RM20 feedback

[ricky]

I bought an RM20 recently and it has an issue that we can't figure out. It initially sounded like a microphonic tube....squealing and stuff, so I swapped out tubes and no changes. It doesn't matter whether there is a guitar plugged in or not. The tech thought to try lower gain tubes so I went to 12at7s. It changed the nature of the sound to more of a mid-range feedback. He then tried 12au7s and it still does it. It does not have to be turned up that loud in order to get the noise...basically if I hit the boost button and start turning up the boost gain the feedback will start in. The boost volume does not have to be up high to get the feedback. I can crank the preamp and get the sound also. I have tried three different modules in there so it is not the modules. Also swapped out power tubes and it is not them. My question is what can make a tube noise w/o it being the tubes? It seems to have something to do with gain.
any ideas?

 

[bnorton4]

You should contact Rob over at Jaded Faith. He probably has some ideas as to what it could be.

 

[ricky]

He didn't want to guess with out having the amp on his bench. it is a curious thing...since posting this the really loud squeal/buzz has stopped. The tech took it out of the chassis to look at it and put it back w/o changing anything and then it worked for a while. Now there is some problem with it making a noise ONLY when the internal speaker is connected...like it vibrates the cab and then sounds like a loose tube. I don't want to have it shipped across the country and maybe there is a guy here in Montana that understands this MTS stuff.