Princeton amps: differences btw. brownface and blackface?

I have a blackface Princeton amp (non-reverb) that isn't very loud at all and stays clean at full volume. My friend has a brownface Princeton amp (non-reverb) that distorts beautifully and is loud enough at full volume to cut through a band even if the drummer is fairly loud.

Does anyone know the technical differences between blackface and brownface Princeton amps? Are the circuits very different? Are the output transformers different? Is it feasible to modify a blackface Princeton to brownface specs?

Kris

Hey Kris, I just bought a black tolex w/silver grill cloth Princeton after hearing a guitar player at a jam session using one. Mine has tremolo, no reverb, and 1-10 speaker. I don't know what the wattage is on one of these but I've used it with an A/B switch along with my DeVille and it's plenty loud. I like the chicken pickin' sound I get with my Tele with this thing. When I crank it up full on it distorts just a tad and sustains pretty well. I'm thinking of looking for a JBL 10" speaker to put in the thing and see what that would do. JH in Va.

I'm not sure about all the differences, but a key one is the tone circuit. The brown Princeton has a simple tone control, the BF has a Treble/Middle/Bass tone stack (no Middle control, just a preset resistor). That BF tone stack gives the characteristic clean/scooped tone that many of us love, but it also shunts a great deal of the signal to ground. So after the tone circuit, the BF amp has much less signal to work with than the brown amp. And the brown amp has a midrange emphasis that often sounds louder.

Bob Arbogast

I have heard it described like this. The pattern on an eq would look like a;

a) frown for Tweeds
b) semi-smile for browns
c) happy face for BF's.

If you look at the schematics, the brown princeton is almost exactly the same amp as the tweed vibrolux, both different animals than a BF or SF Princeton, which is a clean amp indeed. The BF and SF princetons (non verbs) are great amps in their own right, I'd say don't mod it!
Jef

my '63 Princeton is still hands down, far and away the best amp i've ever heard, it's absolutely bone stock, and i use it every day, it's actually my main gigging amp, run in stereo with a recent Blues Jr......

the Bf & Sf Princetons are good too, but cleaner, as noted.....

i'm unsure if it (your amp) could be modded to brown specs, without major revamping....law of diminishing returns etc....might be best to try finding one....

i.m.h.o. & y.m.m.v.

...and the ohnly prescription...... is mohr cowbehl!"

Love that skit Jef. I just saw that again! Absolutely hilarious.

....ok back to regular programming....

Kris, if you wanted to get closer to the Brown amp sound, there are some tone cap changes that will take you in that direction. The treble cap needs to change to a silver mica. that will smooth out the high end. Change the .1 and .047(bass and mid) caps to .022. this will yield you an amp with more of a midrange emphasis compared to the BF. One might look into changing the gain in the preamp, and I would not be afraid to increase the negative feedback resistance.
Also, the Stokes mod might give the amp a bit more oomph. http://people.cornell.edu/pages/mt24/Amp/mm/PRtweak.html

If your BF Princeton sounds weak, it may need service. It probably has one or more weak tubes, and may be due for new capacitors in the power supply as well.

Thank you all for your suggestions (keep 'em coming, if there is more!).

I'll probably do things step by step.

First, I'll have the amp serviced. The tubes are good and new, but I don't know about the caps etc.

If that doesn't do it for me, I'll look into tone circuit mods.

Finally I may consider the Stokes mod.

Thanks all!

Kris

Here's something I copied from the Weber amp board. It was posted by David Allen--one of the best and most thoughtful techs/amp designers I know. Hope this helps.

Dr. Bill

______________________

I worked on a non reverb Princeton over the weekend where the customer wanted more headroom and gain so he wouldn't have to mike the amp in his church band. I always wanted to know the exact impact in power of certain mods so I checked the output power as I made some changes. I thought some of you might find this interesting.

With a tired looking set of RCAs mismatched about 7ma and a low wall voltage around 116V I got 12 clean watts out of the amp before clipping. I did the Stokes and Paul C mods including new 51K metal film plate and cathode resistors, 2.7M/1M resistor divider for the grid and changed the first 18K power supply resistor to a 10K. I measured 15.5 watts before clipping.

Then I installed a well matched pair of JJ 6V6s and changed the rectifier from the stock 5U4G to a 5AR4 as I felt confident the JJs could handle the higher B+ voltage which will likely not exceed 450V. I biased the tubes around 23ma and measured 18 watts before clipping.

For more gain, I changed the 100K plate resistors for V1 to 220K and got about 30% more gain at the power tube grids with the volume dimed and the treble and bass on 5. Disconnecting the negative feedback increased the gain about 70% but I ended up changing the 2.7K NFB resistor to 6.8K which netted about a 30% gain increase.

The real kicker was installing a PEC 2W 250K audio pot where the vibrato pedal jack was located on the back to replace the 6.8K middle resistor. This particular pot has a very gradual taper (about 6-10%) so you get a middle control up to about midway and then you get lots of gain as you impede the tone stack's path to ground. I put a chickenhead knob on it so you could feel where it was set from in front of the amp.

The amp sounded very good. It was very quiet with more clean headroom plus the ability to overdrive it with the new found gain.

So I think it is reasonable to assume you can get 18W from a Princeton or Princeton Reverb with matched JJ 6V6 power tubes, Paul C and Stokes mods to the phase inverter and 120V input voltage.

David