Princeton Reverb

I haven’t released any mod kits yet for the Princeton Reverb reissue, but these wonderful little amps suffer from a common problem: cabinet resonance and vibration on the low notes.

Most often the problem is the baffle. The easiest fix is to get some heavy aluminum angle stock from a hardware or home improvement store and screw it across the top of the baffle on the inside. But the baffle board is made of medium-density fiberboard (MDF), which is not noted for holding screws particularly well. So I came up with a slightly different solution: I routed a 1/8 inch slot about 3/16 inch deep into the baffle and epoxied a piece of 1/8 by 1 inch aluminum into the slot.

Here’s what it looks like:

Stiffener

You get far more stiffness from height than width, and the result is exactly what you’d expect: It. doesn’t. move.

Before adding the bar, the panel would vibrate visibly and resonated loudly on low E through F#. Now it’s rock-solid. If you have a PRRI and a router, this is a relatively quick and easy fix. You need to set the router height from a pair of spacer bars and keep the router base off the baffle itself. otherwise it will lift up when it gets to the grille cloth and won’t be uniformly deep.

After I tested the amp with the baffle stiffener I heard a new, higher buzz, up around Ab or A. It was the upper back panel. I routed a groove in it, as shown below:

rout back stiffener

This time I used a piece of oak to stiffen the panel. I happened to have this piece around from another project and it was the right size. Here it’s being glued in, using Titebond wood glue.

glue back stiffener

Here’s what the finished product looks like:

back stiffener finished

As with the baffle board, it’s completely rigid. The height is much more important than the thickness in providing stiffness, and even a 1/8 inch piece of oak is not going to flex. The Princeton Reverb cab is now silent.

20 Comments

  1. I’ve followed your postings on the FDP, (I’m Tee Pee). My Princeton Reverb is giving me fits with noise. I’m particulary interested in your comment that the baffle is made from MDF, which doesn’t take screws well. Is there any other way to secure the baffle to the cabinet, other than the fairly thin, inadequate screws? One of mine is stripped, and I’m thinking of replacing all of them with larger ones. My top back panel vibrates also, and I thank you for posting your fix.

    • bill says:

      If the screw stripped, it was probably overtightened. The existing screws should hold the baffle firmly under normal circumstances. You can repair the stripped hole by filling it with a couple of toothpicks and some wood glue or super glue (CA or cyanoacrlylate). You can strengthen all the holes by putting several drops of thin CA into each hole. Let it dry thoroughly before you put it back together.

  2. Legg's Lane says:

    I too was plagued with the dreaded low F buzz. I added 4 more screws to the speaker chassis (the holes are already there, unused) to fix it to the baffle more securely, I placed strips of vinyl kitchen cushion flooring between the baffle and the cabinet, and I mounted the reverb unit onto the back of the amp so that it stiffened the lower back panel and removed the possibility of resonance and feedback within the reverb ‘package’. I also filled the cabinet with polyester acoustic absorption material as is used in most speaker cabinets. The combination of all these mods has produced a clean, buzz-free non-resonant amp that sounds fantastic. The original old amp cabinets were, I think, made wholly of pine and thus had different vibrational tendencies from the modern re-issues.

  3. Ken O says:

    I had to make a new baffle all together. I used 3/4 plywood and I routed the relief for the grill cloth. I tried a piece of 1/8″ angled aluminum stock to brace the original baffle after I saw this page. I epoxied and screwed it to the baffle and it just changed where the buzz happened. I obviously should have used Bill’s method here. It worked out. The 3/4″ plywood made a fabulous baffle and it even seems like it improved the overall quality of tone. Very subjective of course but it was my preception. I also had to brace the back covers. Not a single buzz or vibration now. This is absolutely the first upgrade for PRRI owners! If your cabinet does not buzz now, it probably will some time in the future so enjoy it while you can! Next stop, the T020 OT. I let ya know!!

  4. Tom H says:

    Is anyone having trouble with cabinet resonance and vibration on blues juniors? I’ve installed the basic mods on my BJ, and ever since it has been buzzing very annoyingly when I play low notes, particularly when I have the bass turned up. With the chassis removed the buzz seems to be coming from the cabinet, mainly where the speaker is screwed down (and it’s screwed down pretty tight), and when the chassis is installed the power tubes and their housing seem to be vibrating as well. What should I do to stop this? I don’t want to tighten the cabinet screws up any further in case I strip the holes.

