Princeton Reverb Conversion
The Blackface Princeton Reverb was Fenders smallest amp with reverb and tremelo. It came equipped with 1-10″ speaker pumping out about 12 watts. Designed as a student and practice amp it sounded great and was easy to haul around. The only problem with the amp was the 10″ speaker just didn’t have enough range to handle the guitar in a full band situation. Headroom was also a problem if the band got too loud.
That was in the 60’s. There are a couple of things you can do to your Princeton to compete with most any amp on the stage. First off, you will need to change the stock speaker to a good 12″ speaker that is efficient. I recommend the Celestion Greenback, Vintage 30, Jensen P12N, Eminence Texas Heat and the Weber 12F150. I have an original Celestion 30-H “Blackback” from the 70’s in my amp and it is a fine speaker.
In order to do this 10″ to 12″ conversion you will need a new baffle made to accommodate the new 12″ speaker. The baffle changeover will work on all Blackface and Silverface Princeton amps up to and including the 1970 year models with a removable baffle. The later Silverface amps will not work due to the fact that the baffle is dado joined and glued in and cannot be removed. It is possible however to have a cabinet shop remove the glued baffle install cleats and then install a Blackface style baffle. This will cost a few bucks due to the work inolved, but may be well worth it. The other route is to have a brand new cabinet built with the baffle cut for a 12″ speaker. Also a baffle made from good quality plywood will help the overall tone and performance of the amp as opposed to the stock particle board material.
I am sure by now you are wondering, do I need a centered speaker or do I need to offset the speaker to avoid the output transformer or filter cap can ? Well, most speakers will fit centered with no problem. This includes all the speakers I mentioned above plus JBL’s, current production Jensens, Vox Bulldogs and various Eminence models. The only speakers I have had to offset were the vintage Oxford and CTS speakers plus the hefty EV’s. Some Jensen speakers will not work centered. These older speakers don’t have an open frame and usually have “hotdog” slots or small round holes in the back of the frame. The EV has a large spoke frame and may interfere with the reverb tank even if offset.
So now after the new baffle has been installed we have a good 12″ speaker in our amp and things have changed quite a bit. Bigger bottom, a much more open and spacious sound and more headroom. For best results I recommend you have your amp serviced with a fresh electrolytic cap can and a good set of properly biased 6V6’s.
Now it is time to put the amp to the test on the gig or with the guys in the garage. At this point the amp sounds great but I still need a bit more headroom for those jazz or country licks after we all have had a couple of beers and it is near midnight.
I would recommend the Paul C. phase inverter mod. You can find more information about this mod most anywhere on the internet or your tech will have it for sure. The mod will give you tons of headroom and a smoother breakup. You will loose some degree of harmonics in the process but it will be worth it with the overall improved preformance. I personally play in a 5 piece Blues band and play my amp at 3-4 and it has the finest Fender chime and clean tone you could wish for. I have played larger rooms at 6 and the amp did not even begin to break up. Try the mod and if it does not suit your needs or if the bar owner tells you to turn down, then it can be easily reversed. Also the Paul C. mod includes changing one resistor to greatly improve the tremelo. Follow the layout below. On the left is the stock circuit. On the right is the modded circuit. This mod should not take very long and is easy to return to stock if you don’t like it.
Click on the above image to enlarge. Please note that the wire coming from the cap can feeding the plate of V4 will be removed from the 56k resistor as well as the wire feeding the plates of V3. These wires should be soldered together and insulated with heat shrink tubing.
Click on the above image to enlarge. This is the stock layout.
The amp pictured and used for the conversion is a ’64 Princeton Reverb I purchased in totally trashed condition. The tolex was hanging off the amp from being stored in a barn here in southern Florida where the humidity is usually 100%. The cabinet was out of square and had to be dismantled and re-glued. The amp was then recovered with new tolex. The original baffle was gone and someone installed a makeshift baffle in its place. I built a new baffle and used an original piece of grill cloth to make the amp look correct. The faceplate was corroded and required replacing. I sprayed a patina coat on the new plate for a vintage look. The electronics were in poor condition and required an output transformer. Most of the resistors were rotten and were replaced as well as the filter cap can. The speaker is an old Jensen P12N but now sports a 1976 Celestion 30-H Blackback. It took a lot of work but well worth it, this is my favorite gigging amp and it sounds amazing.
I hope this article has been informative and can help you to get more from your Princeton. If you play with a full band and need a small amp with lots of power and tone the Princeton can fill the bill if set up properly. If I can be of any further help with any of the topics mentioned above please feel free to contact me for more info.
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WARNING: IF YOU DO NOT HAVE ANY EXPERIENCE WORKING ON TUBE AMPS, LEAVE THIS TO SOMEONE WHO DOES!!! THE WRONG MOVE IN A TUBE AMP CAN BE PHYSICALLY PAINFUL, DAMAGING, OR EVEN FATAL!
Disclaimer: 300Guitars.com assumes no responsibility for any accidents occurring as a result of performing this mod.