This is a nice, clean example of a 'silverface' Fender Princeton Reverb produced by Fender Musical Instruments from 1968-1981. These amplifiers were preceded by the nearly indentical but more collectible 'blackface' model produced from 1964-1967. They used a pair of 6V6 output tubes to produce roughly 12 Watts through an 8-inch speaker, usually an Oxford or CTS model.
This particular amp, serial number A28864, was equiped with an Oxford speaker and was date-stamped APR 13 1973 inside the chassis near the power supply transformer.
The owner stated that the amplifier sounded good for a short time after it was first turned on, but soon stopped working correctly.
We subsequently found several problems that explained this behavior, including a bad V1 preamp tube, mismatched power tubes, and a classic example of 'tweed disease': a warped circuit card that, in this case, had broken the 47 Ohm resistor in the output section's negative feedback circuit.
Besides repairing the items mentioned above, there are other issues with any older vacuum tube amplifier that require attention. For example, most amps this age need all of their electrolytic capacitors replaced. CBS used the typical cheap ceramic disc and blue poly coupling caps seen in silverfaces, so we decided to change these, too, upgrading to silver mica and 'Orange drop' units as appropriate. Despite a quite explicit warning on a hang-tag, someone clipped the ground lug off the power cable. So we replaced it with a new 3-conductor molded plug, retaining the hang-tag warning label to ward off future users from repeating this folly. We replaced the preamp plate load 100k Ohm resistors with metal values to help reduce noise. We made a few minimal changes to restore the circuit as much as possible to the 'blackface' version. In particular, we removed the 2000 pF capacitor strapped across the 220k Ohm input resistor of the reverb return amp.
Work performed (August-December 2013):
Any copyrighted material on this page is the property of its respective copyright holder and full acknowledgement is made thereto by the authors. Vacuum tube amplifiers contain very dangerous high voltages and can kill you. Do not attempt to build or modify these devices unless you are a qualified electronics technician. The authors are not responsible for any damages or harm that may ensue if you attempt to build or modify the device described here.
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