FDP Forum / Princeton Mods, Bring 'Em On/ 27 messages in thread.

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wrnchbndr

Contributing Member
****

New Jersey

saving the otters one guitar at a time
May 20th, 2008 08:48 AM        

A client and good friend has brought me a working black face Princeton AA964 that he wants to mod. He's looking for a bit more headroom and richer tone. Along with the amp, he has provided a new Vintage 30 speaker and I'm going to make a new baffle plate for it. We're upgrading to a grounded power cord. Even though there is no problems with hum we're going to replace the original filter caps. We're also going to upgrade all of the coupling caps and install a tone lift via a push-pull pot on the volume knob. I have heard that there is a known mod for the phase inverter that addresses headroom. I'm looking for people who have done mods that are successful and links to explanations and diagrams.



jazzguitar

Contributing Member
*

Delicious tone !

Looking for the perfect job
May 20th, 2008 09:23 AM        

"We're also going to upgrade all of the coupling caps"

What on earth for? Best leave that amp alone (aside from maintenance like new tubes and electrolytic capacitors).

The phase inverter mod (a.k.a. "Stokes mod") may not do anything good as the preamp of this one hardly drives the power circuit to saturation.



Gene Dunn



USA, Sacramento, CA

Ono Boy says: "good n' swampy"
May 20th, 2008 09:48 AM        

I have a 69 Princeton Reverb, which is slightly different, but I likewise wanted maximum clean headroom. I think you can easily get what you're after without major circuit mods.

Just replace the V1 tube with the lower gain 12AY7 and retube the other positions with quality NOS or UOS tubes. I put in an Electrovoice Force 10 speaker, which is a 150 watt rated ultra clean speaker. I have headroom for days now. In fact I run my amp generally full on and just control everything from my guitar volume knob. It cleans up beautifully.

You didn't mention if your amp was a reverb model.... I assume not.... so without the reverb stage, you'll have even less gain and the 12AY7 substitution should really warm up your tone, yet stay sparkly clean. The 12AY7 is brighter sounding than 12AT7s, 5751's ..... Just do the recap and 3-prong



Fritz_D_Cat

Contributing Member
****

was the coolest cat

in any toon genre
May 20th, 2008 12:47 PM        

Stokes mod DOES too make a huge difference(IMHO of course). That and a 10F150T Weber, and a 3 prong cord. Change the electrolytics. All done !



mbratch



Webster NY

May 20th, 2008 12:51 PM        

More headroom in an AA964 (Princeton NR)?? They already don't really overdrive, at least very little. Unless you really mean "more volume"...

Another interesting very simple mod is to put a 250K pot in series with the Negative Feedback (NFB) resistor (or switch off the NFB with the ground switch). It will get much more volume with less NFB, but will overdrive more as well (maybe not what you want). It will also have more presence and more bass (almost to a fault).



RivEraEra



usa/east bay sf

Ain't no time to hate
May 20th, 2008 01:12 PM        

MBratch - which is th NFB resistor - also, any links to the Stokes mod? I'm wondering if it's the same thing or similar to what Torres calls the Perfect prince mod.



1958Bassman

Contributing Member

USA

May 21st, 2008 08:10 AM        

Riv- look at the schematic and you'll see a 2700 Ohm resistor at the top of the diagram, going from the speaker (and ext spkr jack) to the second half of the 7025 tube. That's the NFB for the amp and it's a common resistor value. 250K is way too high of a value- adding resistance will decrease the feedback (and make the sound grungier/less headroom), while decreasing the resistance will make it sound sterile. 25K -50K should work fine and the changes will be more obvious, since a 250K will pass through the normal NFB value very soon after moving from the minimum value and then it'll be hard to hear much difference after about half way.



RivEraEra



usa/east bay sf

Ain't no time to hate
May 21st, 2008 09:48 AM        

Thanks '58.



mbratch



Webster NY

May 21st, 2008 02:39 PM        





SoK66

Contributing Member

USA

We had the hit but Van got the money
May 21st, 2008 08:49 PM        

" any links to the Stokes mod? I'm wondering if it's the same thing or similar to what Torres calls the Perfect prince mod. "

Not the same thing. Stokes mod moves the PI power feed to a lower resistance node of the filter cap can, increasing vltage to the PI. The Torres "Perfect Prince" kit supposedly balances the resistance of both sides of the PI using Dale resistors, and, IIRC, adds 470k screen resistors to the power tubes.



imjb1911

Contributing Member

Chicago

BFPR fan club member
May 22nd, 2008 08:00 AM        

A lot of knowledgable people feel that the Stokes mod fixes a flaw in the original design of the Princeton. There is no good reason NOT to do it, JMO of course.



