Paul C and Stokes mod

traynor_garnet

Silver Supporting Member
Messages
3,132
Paul C's mod changes bias method of the inverter, again for slightly more gain.

The problem with the Princeton Reverb [where this mod is most usually used] is that the inverter distorts before the power tubes, causing a rather raspy distortion. By giving the inverter a bit more umpf, the amp delivers a bit "more".

I have a SF Princeton (Nonreverb). Will the Paul C mod give me more gain and a bit more volume? Or, is it only applicable to the Princeton Reverb ?

I am looking to get "more" out of the amp but don't want to fundamentally change it or make it something it is not.

TG
 

mark norwine

Member
Messages
17,246
I have a SF Princeton (Nonreverb). Will the Paul C mod give me more gain and a bit more volume? Or, is it only applicable to the Princeton Reverb ?

Nowhere in the Fender BF lineup is there more disparity between rev & non-rev models than with the Princ. & PrincRev. Because of the lack of reverb, your amp lacks a recovery stage and, as such, a lot of gain.

IMO, your amp "is what it is" and neither mod discussed herein (PaulC or Stokes) will do anything of significance to change that.

There *IS* a way to get a non-rev princ. to come alive, but it's an invasive mod [sacrifice the trem in order to make a LTP inverter], but you said:
I am looking to get "more" out of the amp but don't want to fundamentally change it or make it something it is not.TG

Getting more without changing it isn't going to happen. As I said before in this thread,
If you want a bigger amp, buy a bigger amp.
 

Gris

Member
Messages
400
I have a BF non verb Prince and it is my fave above all the others - brown, black, verb, non-verb, etc. (except one other modded PR i had). It has an Allen TO20 and a 12" spkr, as well as a diff filter section. Also removed the resistor from the back of the bass pot and put another 'mid/raw' pot in the ext spkr hole in back. No drilling required, all easy stuff and the amp is super versatile and screams... :)
 

Paulmc

Member
Messages
4
I have a BF non verb Prince and it is my fave above all the others - brown, black, verb, non-verb, etc. (except one other modded PR i had). It has an Allen TO20 and a 12" spkr, as well as a diff filter section. Also removed the resistor from the back of the bass pot and put another 'mid/raw' pot in the ext spkr hole in back. No drilling required, all easy stuff and the amp is super versatile and screams... :)
Gris, your inbox is full.
I have a 78 Fender Princeton non reverb. I've rebuilt the eyelet board and got a new cap can. Would you be able to explain how to get more gain from the amp please? Its clean all the way to 10 on the volume. Cheers Paul
 

macmax77

Silver Supporting Member
Messages
13,686
This is such a great thread for me, I do not understand s ^&*.

And yes, I am eating and just wanted to share my hurt feelings.
 

dcbc

Member
Messages
2,376
David Allen did the phase inverter mod to my old 76 PR and got rid of that rasp/ratty distortion. I thought it sounded absolutely fantastic. I don't own the amp anymore, but my friend who is its current owner can't say enough wonderful things. No reported ill effects from the mod several years later.
 

HotBluePlates

Member
Messages
13,599
... Was looking for help getting some more gain out of my princeton.

Can you tolerate more midrange?

If yes, look at the Bass pot. There is a 6.8kΩ resistor soldered to it running to ground. Replace that resistor with a pot between 25kΩ to 250kΩ (wired as a rheostat; use one lug grounded, and the wiper).

If you go for a 250kΩ pot, turning the pot to max will isolate the tone circuit from ground, boost mids a lot, and increase overall signal level to drive the output section. You might expect a ~20dB signal boost.

You could mess with a master volume, but you're not likely to get great distortion from the Princeton's phase inverter anyway.

If you need more gain/distortion than the above provides, you really want a different amp with a different preamp & phase inverter architecture.

_____________
EDIT: All I did here was describe the Allen Amps "Raw" control, meantioned earlier in this thread.
 
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Jeff Gehring

Gold Supporting Member
Messages
7,209
You know, it may be heresy, but the Princeton I liked the most was a modified one I rebuilt for a customer with a long tail PI (courtesy of giving up the tremolo tube). Kept the tremolo in the amp by adding a MOSFET tremolo, a la RG Keen. To me, it sounded more like a "little big amp" with the long tail PI rather than sounding like a "little amp" with the cathodyne. To be fair, though, we also replaced the power transformer with David Allens excellent TP25...
 

