Saturday, 21 January 2017

1968 Princeton Reverb Repairs

1968 Princeton Reverb Repairs — A reprised post from my old QRP HomeBuilder website

This somewhat rare 1968 silver face Princeton Reverb had aluminum trim surrounding the grill cloth. We verified its age by the output transformer code, the trim and by serial number. This particular amp ran a GZ34 rectifier in the rectifier tube socket.

Boy, it sounded terrible!  According to the owner, it 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 B+ chain resistor, and the speaker --- this amp sounded pleasant once again.

During physical examination and testing, I found the following problems:

  1. A 4 amp fuse was in the fuse socket
  2. Blown speaker
  3. There were 2 different brands of 6L6 tubes
  4. Loud hum, noise and microphonics in the AF chain
  5. The rectifier tube was audibly clinking and stinking
  6. Low AF output power
  7. The first 1K resistor on the B+ appeared to be temperature distorted
  8. I heard an approximately ~1 KHz intermittent oscillation

Above — Post CBS Princeton Reverb. The Princeton Reverb was essentially a poor man's Deluxe Reverb with a 10 inch speaker. Rated at  around 12 watts, it ran a much smaller output transformer and as a result, offered much less head room than the Deluxe Reverb. The tolex on this old amp looked in incredible shape. All it needed was a good cleaning.

Above — The look on our jazz cat's face says it all. This amp sounded terrible. He
demanded that I make repairs as soon as possible and return it to the owner.

Above — The chassis removed from the wood. The PR chassis proved very compact, lightweight and easy to work on.

Above — A view inside the chassis. This circuit and its schematic looked very much like the blackface pre-CBS version. A few newer, non-Fender parts gave evidence of previous repair(s). This can get scary. Fixing some else's bad repairs may open a Pandora's box. For this particular amp, the previous repairs seemed OK.

Above — The 4 main electrolytic B+ 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 it and measured 1K6 ohms.

Above — Removal of an old multi-section capacitor is never fun. I used an 80 watt soldering 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 I cleaned and buffed the local chassis area.

Above — The new multiple section capacitor. It's diameter measured 1 - 3/8 inches. This means obtaining a special part as modern cans are greater in diameter and won't fit in a vintage amp. I got this cap from Antique Electronic Supply. The number = C-EC20X4-475 and it's a 20/20/20/20 uF @ 475 VDC. Apparently they're manufactured on original Mallory equipment.

Above — 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, I palpated it immediately after the power was switched off and the high voltage was bled to ground. Nowadays, I measure temperature with an infrared thermometer.

Above — Replacing the bias capacitor removed the intermittent ~1 KHz oscillation and decreased hum. I only had an axial type on hand and it worked fine.

Above — The old and new 6V6 finals are shown. A matched set of Electro-Harmonix 6V6s were installed. I loved 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. Further, a new Sovtek rectifier tube replaced the clanking old rectifier tube. The other tubes seemed OK.

Above — 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. The old speaker cone and surround were cracked and separated. Further, the cone felt stiff + brittle --- and a new speaker proved the best way to go. Fender didn't put a great speaker in the PR to begin with.

Above —  A rear view of the final tested chassis back in the wood and connected to the new speaker.

Above —  The jazz cat now mesmerized by the sweet tone of the restored Princeton Reverb. The parts total ran about $200 and much of it arose from shipping/handling/duty and tax costs to Canada. The owner only wanted to spend a maximum of $200, so it felt good to come in on budget for a change.

Above — My bench back in 2008. I built and repaired many tube amps here.


Tube amplifiers operate at high DC voltages. Repairing, modifying or building tube amps can be dangerous, or in some cases, fatal.

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