Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ElPedro on May 13, 2017, 09:33:22 am
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Hey all. Having trouble with my Fender Twin. The Vibrato takes a little bit to engage after I turn it on. Once it has come on, I can turn it on and off and it works fine.
When I first turn it on, I can hear the vibrato thumping, but the guitar signal isn't getting through to the vibrato.
I'll try swapping the tube, but are there any other suggestions?
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Which model twin are you talking about? There's a big difference in the vibrato circuits of different models.
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Sorry...
It's an ab763. Not a reissue.
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Replace the tube first. Then replace the roach (optocoupler).
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> Vibrato takes a little bit to engage after I turn it on
Slow-to-start is more likely tube or cathode cap.
It *could* be that once it comes on, it holds a charge for a while so then it goes off/on quickly, but not if it has been dead a while.
Without looking, if this model has a kick-start (negative bias when off), that could have failed, but unlikely.
I would rate opto-coupler low on a list of slow-to-start reasons, but optos do have some strange failures.
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Thank you for your thoughts and ideas. I will try the tube and cap tomorrow. This is a kickstart model - I remember the pin on the back of the rca jack read somewhere around -55vdc when I checked it.
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> Vibrato takes a little bit to engage after I turn it on
Slow-to-start is more likely tube or cathode cap.
It *could* be that once it comes on, it holds a charge for a while so then it goes off/on quickly, but not if it has been dead a while.
Without looking, if this model has a kick-start (negative bias when off), that could have failed, but unlikely.
I would rate opto-coupler low on a list of slow-to-start reasons, but optos do have some strange failures.
+1
if new tube doesn't do the trick, replace cathode cap with a 47uF 25V.
--pete
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Would a 50u 50v work ok?
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yes
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Thanks, all. It was the roach.
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So Sluckey was right.