Fender twin reverb farts out after switching to standby - diyAudio
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Old 2nd July 2008, 02:52 AM   #1
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Join Date: Aug 2006
Question Fender twin reverb farts out after switching to standby

Hello, I have a Fender silverface twin reverb circa' 1969, it started to make an strange kind of a fart out noise shortly after switching to standby mode having the amp on.

I visually inspected the power tubes and discovered that one of them had a blue haze, which I think it was not the problem, however a couple of days later and soon after I turned the amp on a loud buzzing noise was heard which became gradually louder and I noticed that both power tubes had the same blue haze but this time as the buzzing sound increased its volume the power tubes' glow became more blueish and almost the entire tubes were glowing blue, then I switched the amp off and checked the power tranny which I believe was hotter than normal.

Then I changed the offending pair of power tubes with a fresh set but the same thing happened, I noticed that if I turned the amp on on standby mode the blue glow wasn't there and until I defeated the standby switch the blue glow appeared on a pair of power tubes.

I checked the B+ and seem to be normal around 430V, checked the bias voltage and it was around 50V. About 6 months ago I recapped the power supply and it was working fine until now. The vibrato effect doesn't work as well, but my main concern right now is the power tube section problem, any advice will be appreciated, thanks in advance.
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Old 19th July 2008, 05:42 AM   #2
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Join Date: Jul 2008
Location: Orange County
I'm not an expert amp repair technician, but no one else has answered... (and I work with one), have you tried swapping you're preamp tubes yet? One of them should be driving your vibrato section... if there's something wrong in the preamp section of your amp it could be affecting the problem in the output tubes, hope this helps...

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Old 19th July 2008, 06:52 AM   #3
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checked the bias voltage and it was around 50V.
Is that +50V,or -50V? (it should be -negative!)
It sounds like it might have lost bias,hard to say.

Looks like there are some schematics at the bottom of this page:
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