RivEraEra



usa/east bay sf

Ain't no time to hate
May 22nd, 2008 11:37 PM        

I started to do to do the Stokes mod, but haven't tested it, got waylayed by other anomalies I found in there. The amp is a pull boost so I realize it's not a blackface circuit, but found a couple of things that didn't seem right. The most unusual thing was a .047 blue drop cap attached to pin 6 of the reverb driver tube (edit - I've never seen a blob type cap attached to a tube pin) and a 1.5M/22pF combo where the 3.3M/10pF combo usually are - unfortunately don't have a 3.3M resistor in my parts box so have to wait til tommorow to pop that in - I also pulled out the pull boost leads.

I also added a 47k resistor to the NFB, although it seems like I should be able to just replace the 2.7k altogether and just put the 47k in it's place.



rfrakes331K

Contributing Member
*******

IL USA

RonHalen Jokingly He Says
Jul 12th, 2017 03:50 PM        

<br /> I am thinking of modding my plain Prince. I've looked at the Stokes mod, but unless you put in the beefier Deluxe OT, you may blow the stock OT. Then there is the Paul C. mod. And the Torres Perfect Prince kit. Torres has retired, but the kits are still available. I would like to take it to Tweed as i am not interested in more power, just better breakup, odd order harmonics. I don't see any Tweed mods out there though?<br />



Hammond101

Contributing Member
**********

So. Cal. USA

Jul 12th, 2017 04:25 PM        

rfrakes331K,<br /> <br /> I don't see the value in modding a cool BF Fender amp, however....this is pretty non-invasive and reversible.<br /> <br /> I think your best success would be playing with negative feedback resistor values to get the amp to break up a bit more. I believe the stock value is 2.7K ohms. You might try raising the value by 25% and 50% and see if you like the changes.<br /> <br /> You can use your unused ground switch to switch resistor values if you have put in a three conductor power cord. This switch is then no longer needed.



rfrakes331K

Contributing Member
*******

IL USA

RonHalen Jokingly He Says
Jul 12th, 2017 06:45 PM        

Thanks Hammond. It is a 75 Silverface Princeton non reverb. Anyone else? Any point in putting in a bias pot?



ejm



usa

Jul 13th, 2017 08:17 AM        

FYI: FWIW, this thread was recently resurrected after 9 years dormancy.<br /> <br /> And JMO, and not trying to be the vintage police or anything, but I would not mod any of the "collectable" Fender (or other) amps. And I guess that means we're getting older, because the 1970's and some 1980's Fender tube amps are collectable.<br /> <br /> And if I did do any mods, I would make sure that they are reversible (for possible resale at some later point).<br />



rfrakes331K

Contributing Member
*******

IL USA

RonHalen Jokingly He Says
Jul 13th, 2017 04:17 PM        

ejm, I would agree, but the amp will stay in the family. Hammond has made a good reversible mod suggestion. I will start there. A 50% increase would be 4050 ohms. I could see a 50K pot stuck there as a Presence control like on the Bassman.



rfrakes331K

Contributing Member
*******

IL USA

RonHalen Jokingly He Says
Jul 13th, 2017 07:16 PM        

But then, it has always lacked that Magic Fairy Dust Tone! In a YT Tweed Princeton, it had that Tone!



Hammond101

Contributing Member
**********

So. Cal. USA

Jul 14th, 2017 10:44 AM        

Without major surgery the tweed front end ain't happening. Then you still have the SF power section. Want a tweed Princeton? I'd buy a kit and build one. (Actually I turned an old radio into a tweed Princeton)<br /> <br /> Try the negative feedback thing 1st. You may be real surprised at what it does to make the thing hair up and sing.<br /> <br /> What ever I would do, I would drill no holes.



rfrakes331K

Contributing Member
*******

IL USA

RonHalen Jokingly He Says
Jul 14th, 2017 07:22 PM        

I did revive this old thread so as not to create another. What about modding a basement floater Blackface plain prince? It was a rusty mess when I got it. Many parts had to be replaced. So it is not all original or unmolested.



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