Need Help - Twin Reverb

David Ward · From: White Rock, BC,Canada

I have a fairly new Twin Reverb Re-issue. Today, it started to make this high pitched hum (I held my tuner up the it, and it was a
B natural - if that helps). The hum is connected to the reverb in some way, as it only does it with the reverb on. It will do it with no instrument plugged in, as well as with a load. The volume control has no effect on the loudness. If I tap the amp, it will go away for a little while and then slowly start to come back and get louder.

To complicate the issue, it is also making
a 'static' type noise that sounds very simular to a dirty pot being turned. Again,
this is regardless of whether there is anything plugged in or not, as well as not being effected by the volume control.

I can certainly take it back for warranty, but if it's something minor, I would prefer to do it myself.

I would appreciate any suggestions.

I did check all of the tubes/connections.

Thanks,

David

Gene Jones · From: Oklahoma City, OK USA, (deceased)

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[This message was edited by Gene Jones on 06 May 2002 at 09:45 AM.]

Donny Hinson · From: Glen Burnie, Md. U.S.A.

To me, it sounds like one of your output filter caps is shot. This will often cause hum, squeals, and "motorboating" sounds. In the Twin Reverb, there are 3 or 4 capacitors in the power supply that are rated about 20mfd. at 500V. One of these may be going bad, and this allows "feedback loops" to be created, which is what causes all those weird sounds. Best bet is to take it back to the shop if it's under warranty (since any repairs you make yourself might void said warranty). If it's out of warranty, and you want to fix it yourself, I would just replace all the bypass caps at once. They're not too expensive, and chances are that if one is bad...soon another will crap out too.

J Fletcher · From: London,Ont,Canada

Sounds to me like your problem is one of two things. Either the reverb springs are feeding back on themselves, or the reverb recovery tube is too microphonic. The rev recovery tube is the 4th tube from the right while looking at the back of the amp. Should be a 12AX7 or a 7025. Try a different tube in this spot. Switch this tube with the 1st one on the right, and see if that cures the problem...Jerry

Tele · From: Andy W. - Wolfenbuettel, Germany

David,

that hum you mentioned must be a a bad tube in the reverb circuit, while the static noise sounds like a bad cap. TAKE IT BACK TO THE SHOP ....strange, that a new amp has that failure so early.

Andy

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Len Amaral · From: Rehoboth,MA 02769

The reverb tube should be a 12AT7 and not a 12AX7a. A 12AX7a tube will work in the reverb circuit but the signal will be too hot and the reverb will sound boingy. The driver tube is also a 12AT7 (the first preamp tube nearest the power tubes) and you can swap that one in place of the reverb tube to see if it is the problem. Hope this helps?

Jay Ganz · From: Out Behind The Barn

Your won't usually have a bad filter cap
in a relatively new amp. Besides, you
say it's only when you turn the reverb
up. Did you check the cables leading
down to the reverb spring unit or
where they plug into the circuit board?
Also, in general, those amps are notorious
for bad solder joints. Depending on where
they are in the circuit will determine
what kind of problem your amp will experience. That's the problem with
those reissue amps. You have to take them
completely apart to get to the underside
of the darn printed circuit board!
And, of course when it's apart you can't
play thru it while you're probing around
trying to find the bad solder connection!

J Fletcher · From: London,Ont,Canada

Len
The reverb recovery tube is a 12AX7, and is prone to be noticeable microphonic as it is in a very high gain spot in the circuit. It amplifies the very small signal coming from the rev tank. They will often "whistle" as the rev control is turned up.
The rev driver tube is a 12AT7, but doesn't usually have problems with being microphonic. It has it's own set of problems...Jerry

David Ward · From: White Rock, BC,Canada

I want to thank everyone for your input.
It ended up being a 12AT7 tube that was
causing the problem and I have it fixed up
now.

It's certainly nice having this resource to
address problems through. Thanks again.

David

J Fletcher · From: London,Ont,Canada

Me...wrong again. Glad you got it sorted out...Jerry

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