Paulmc

Member
Messages
4
Thanks for the info, thats great. Im building a 5e3 next so will probably keep my princeton clean as you recommend. Is the 6g2 similar sounding to the 5e3? Cheers Paul
 

hasserl

Member
Messages
4,708
Hey guys, just came across this old thread in a google search. FYI, my old PR with the Stokes mod is still doing fine, with the original power transformer, and it gets quite a bit of use. I regularly put it on the backline of a jam I host, and for 4 hours it hardly gets a rest. Great amp, sounds fantastic! I don't doubt Mark has replaced some power transformers, just saying what my experience has been on my own personal amp. It already had the Stokes mod done years before this thread started, and that was over 10 years ago. The power transformers are small, but maybe not as delicate as they would appear to be.
 

JDW3

Member
Messages
3,637
OMG, has it been 12 years...?

With all the fear of blowing transformers, I stopped using my modded Princeton...about 12 years ago.

About 13 years ago I installed the Stokes mod, and the LTP.

My tastes changed, I don't run my amps quite so loud, and began using pedals again. What to use for a pedal platform thats easy to carry?

I pull the old Princeton out, and its absolutely killer. The amp already had a mid control in place when I bought it. So its all usable guitar frequencies. Even running it really hard, the transformers aren't really hot. No worries.

Theres also no bass flub which is nice. I'm using a Weber F150T. It might be close to broken in now. 8)
 

jordan86

Member
Messages
3
Thanks for the zombie thread revival!!! I've just built my first amp, the AA1164 and in the process have read EVERY SINGLE stokes and paul c thread I could find on any and all forums.

I understand the technical aspects of what the mods do, but I would love to know more about the logic behind this mod being "bad" or "stressing the PT". How exactly is it doing that? Is it because you are now pulling more voltage/current through the PT via the previously untapped filter capacitor?

It seems David Allen does these mods with great success on his Sweet Spot amp. I did put some slightly upgraded iron in my amp. Allen TO20B for the OT, and the Hammond 290AX PT. The Hammond does supply 100ma at 325v on the HV tap. Seems the "stock" Princeton PT's only supply 70ma. So there is some improvement. What are the "risks" I am facing?

Would I reduce "risks" with a different rectifier or by using the lower HV tap on the PT (my Hammond does 275v and 325v).

FWIW....I temporarily rewired the amp last with the Stokes. Now using the 350v cap vs the 280v cap. My plate voltage on my PI went from 207v to 260v. I do enjoy the added headroom. PT seems to still be running cool.

As of this morning, I have the stokes mod in place. 250v on the PI plates with a NOS 5V4. 6v6's are running at 24ma and 414 plate voltage (almost 10w dissipation). Sounds GREAT! Only downside is the dirt isn't as nice on 9. Thinking I could maybe put the 275v and 325v HV secondary options on a switch to cure that?

THANKS IN ADVANCE!
 

HotBluePlates

Member
Messages
13,599
... I understand the technical aspects of what the mods do, but I would love to know more about the logic behind this mod being "bad" or "stressing the PT". How exactly is it doing that? ...

Since @mark norwine mentioned burned PTs related to the Stokes change, you'll have to see if he can clarify.

You have a 12+ year test run from another member below reporting no issue, and with a vintage Princeton PT rather than a modern upgraded part. My hunch is you need not worry.

Rick, FWIW I've gigged a SF PR with both mods, along with a few others, many times with no problems. I tend to think the concerns are over stated. ...
Hey guys, just came across this old thread in a google search. FYI, my old PR with the Stokes mod is still doing fine, with the original power transformer, and it gets quite a bit of use. I regularly put it on the backline of a jam I host, and for 4 hours it hardly gets a rest. ...
 

mark norwine

Member
Messages
17,246
The Hammond does supply 100ma at 325v on the HV tap. Seems the "stock" Princeton PT's only supply 70ma. So there is some improvement. What are the "risks" I am facing?

I have no idea. You don't have a stock Fender amp, you have …."something else entirely" by virtue of different voltage & current. You don't risk the PT meltdown I wrote about (in 2008! Has it really been that long? jeeze..), but whatever else your design may risk is unknown to me.

If you speak to David Allen, tell him I said "hi".
 
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