HOFFMAN PLEXI w/HOTSWITCH - WEIRD PROBLEMS

[guitarmyofone]

Hi all, thanks for the forum and thanks in advance for any advice to my amp problem. I built a plexi based amp from Doug Hoffman plans and parts. I recently installed the hotswitch mod which made it switchable from stock to cascaded preamp / master volume. Since then, I have played the amp for probably about 30-50 or maybe more, gigging, practicing, recording. Most recently, I am noticing an intermittent problem:
When i switch the standy to start playing, I hear a static sound, not unlike that you would hear from a tv on a with no reception. This starts out somewhat loud, but not overpowering, and gradually fades out over a period of about 10 seconds. If I play over the static, I can hear it fine, and the tone seems unaffected. Sometimes, after experiencing this, I adjust the post volume without effect, until I reach about 4 o'clock, at which point the volume seems to jump to full bore. The last time this happened, I opened it up and simply wiggled the tubes around a little, turned it back on and it was fine. The time before, I beat the amp on the top of the enclosure with my fist like a an old junk tv, and it remarkably worked! The problem is intermittent. I suspect there is a loose solder joint somewhere or possibly (and less likely) a defective pot. Would a bad cap explain the gradual 'discharge' of static over the initial period of approx 10 seconds? The weirdness of the symptoms is causing me to formulate crackpot theories and such. Any thoughts before I fire up the iron? thanks

[bigsmitty]

chopstick

[Structo]

Bad preamp tube can do that.

Also loose socket pins can be intermittent like that.

I suggest you pull the tubes, discharge the filter caps and re-tension the socket pins with a dental tool.

Also, I recommend you clean the sockets and tube pins with some Caig contact cleaner.

[guitarmyofone]

Thanks for the replies. This is a testament to the chopstick method: I opened the amp up and turned it on, the first thing I touched with the chopstick (a lug on my post volume pot) revealed a cold solder joint. I reflowed the joint and all the symptoms disappeared! Wouldn't it be nice if everything worked so easily?

[jaysg]

...the first thing I touched with the chopstick (a lug on my post volume pot) revealed a cold solder joint.

This is something I've always done wrong. For real manufacturing, you're supposed to make a good connection with the wire and lug, such that adding solder doesn't improve the connection, it just seals it. I've never built two of anything, so I'm making connections with the solder, which is bad form but a lot easier when I have to tear something out or re-do some bits.