When my friend Mark acquired this Fender Pro Reverb guitar amplifier, it had a number of serious problems, but it looked good and seemed to be a promising project.
"Silverface" Fender amps have a mixed reputation, as do models such as this one which use an ultra-linear output circuit to boost its power to an advertised 70 watts.
Most of the components in this amp had 1979 date codes, but a few had early 1980 codes. My best guess is that the amp was first sold sometime in 1980.
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Here is the Pro Reverb chassis removed from the cabinet. It features a "Normal" channel and a "Vibrato" channel, which also includes a spring reverb. The "Vibrato" channel is somewhat mis-named, since it actually varies the amplitude of the guitar signal and not the pitch.
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The electrolytic filter capacitors mounted on the top of the chassis typically go bad after several decades, and these were no exception.
This Pro Reverb uses the highest B+ voltage that I have ever encountered in a guitar amplifier, 500 volts DC according to the schematic.
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Here I've replaced all of the electrolytic filter capacitors.
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Unlike earlier Fender amplifiers which featured a bias adjustment potentiometer, this Pro Reverb and its related models were built with a "bias balance" pot.
Older bias adjustment circuits allowed adjustment of the absolute value of the bias voltage, thereby adjusting the plate current in the output tubes.
The "bias balance" circuit only allowed balancing the bias voltage supplied to the two 6L6GC output tubes. It was not possible to increase or decrease the value of the bias voltage. Today it is easy to obtain matched pairs of output tubes, for which a traditional bias adjustment is more useful.
As-found, the bias voltage allowed for a plate current that was too high for good tone or long tube life.
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