Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jimi113 on September 27, 2015, 01:41:19 pm
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Hello everyone! A Few days ago the V3 25uf 25V capacitor in my Fender super reverb AB763 shorted and blew up taking with it the 2.2K bias resistor... I have replaced the tube and of course both cap and resistor. Amps works but I get no volume unless volume and reverb knob are turned up... Not many parts in the reverb chain... All V2-V8 tube voltages are as they should be per my super reverb blackface amp schematics. this happens with or without reverb in connected. I think that possibly the tube oscillated and sent way more than 25 volts to the cap. So I suspect that the reverb transformer has crapped out also.
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A shorted cathode cap won't kill the cathode resistor.
A sick RT will not hurt the clean (un-reverb) signal.
In an AB763, the Volume and Reverb pots are not near each other signal-wise.
Can't say what is really wrong. More investigation needed. What *other* parts may be bad?
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I have changed V3,V4 nothing else left except a 1M to ground and 220K @ the reverb out rca jack??? I guess
I can check the 470K in series with reverb pot and 220K to ground further down the signal path? or the pot itself?
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Most likely the 2.2K resistor opened-up first allowing the voltage to rise on the 25uf cap until it exploded. Pretty exciting, but probably not related to the current problem.
If the 470K on the wiper of the reverb pot is shorted or nearly shorted, that would cause the symptoms.
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Thanks I will check all of the parts in the circuit. I just checked the reverb pot other than being just a little short of 100K ~95K @max and about 20 Ohms at min it seems okay? I will post when its fixed... It's always good to have a log of odd ball problems.
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Does the normal channel work?
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Yes the normal channel works fine...