K & L Tube Works : 1973 Fender Princeton Reverb

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.

Front view
Chassis before:
Chassis view, before Chassis view, before 		Chassis after:
Chassis view, after Chassis view, after
Front view:
Front view
Back view:
Back view
 Tube chart:
Tube chart
Serial number:
Serial number
Date stamp:
Date stamp
Power cord warning label:
Power plug
Bias circuit:
Bias circuit
Princeton Reverb AA1164 Schematic Princeton Reverb AA1164 Layout

Work performed (August-December 2013):

Replaced power supply electrolytic capacitors (the 'Cap Can').
Replaced all cathode bypass electrolytic capacitors.
Replaced all signal chain coupling capacitors.
Added 'Mojo Knob' (25k Ohm middle tone control).
Added adjustable output tube bias circuit.
Replaced power cord, corrected wiring of courtesy outlet.
Corrected out-of-phase tube socket filament heater wiring errors.
Replaced pre-amp tube 100k Ohm plate load resistors with 1W metal resistors to reduce noise.
Replaced damaged 47 Ohm resistor in negative feedback circuit.
Replaced mismatched output tubes with pair of Electro-Harmonix 6V6 EH tubes matched @ 30mA.
Replaced bad 7025 V1 preamp tube with 12AX7.
Cleaned chassis, faceplate, cabinet, speaker, potentiometers, and tube sockets.

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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