. After less than a minute, they lit back up and started working again. What is going on?

[maxkracht]

Here is something new, before I went to try swapping out the preamp tubes one at a time with an extra I have, I turned the amp on to play a little. The crackle started immediately, then it went totally silent and passed no signal. I looked in back and the power tubes were all dark. After less than a minute, they lit back up and started working again. What is going on?

Those are a real pain to work on, pcbs are in an odd 3d U shape, so testing things is no fun. If tube swapping doesn't work, I would suspect bad solder joints before anything. Tube sockets, jacks, pots, and jumpers between boards are a good place to start. Just reflow anything that looks remotely suspect. Probably needs to be recapped while you are in there and it's not a bad idea to modify the power amp and give each el84 its own screen resistor. I think there are a couple low value resistors that tend to cook the board as well but I can't remember which ones off the top of my head.

PCB mounted tube sockets aren't a great idea, especially on a single sided board, could also be jumpers between boards or a bad heater quick connect. Poking with a chopstick will confirm a mechanical issue and might point you toward the problem, but I don't think it's necessary in this instance. You don't want to open it more than once, so recap the thing while you are repairing the bad solder joints.

[A1Sensei]

So here is where I am at so far;

While the problem(s) are almost constant, the static sometimes goes away for a while, and the power tube shutdown only happens here and there, which may make finding the problem difficult.

The static is not a tube problem. If it was V1 the volume control would effect it. If it was V2 it would only be on the gain channel. I have replaced V3. If it was in the power tubes the treble control would not effect it, and it does. So the problem is somewhere on the board after the input gain stages but before the eq section. Right?

All of the power tubes going dark must be either a break in the circuit between the power transformer and the common tube power input or else on the other side, the common power tube ground, am I right? I should be able to find this I think, or at least just re-heat the solder connections.

The weird thing is both problems appeared at the same time. Yet they seem unrelated.

Where I really need help is finding the static issue. We know the area in the circuit where the problem is. What kind of component or circuit failure would cause such a sound (random crackling).

So where on the circuit board (which components) should I test and how?

[lonote]

I looked in back and the power tubes were all dark.

This seems like a problem.


If it were me, I would start here, looking for bad connections.

[Stephen1966]

So here is where I am at so far;

While the problem(s) are almost constant, the static sometimes goes away for a while, and the power tube shutdown only happens here and there, which may make finding the problem difficult.

The static is not a tube problem. If it was V1 the volume control would effect it. If it was V2 it would only be on the gain channel. I have replaced V3. If it was in the power tubes the treble control would not effect it, and it does. So the problem is somewhere on the board after the input gain stages but before the eq section. Right?

All of the power tubes going dark must be either a break in the circuit between the power transformer and the common tube power input or else on the other side, the common power tube ground, am I right? I should be able to find this I think, or at least just re-heat the solder connections.

The weird thing is both problems appeared at the same time. Yet they seem unrelated.

Where I really need help is finding the static issue. We know the area in the circuit where the problem is. What kind of component or circuit failure would cause such a sound (random crackling).

So where on the circuit board (which components) should I test and how?

You have a major connection problem if the tubes are going "dark". The heaters are what give tubes their glow so chopstick the heater supply.

If it comes and goes, it's not a fuse.

If it's a power issue it could come from any of the power connections to the primary side of the transformer or from the secondary side to junctions and traces on the board. Start with the mains power cord and work to the power transformer, then from the transformer to the heaters.

Chopstick everything when it's working. Old solder, heat damaged traces. Isolate the issues and fix as you go along.

[maxkracht]

Seems like nothing I am saying is getting through, but I will try one more time. The way the boards in that amp are arranged make testing very difficult, probably near impossible to do safely if you aren't experienced, so don't bother, just open it and start soldering. Even chopsticking might be a waste of time due to the board arrangement. Your problem(s) are most likely a bad solder connection, and that connection is most likely related to the heater. The heater will not shut everything off instantly, tubes stay warm for a bit with no power. I suspect the static is losing and gaining the connection again before the tubes have time to cool, could also be another connection failing at the same time.

You need to open the amp, resolder anything that might have mechanical stress (I already pointed out those locations). Check for burned spots, scrape out the burned stuff, replace burned components. I think the low value resistors I previously mentioned might be in the heater circuit, so there's another thing to look for. Those need to be elevated for air circulation and given appropriate strain relief so the solder joints don't snap.

[bepone]

one more word too, to work on this amp is PITA, and there are many many problems with those pcbs..

[Stevem]

You have failing and or bad solder connections on the tube sockets, as such is common with this amp, and I would say at least one bad plate load resistor.

And yes, these amps are a flat out pain to work on!

[A1Sensei]

You have failing and or bad solder connections on the tube sockets, as such is common with this amp, and I would say at least one bad plate load resistor.

If it were a solder connection on the tube sockets wouldn’t it just effect the tube in that socket? All of the power tubes went dark and came back on together.

Is the plate resistor what you feel is causing the static?