Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Hello everyone! I have an amazing Sunn concert controller pa, but it has recently developed an unsettling problem. When first powered on, the amp usually works fine. However, usually after a few minutes of playing through it (with any instrument,) the amp begins to produce a squealing feedback. This issue will occur even at the lowest volumes and continues even if the instrument is unplugged from the amp.

I've messed around with the amp and im fairly positive that the issue is in the preamp. The issue persists on all of the channels which leads me to believe that it is something to do with the master eq section.

Does this sound like anything you've ever heard of before?
Thanks for your time

Turn ALL the reverb controls to zero.  ANy help?

Reverb didnt change anything. I opened it up again and when i closed it back up and powered it on, it started working fine again. There is a decently loud distorted hissy sound at lower volumes though. it has been there, but it seems worse.

I have no way of knowing if your hiss is more than normal, but we can isolate it.

MY gut reaction is that in poking around inside, some loose connection wound up restored.  Get a wooden chopstick and with the amp running, poke at the parts on the boards to see if anything reacts.  ANything that does probably needs to be resoldered.

SO there is a hiss.  Do ANY of the controls affect that hiss in ANY way?  In other words is it present even with ALL controls at zero?  Or does it only come up when the master volume is turned up some?  Or maybe an input level has to be up from zero.  Or perhaps the EQ or tone controls affect the tone of the hiss.   ALl those help define wher in the circuit the hiss is coming from.

Ok so it seems the problem is connected to the reverb section of the amp. However it has just taken to creating a very loud oscillation upon turning on :( If any of you have any suggestions as to what may be wrong or how to fix it please let me know.
At this point though, I may just need to take it to a professional repairman.