1967 Princeton Reverb Refurb

A 1967 Princeton Reverb came in last week. It sound weak, anemic. No strength in the bass, shallow tremolo, lots of background hum and noise. One of my modded Blues Juniors blew it into the weeds. The owner wanted the old tone, more clean headroom, proper tremolo, and a 3-prong outlet. He had already installed a 12-inch baffle and was running a JBL D-120 in there.I’m sure a lot of purists would cringe at what I did, but now it sounds absolutely huge.

The amp had the original .02μF ceramic disc coupling capacitors, an incredibly lame choice on Fender’s part, even back in 1967. The best of them read less than .015μF, so I didn’t feel bad about replacing them. I used film-and-foil Orange Drops, which are very rich and toneful.

It's important to form the leads so there's no stress on them or the epoxy shell

All of the caps in the tremolo oscillator and bias coupling read low too, so I replaced all of them with Orange Drops. Ceramic discs are actually fine for this application, but the ODs are very stable. There was an immediate improvement in tremolo speed range, depth, and stability.

The last tech had updated the output coupling caps, bypass caps, and tone stack caps. I left them alone. Checked the plate resistors for value and noise; they were fine. So the board is a colorful mix of new and old tech.

Axial caps had replaced worn-out cap can, but they're too small.

The original can cap had four 20μF stages; the tech had replaced them with four 16μF caps inside the chassis, neatly stacked, two on two. I replaced them with 47μF in the first stage and 22μF in the remaining stages. 47μF may be a little bit of a strain for the rectifier to fill when it’s cold, but it provides lots of current for good low bass and clean tone the rest of the time.

I also did the Stokes mod, which moves the plate supply for the phase inverter from the lowest supply voltage to the next higher one. This gives a bigger voltage swing in the PI, less compression, more headroom. It also lets the 6V6s generate a bit more power, all the more reason to have that 47μF cap feeding the output transformer.

The ’67 Princeton Reverb uses a small output transformer, the same size as found on a Blues Junior. The TO20 OT is significantly larger and heavier, with better materials (M6 grain-oriented steel core laminations) and much better clean tone and bass notes, although not as good at raspy grind. One of the screws for the circuit board interferes with the wider transformer, so I added a couple of fiber washers to bring the screw up flush with the other side of the chassis.

The amp had 47Ω filament balance resistors (virtual center tap) that weren’t very close in resistance. I replaced them with 100Ω 1 percent resistors for a much closer match and better hum rejection.

 

 

 

 

AC line voltage has gone up steadily over the decades. The amp doesn’t mind the higher plate voltages, but the bias  supply was not tracking accordingly. Also, the customer wanted to be able to switch between NOS and new production tubes, so I built an adjustable bias supply. The stock bias was very hot, not the sort of thing you would want to subject expensive NOS tubes to.

It’s functionally identical to the stock bias supply except that it adds the 25-turn trimpot (in the foreground) for fine bias adjustment.

The bias affects the tremolo in Princeton Reverbs because it’s a bias-vary tremolo. The hot bias gave very limited tremolo depth; the circuit couldn’t pull it down enough for good, throbby cutoff. Between the fresh capacitors in the tremolo circuit and the adjustable bias, the tremolo now works great–deep and powerful, with that swooshy tone that you only get from bias-vary tremolo.

Other mods included a 3-prong outlet and 470Ω screen resistors.

The results speak for themselves: great, commanding clean tone, louder, more dynamic, more responsive. The amp is quieter, with hum and hiss reduced. It’s ready for another 40 years of great tone.

2 Comments

  1. Tongo says:

    Hey Bill, I just built a ptp princeton 11a64 and used your tp20 OT .. it sounds fantastic. I am wondering if the amp could benefit from a presence control or if its even possible?

    Thanks!

    • bill says:

      The Princeton does have a negative feedback loop. The very simplest thing you could do would be to set up a 5K pot as a variable resistor and put in in place of the 2700 ohm resistor, which would make the NFB variable from “less” to “more.”

      The presence control in the Blues Junior and most other amps is actually a tone control on the NFB. If you dig through some other Fender circuits, you can find one, such as on the 5E4 and 5F4 Tweed Super schematics. Note that the NFB resistor is much higher, but there is a simple tone control to drain highs from the feedback loop.

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