Twin Reverb Hum problem

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eanderso22193
Junior tube assistant
Posts: 94
(3/4/04 10:17 am)
 Twin Reverb Hum problem
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 I'm working on some sort of Twin Reverb...stamped with the number 10765 on the chassis and on the tube chart. Looks unlike any other Twin or Fender I've seen. Construction is crap on it, but anyway...
It has a bad hum in it that gets much worse when I turn the reverb up, even with the reverb tank not attached. I've pulled tubes the only one that affects the hum is the 12AT7 for the phase inverter. Tried a new tube there, same thing. I've checked all the components and they all seem to check out alright. The filter cap looks ok and reads ok, but might this still be the problem? Something else, maybe? I find it strange that the reverb level effects the hum but it doesn't seem to be coming from there.

Eric
 
Lucid Alice
Senior tube assistant
Posts: 172
(3/5/04 12:01 am)
 Re: Twin Reverb Hum problem
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 I just reviewed the schematic for the AA270 and there is a stage before the PI where the reverb return is mixed in. V3 is a 12AT7 on the reverb send. V4 is a 7025, 1st half is the reverb return, and 2nd half is the mix with the dry signal. Wouldn't it make sense that if turning up the reverb increases the hum that it is coming from that circuit? (In other words it is mixing more signal from the reverb return into the dry signal.)


 
eanderso22193
Junior tube assistant
Posts: 95
(3/5/04 9:02 am)
 Re: Twin Reverb Hum problem
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 I thought it might, but wouldn't pulling that 7025 kill the hum then, since its feeding the reverb signal back into the phase inverter? The only tube that affects the hum is the phase inverter 12AT7.

Eric
 
Lucid Alice
Senior tube assistant
Posts: 173
(3/5/04 12:24 pm)
 Re: Twin Reverb Hum problem
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 If it's old I would try new filter caps.
 
 
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