Introduction
This somewhat rare 1968 silver face Princeton Reverb has aluminum trim surrounding the grill
cloth. We verified its age by the output transformer code, the trim and by serial
number. This particular amp had a GZ34 rectifier in the rectifier tube socket.
It sounded terrible. Apparently it had sat in a closet for 15 years, but prior to
that some teenagers had borrowed it to "rock it hard", but brought it back and stated
that it did not sound very good.
After replacing 4 tubes, the power supply capacitors, the bias capacitor, a resistor,
and the speaker, this amp sounded pleasant once again.
During physical examination and testing, I found the following problems:
- A 4 amp fuse was in in the fuse socket
- The speaker was blown
- There were 2 different brands of 6L6 tubes
- Loud hum, noise and microphonics
- The rectifier tube was audibly clinking
- Low output power
- The first 1K resistor on the B+ appeared to be temperature distorted
- An approximately 1 KHz intermittent oscillation was noted
Mouse over the images below to view a full size version.
The Princeton Reverb was essentially a poor man's Deluxe Reverb with a 10 inch speaker. Rated at around 12 watts, it has a much smaller output transformer and as a result, much less head room than the Deluxe Reverb. The tolex on this old amp is in incredible shape. All it needs is a good cleaning.
The look on the jazz cat's face says it all. This amp sounded terrible. He demanded that repairs be made as soon as possible.
The chassis is removed from the wood. The chassis is very compact, lightweight and easy to work on.
A view inside the chassis. This circuit and its schematic are very much "black-face" Fender.
The 4 main electrolytic power supply capacitors are housed inside 1 can which is referred to as a multiple section capacitor. This is the old multiple section capacitor. Note the heat disfigured 1K ohm resistor to the right of the capacitor. I removed and measured it at 1655 ohms.
Removal of an old multi-section capacitor is never fun. I used an 80 watt solder iron, solder wick and a flat screw driver to unlatch the anchoring tabs from the main chassis. After capacitor removal, the remaining solder was removed and the local chassis area was cleaned and buffed.
The multiple section capacitor diameter is 1 - 3/8 inches. This means
obtaining a special part as modern cans are a greater diameter. This cap was obtained from
Antique Electronic Supply
The part
number was C-EC20X4-475 and it is a 20/20/20/20 uF at 475 VDC. Apparently they
are manufactured on original Mallory equipment.
The new multi-section capacitor is soldered in. I replaced the disfigured 1K resistor with a NOS version. It ran cool to touch during testing. For safety purpose, it was palpated immediately after the power was switched off and the high voltage was bled to ground.
Replacing the bias capacitor removed the intermittent ~1 KHz oscillation and improved hum. I only had an axial type on hand.
The old and new finals are shown. A matched set of Electro-Harmonix 6V6s was installed. I love the sound of these Russian-built EH 6V6 offerings. I also put in a new, low-noise, Sovtek 12AX7a in the number 1 preamp tube socket. A new Sovtek rectifier tube replaced the clanking old rectifier tube.
The old and new 10 inch speakers. The speaker choice was made to suit the owner. He wanted maximum head room and plays only clean guitar. I chose a Fender 099-4810-004. This speaker is actually made by Eminence, a company I like and whose products I have used on other projects. The new speaker sounds great and most importantly, pleases the owner.
A rear view of the final tested chassis back in the wood and connected to the new speaker.
The jazz cat is now mesmerized by the sweet tone of the restored Princeton Reverb. The parts total was about $200 and much of it was from shipping/handling/tax costs to Canada. The owner only wanted to spend a maximum of $200, and it was good to come in on budget for a change.
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Warning: Tube amplifiers operate at high DC voltages. Repairing, modifying or building tube amps can be dangerous or in some cases fatal.
