1977 Princeton Reverb Repair Part 2

Part 2

The missing bypass cap may have been intentional. The work was not subpar, it may have been a “fix” for the washed out reverb that happens on Fender amps above 4, generally speaking. I had never thought of that one, so there again, someone knew what they were doing, electronically speaking. I brought the entire pre amp section of the amp back to vintage specs.

I then moved on to the power supply cap which was in proper working order, so on to the power and rectifier tubes:

The Rectifier tube was an old Chinese made tube, that honestly, I was shocked it still functioned…The power tubes were generic Russian made and were WAY out of range from each other, like one was reading 28mA and the other was over 50mA…This would explain the “after it is on for an hour, it loses power and dynamics” issue. Fortunately, I had just received a new order of TAD 6V6 tubes…a box of 50, all matched within 3mA! The new Red Base TAD products are really impressive…The quality control going on over there is top shelf, and the tones are very vintage to my ears. Once those new tube were installed, everything came back to life on the old ’77.

Most of the pre amp tubes were older Sovtek short plates tubes, not the best tones I’ve heard from a new pre amp, but very reliable and they all tested good and were quiet, so no changes there. The fiberboard was warped (they all are basically) and had a big hump in it and only one screw “holding” it in the chassis, so I added a screw and locking nut to the other end of the board to secure it better.

After some de-oxit on the jacks and tube sockets, I hit all the pots with lubricant to assure a nice quiet sweep in those. She was just about ready to fire up, but I always use my current limiter to fire amps up, new, used or recently repaired, just in case there is an issue, it is at half voltage. This is also how I “form” the caps of a new amp, bringing up the voltage slowly so as not to hit a new unloaded cap with 400VDC right off the bat. All checked out in that department as well!

Once at full voltage, I checked the bias…I had swapped out the resistor in the bias supply as the old PR’s did not come with a Bias adjustment pot. This is one small change I made with the Lil King Amp, added a bias pot so it is much easier to dial in where you want to be on current, while not changing the tone or dynamics of the original design. Having noticed the current and DC voltage was so much higher on this amp when I fired it up originally, I changed the bias supply resistor from what is usually a 22k resistor to a 27k…I knew that the bias would be way too hot with a 22k. I was also hoping the DC voltage would come down a tad as well with the new rectifier, and it did. It was originally reading 440 VDC, which is what Deluxe Reverb amps usually read, whereas a Princeton Reverb is usually in the 400-425 VDC range. Bias is always a sticky talking point for many techs. If you go by the old manuals a set of 6V6 tubes in this amp should be set in the low 20’s…I prefer the upper 20’s and closer to 30mA per tube. This amp was pulling 32mA per tube at 420 volts so the change of the bias resistor and rectifier tube helped! Definitely on the hot side of things, but these TAD tubes can take a beating, almost the same way a JJ can. The JJ tubes are like tanks and you can basically run them at 440 VDC with few issues…I would say the TAD tubes are close to that, minus the filament rattle. I love the JJ tubes, but they rattle pretty badly on low notes. I use dampening rings to help with this on the JJ GZ34 rectifier I used. The TAD 6V6 were dead quiet and no rattle at all. I burned in the amp on the bench for a solid 24 hours to assure all was stable. When I do this, I will randomly plug in a guitar throughout the day to test the amp a few times, I also measured voltage and current several times.

The next day when I sat down at the bench, I found the amp to be set exactly where it was the evening before! All was ready to load back into the cabinet.

A word about these late 70’s Fender amps…the cabs are not great! In the old days you had the baffles made from higher quality plywood and you could remove them aka “floating baffles” These later versions still had pine shells in most cases, but the baffles and back panels where made from plain old particle board and the grill cloth was mounted on a thin frame and held to the baffled by Velcro. NOT my favorite design, but it works. They are also heavier…And honestly, a little stiff as the baffle doesn’t seem to have the same throw as an older plywood baffle. We use void free baltic birch for the Lil King and all other cabinets, in the period correct floating baffle style. To me this is a BIG part of the old tones, cab size and material plays a big role in the tone of an amp, and the weight!

Once I loaded it all in, I had a chance to try the amp out next to a recently completed Lil King with the stock 12” Alessandro Eminence speaker I use. I ran a speaker cable to both amps so I could run the ’77 into the Lil King’s speaker and the LK into the ’77’s vintage 10” speaker. I put some video below for you to discern the differences. I am pleased with how the LK sounded and very pleased with how much better the ’77 sounded.

This is NOT a competition, tone to me, is as subjective as looking at a painting or a sunset. It is what speaks to you as a player or listener. To my ears the PR is a little bright for my taste, but to others, that is what they love about a Silver-face! What do you think?

I met the customer to drop off the amp, and this rare repair job was complete! All reports back are that the amp sounded better than he had heard it and it was ready for some work on a new PBS soundtrack!

Thanks for reading this and please tell your friends about the blog if you think this will be of interest!

Wayne