Amp Hiss

David Mullis · Post by **David Mullis** » 8 May 2000 4:55 pm

One of my 2 Christmas Tree amps has a bad hiss in it. It only does it when plugged into the Profex II, either by itself or with the other amp plugged in. The other Amp does not do this. I tried just plugging my acoustic straight in, no Profex, no hiss. Is it the Profex? I'm guessing not since the other amp doesn't have this problem. I talked to a guy today, who told me to check a few things and call him back. He suggested maybe the filter caps in the amp?
I'll call him back tomorrow, but in the meanwhile, does anyone have any ideas?

Thanks Fellers
David

Mike Brown · Post by **Mike Brown** » 9 May 2000 6:03 am

I would say that it was one of your Christmas tree amps since one does it and the other one doesn't.

David Mullis · Post by **David Mullis** » 9 May 2000 6:54 am

That's what I'm thinking Mike. It doesn't do this if I plug the guitar straight in to the amp. Maybe it's just something with the Profex that brings it out. I'm going to approach in as a problem with the amp. Does anybody know if filter caps would cause something like this?

Thanks
David

Donny Hinson · Post by **Donny Hinson** » 9 May 2000 7:31 am

Filter caps usually don't cause a hiss problem, they usually hum. Hiss problems are usually traced to transistors with a lot of "leakage". But normally, a "noisy" transistor is noisy when it's new. It's not a problem that gets worse with age. A lot of gain at the higher frequencies (due to component "drift") might exacerbate the problem, though.

David Mullis · Post by **David Mullis** » 12 May 2000 6:23 am

Thanks for the info guys. I'm probably going to do one of two things. Take it to my local electronics guy, or just wait till I get back up towards Nashville and drop it off at Benchcraft. I've got a pretty good idea that it's one of the FET's in the first stage, but I don't know how to check these and I'm too chicken to just start replacing components.

IF I can find the correct FET's here local, I may go ahead and replace those, other than that, I'll probably leave it to the Pro's.

David Mullis · Post by **David Mullis** » 15 May 2000 10:48 am

Well, in case anyone wants to know, I replaced all of the FET's in the 1st stage of the preamp and this hiss got alot quieter. I noticed the volume pot was a little noisey so I hit it with some electronics cleaner and the hiss went away completely. Of course after all the cleaner dried it came back. Guess I'll replace the volume pot next.