Troubleshooting help needed-VHT amp

[pine]

I have a VHT Super 30, which when I play it for a few minutes or so, starts changing volume; I don't mean it slowly gets louder and softer, it's as if I was stepping on a boost switch, very abrupt, large changes in volume. I have so far cleaned all the input and effects loop jacks with de-oxit, and the tubes test ok, though I ordered new power tubes. This is a PCB amp so I am having trouble finding the problem. You guys have any suggestions?

[ricach]

Did you clean the volume pot as well? Does this occur with the volume pot set at different levels?

[pine]

I didn't try that; the pot is not scratchy sounding. The amp is so naturally loud the volume is always set low, I'll experiment, good call.

[pine]

Did you clean the volume pot as well? Does this occur with the volume pot set at different levels?

Well, I cleaned both volume pots, and the same problem persists. It also occurs regardless of the channel I'm in or whether I use the low or high input.
A tech has tried unsuccessfully to fix this. Are there any other things I could try short of sending the amp back to VHT?

[52 Bill]

Do you know what the tech checked when he had it? Is the volume change random or cyclical? Does giving the amp a good Enzo whack cause the problem to come or go?

Volume changes can be caused by so many things, that it's hard to know where to start. If you check the supply voltages, do they change with the volume drop?

[Rick Erickson]

A tech has tried unsuccessfully to fix this. Are there any other things I could try

You could try another tech. Ask the local music shops and/or local pro musicians who does the best work in your town, and chose one based on their reputation. If I can't find a problem like this in an amp - I start questioning the customer about their other equipment: pedals, guitars, cables etc.. If necessary I have them bring in everything and demonstrate the problem to me so I can locate the cause. Does this amp have an effects loop?

RE

[drewl]

Are the tubes mounted to a pc board?
I had one with a few intermittant pc board connections that was fixed by simply reflowing the bad joints.

[Enzo]

Isolate the problem.

When it does this, send the signal from the effects send to another amp to monitor it. Does this signal also vary? If so, the problem is in the preamp, if not the problem is in the power amp. Likewise, when it happens, plug a signal, the guitar even, into the effects return jack. This feeds the power amp. If it still does it, then the problem is in the power amp.

Send a steady signal into the front, now scope or otherwise monitor the signal stage by stage through the amp looking to see where it no longer remains stable.

Note that so far whether it is built on a pc board or hard wired has not come up. Pins 1 and 6 of a 12AX7 are still pins 1 and 6 regardless if they are soldered to a board or wired to a terminal strip. Troubleshooting is universal.