  5. Phil says:

    Hi Bill — I am considering a range of mod options for a mid-70’s non-reverb, non-pull boost Princeton amp. In addition to general maintenance, I am planning on installing a 12″ speaker to start. I also thought the TO20 Low-Profile Output Transformer looked like a really interesting mod, and I had a couple of questions: 1. Will it fit in my amp? 2. What sonic differences am I likely to hear in this non-reverb Princeton? Thanks in advance for your help 🙂 Phil

    • bill says:

      As far as I know, the Princeton uses the same spacing, but if you give me the measurements I will check it. You would likely hear more bottom end and get smoother distortion tone. The rectifier and limits on filter cap size may limit the benefit.

  6. Joe says:

    I recently put a 10″ Tone Tubby 40/40 ceramic speaker in my PRRI, which already has a T020 O/T that Bill installed. While I liked the Jensen C10R that comes with the amp, I decided to try to improve the already killer tone of this amp further. Out of the box this speaker sounds great. The Tubby did not change the voicing or tone of this amp at all. The result was a louder, richer, full/deeper tone with even tighter bottom and elimination of any high end shrill, while preserving the Fender clean chime. In addition, while Bill eliminated my baffle rattle with the above fix, the Jensen speaker itself would vibrate when cranked. This does not exist with the Tubby. The only downside is that this speaker adds a bit of additional weight to the amp due to the huge magnet and speaker frame. I anticipate this speaker will sound even better once it is broken in.

  7. Rick says:

    Hi Bill: I just bought a 1981 Princeton Blackface Reverb (CBS). I’d like to put a line out jack into it so I can run into the board like I’ve been doing with my Super Champ XD. Any idea where I’d take the feeds from. Asking a lot I know, but I figured if anyone would know, you would…. From what I can tell from the history I can gather, these were the last 2 years of the Silverfaces effectively but with changed cosmetics and, yes, it does have the “boost” pull switch. Thanks. Oh, and any other suggestions most welcome on this amp if you have any…

    • bill says:

      You could convert the second speaker jack to a line out; the Blues Junior kit would work. Alternatively, you can plug an inexpensive DI like the Behringer GI100 between the amp and speaker. It gives you balanced and unbalanced output, plus speaker emulation.

  8. Ray O'Connell says:

    Hello Bill, Just like to thank you for the mod on the princeton reverb reissue. The resonance and vibration problem causing the low notes to buzz and vibrate. I saw your Mod with with excellent quality photo’s and did as you suggested by putting a strip of 1″x 1/8 alloy bar into the top of the baffle board and it has worked a treat. Thank you for posting that help as it has been driving me nuts for over a year (I contacted Fender UK but they denied that there was a fault, surprise, surprise, but they did send me three altogether, all I may add with the same problem!)The amp now works fine and will be going to its first gig. Can’t wait – he he he.
    Regards
    Ray O’Connell, Liverpool, UK

  9. Jack says:

    I love my PRRI and am soon going to do the output transformer mod. I am also seriously considering upgrading the speaker. The stock is fine, it a little blah at times and seems to have slightly harsh higher frequency to my ear. I tend towards a warmer, richer “Bill Frisell” tone and was wondering if there were any suggestions for a speaker in that range. Maybe the Eminence Delta Demon?

  10. Geno says:

    Greetings Bill. Thanks for all the fantastic, helpful info over the years. My question regards bias level on the PRRI’s tubes. The stock setting was 14ma, and I bumped it to 18. After exchanging info with some folks on the FDP, I took the bias up to 24. These are the large JJ 6V6S. It sounds better set hotter, but how hot is too hot? What’s the best compromise between great tone and reasonable tube life? Cheers!

  11. MAHA says:

    Hello Bill, Can you provide your expertise in choosing a speaker for the Princeton reverb? C-rex?, Celestion gold?, or Alnico blue?, or something better?

    Or are the recommendations for the BJ in the same league. Hope this is not a can of worms?

    • bill says:

      The Princeton Reverb has a 10-inch speaker. The ones most often mentioned as replacements are the Eminence Copperhead and the Celestion C10R. If you’re talking about upgrading to a 12-inch speaker, I’d probably choose a Texas Heat for a full-range, Fendery tone. Although I was playing a ’65 Princeton Reverb through my Swamp Thang last night, and it sounded pretty darn sweet.

  12. bill says:

    A piece of wood will work as well. Aluminum and routing are not necessary. It’s just the way I like to do things.