Noise when standby on or powered off

[Thorny]

It is a Marshall JMP Master style circuit, pretty well a 2204. I have built several of these - all of them great. One of them makes a rather loud "eeerrrrp" noise that almost sounds like a siren when you switch on the standby or power down and the caps discharge. Sometimes it sounds like a tone generator that someone slowly turns the pitch higher as it discharges. I know that marshalls do this, I have heard it before. But this is particularly annoyingly loud on this one.

The amp sounds great while operating. No problems with it. It also does not do the thump that some amps do when you switch them on standby and sometimes you can put a cap across the standby switch to stop. This seems to be something different.

Anyone have an idea how to stop it or what could cause it?

[Thorny]

Fixed the problem. I did three things on this one, but I think fixing a ground fixed the problem. I noticed the output jacks were not grounded well to the chassis because the chassis is a painted chassis. It noticed when I jumpered it to a known good ground the background noise of the amp was quieter. So to took a dremel and removed the paint around the area and tightened the output jack with a lock washer back down - solved that one.

Earier I found a slightly loose wire, and resoldered - but I don't think that was the problem. I also found the voltages were slightly ugly - I was running over 450v on a 450v cap, so I moved from a 120v to the 125v AC input on the power tranny and it dropped the voltages just enough to give me a bit of comfort margin. I don't think that was the problem but it eventually would have been one. Now I have to tweak the bias cap so that I can lower the range of the bias voltage so the tubes will run hot enough.

Thought I would report back.