Ampeg SVT 3 pro snapping and popping sounds

[Bim]

It has had numerous problems in its lifetime but this is the newest. Usually after its been running maybe 2-3 sometimes 5 minutes it will start making snapping or popping sounds. I definitely know its in the preamp as I've fed the preamp into a seperate power amp (that doesnt have issues) and it does it. Ive swapped out all of te tubes for a brand new set of tubes and the problem remains. I haven't tried feeding another preamp into the Ampeg's amplifier section to see if it's just a preamp problem or if it affects the whole amp.

Anyone have an idea what the problem might be? I'll try to get into the amp in the next few days to see if it affects the power amp section also.

[Enzo]

Well please DO feed the power amp with something. You can just plug the guitar into the power amp in if you like. Your preamp is affected, but the problem could be in the power supply, which would affect both halves of the amp. Simple to find out.


Meanwhile, wherever it is, ball up your fist and whack the top of the amp hard. Does that trigger the noise or get any reaction. if whacking causes a response, then there is a loose connection inside somewhere.

Does turning ALL controls to zero stop the noise? ANY control that has ANY effect on the noises is after the source of the noise.

[jmaf]

Also knock all pots on the panel and turn them to see if any are scratchy or have become loose. As Enzo suspects, might be a mechanical problem. Normally pops come out of the power amp so if it's on the preamp the first suspect is some cold solder or mechanical failure.

[Bim]

Turning the controls to zero stops the noise. Pots were cleaned after it first started at a gig but the noise is still there. Smacking the amp doesn't add any more noise or stop it.

Pulling the amp and resoldering everything was my next choice after I plug something into the power amp input to try to isolate it.

[Bim]

Of course when it started my first response was to reach for the master volume on the Ampeg when I was running it as a whole amp and the volume on the power amp when I was using the Ampeg's preamp. I'll have to fire it up again soon to see if any of the other controls affect it.

[Bim]

I'm back!!! Just been too busy with other things to reply.
The problem appears to be strictly in the preamp. I did feed a signal into the power amp section and I got no snapping or popping from the amplifier section although the bias is so far off the output section does nothing but distort.
Pounding on the amp when it was making its noise did nothing to stop it and it didn't make noise when I pounced on it before it started acting up.
Looks like I'm going to have to get some chopsticks or small diameter wooden dowels to start poking and prodding around to see if there's a loose component although I could very easily pull out the pcbs and start reflowing everything.

[mozwell]

Im no expert, (and i stand to be corrected) but can you measure the DC rail voltages to see if the power supply to each preamp tube (or node on the power supply string) is stable when you get the clicks & pops.
Careful here, there are very high DC voltages which can kill you.

Check out the coupling caps between stages, if one of those was partially breaking down it could give you the problem you describe.
Also, try cleaning & re-tensioning the preamp valve sockets if you can

Can you measure the output of each valve, to see where the pops & clicks start from. It may be a preamp tube going weird. Do you have other tubes you can substitute (one at a time) to see if the problem goes away.

[Enzo]

How about the controls i mentioned, do ANY of them do ANYthing to the noise?

[Bim]

The noises are in the preamp only. Nothing is in the output section (the bias is so far off the output signal is totally distorted, which I need to reset).
When I pound the top of the amp the noises don't go away but there aren't any noises added to it.
I need to adjust each control individually to see which one affects it, but I haven't done it yet.
I also need to get some small diameter dowels and start probing around to see if there are any loose components, but to me it's not acting like a loose component, given that it takes a few minutes to start acting up.

[Bim]

So far just the master volume. I'll play with it again today and see if any of the others do anything.

[Bim]

I've already replaced all the tubes with known good ones. The problem remains.

[Enzo]

It is still basic troubleshooting: isolate the problem. It isn't about "finding a bad part", it is all about "find WHERE the problem is". Then when we know that, we can decide if it is a bad part or a bad connection or contamination on the circuit board, or whatever.

If we find the master turns the noise down, the problem is before the master, and if we find the gain control has no effect on it, then it is after the gain control. If EQ controls affect the tone of the noise, they are after the source.We can use techniques like grounding the signal path at various points to see if it affects the noise. When DC is present like on a triode plate, we can add a cap to our grounding lead.

You can also knock together a signal tracer using some other amp in your shop to monitor the signal path step by steep, listening for the noise. Or use a scope in the same way.

[Bim]

It didn't act up at all this afternoon, and I had it running into another power amp like i usually do since i cant trust the amp in the Ampeg. It was running for a good 3 hours with a signal injected into it. The difference this time is that I didn't have the cover on the amp, but another amp in a rack was sitting on top of it. To me it seems like there might be a flex in the PCB from the rubber piece that holds the tubes in place. Ill try removig the rubber piece and screw the lid back down and see if that makes a difference.

[52 Bill]

To me it seems like there might be a flex in the PCB from the rubber piece that holds the tubes in place. Ill try removig the rubber piece and screw the lid back down and see if that makes a difference.

If you want to test to see if closing the top will cause a problem from heat, just place the cover on upside down and wait.

If you are removing the foam to diagnose a bad connection problem, then just press down on the pc board with a wooden dowel rod and see if you can make the problem happen.

[xtian]

I ALSO have an SVT-3 PRO on the bench. First issue was easy--IEC neutral pin came unsoldered. I'll have to remove the damn PCB to get to the issue.

Other issue is, just like the OP, a nasty crackling sound. This issue is only when the "TUBE GAIN" pot is clockwise. When crackling, tapping lightly on the 12AU7 seems to affect the issue, BUT, when TUBE GAIN is set to 0, no cracking, AND tapping/wiggling/manhandling the 12AU7 causes no issue! That's weird. The 12AU7 appears to be a current driver for the power amp? Anyway, the schematics I've found (Index of /~kmg/files/schematics/ampeg/) do not show the TUBE GAIN circuit at all. I can't find it.

Anyone have a schematic that shows the TUBE GAIN circuit?