Paul C and Stokes mod

Discussion in 'Amps and Cabs' started by silver surfer, Feb 15, 2008.

  1. silver surfer

    silver surfer Member

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    Could someone please explain what the Paul C and Stokes mods do to an amp - just in layman's terms as far as how it affects the tone, volume, etc. Thanks.
     
  2. mark norwine

    mark norwine Member

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    The "Stokes Mod" [which John no longer endorses, BTW] raises the B+ of the inverter so that it offers more gain

    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".

    That said, you don't get "more" for free......these mods raise the amp from about 12W to about 18W, but doing so requires that the PT deliver more current.....current that the PT is only marginally-capable of handling. I, and other techs, have seen blown PT's as a result of the PaulC mod and [moreso] the Stokes mod.

    I no longer feel that these mods are "worth it". If you want a bigger amp, buy a bigger amp.

    YMMV
     
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  3. BudLite

    BudLite Supporting Member

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    I had the mod done on my BFPR and thought it was not that big a deal. I don't think it makes the amp sound better(JMO) I do like the idea of having a adjustable bias pot, that is a GREAT mod.
     
  4. hasserl

    hasserl Member

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    To add to Mark's comments, IME the mods help clear and smooth out the distortion. The Stokes mod raises the point where the amp distorts a little higher; then when it does go into distortion the PaulC mod makes it smoother and less muddy, or as Mark put it, less raspy.
     
  5. PaulC

    PaulC Member

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    Well - I've done the inv mod in a ton of amps, and I've never seen a pt blow. Some of those amps have been running for years. But I never did the mod with the stokes mod included, so maybe that's why...

    Like Bud lite said it's not that big of a deal - it just takes a little bit of hash out of the signal. the amp stills sound pretty much like it did. It came about from seeing the bogus way people were talking about balancing the inverters by matching the anode/cathode resistors. that wasn't the problem with the inverter - it was the bias point, and how the different source imps were effecting the pwr tubes.

    The inverter mod (which is an old text book circuit btw - I never said I "invented" it) just biases the inverter to have a little more sym swing reducing the nasty anode clipping of that design. This will have a slight increase in gain, but nothing like what the stokes B+ mod does. Also it will have much less gain than another standard mod which is to kill the vib circuit, and change the inv to a long tail which will blast the pwr tubes hard. I've not heard many stories of iron blowing doing that. But there's always the chance. I've seen amps blow when slamming them with big boosters...

    But yes - the iron in the amps are wimpy. If you want more out of the amps that is a good area to dink with.

    Later, PaulC
    Tim & timmy pedals
     
  6. mark norwine

    mark norwine Member

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    Hey! Long time! Gimme a call sometime.....

    Anyway, "fixing" the bias is a pretty good thing, no doubt. So, yeah, yrou mod is fine. Heck, Ampeg did it for years!

    It's John's mod that, indeed, ramps up the I-draw on the PT, and it really heats the hell out of it.
     
  7. Strat

    Strat Member

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    I think i'll un -mod the Stokes and see how it sounds cause mine is too raspy now as is... or sell it...BF anyone ?
     
  8. JDW3

    JDW3 Member

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    Is the inverter mod the one that Torres offered in a kit awhile back? Just installing a few resistors? I did it and it didn't really sound much different, although the amp had been modded incorrectly before I got it. The vibrato had been disconnected. (Non-reverb Princeton)

    Now years later, I did the long tailed-pair inverter mod, which totally changed the amp for the better. I thought the long tail was the Paul C mod.

    I also added the Stokes mod, which seemed to give the amp more headroom. The amp sounds killer with a SG Jr. plugged in and cranked.
     
  9. silver surfer

    silver surfer Member

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    The amp I asked about is indeed a BF Princeton Reverb that I had both mods done to a few years ago by a well known amp tech in New Jersey. It sounds perfect right now. I was asking the question because someone asked me how the mods affected the amp and I did not know the answer.

    Now, after reading the responses, I may contact this exceptional amp tech in New Jersey and ask him to undo that Stokes mod, particularly since I will be using the amp more now for it's dimed overdrive tone.

    Thanks for all the info.

    Rick
     
  10. mark norwine

    mark norwine Member

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    Paul C is not a long tail. Paul C is about establishing a fixed bias point for the inverter by creating a voltage divider off the B+, and putting a fixed voltage on the grid, a few volts below the cathode voltage.

    Is it done with "a few resistors"? Yes....but that's not to say the "Torres kit" is the same thing. I have no idea what DT offered.
     
  11. mark norwine

    mark norwine Member

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    Was it me?
     
  12. silver surfer

    silver surfer Member

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    Hey Mark - long time no see. Yes, that was your work. The amp sounds excellent along with the BFDR and BFSR you worked on. But after reading this, I may want to undo that mod. I want to use this in a live situation for the cranked overdrive tone - so it will get a real workout.

    Rick (formerly of Mojogypsies)
     
  13. silver surfer

    silver surfer Member

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    We have a time limit here at work and I know I'm down to the last few minutes - I'll catch up with you later Mark.

    Rick
     
  14. BudLite

    BudLite Supporting Member

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    What makes the BFPR sound better than any mod is a 12" speaker.
     
  15. mark norwine

    mark norwine Member

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    You know where to find me....
     
  16. PaulC

    PaulC Member

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    The whole "PaulC" thing always bugged me. It must have been about 8 years or so ago when i talked about this way of setting up a cathodyne/split load inverter on weber and ampage when guys were talking about balancing the inverters by matching the load resistors. I was just tossing out a tweak like everybody did, but for some reason it got my name stuck to it.

    I always said I got it from the old text books, and that ampeg used it in a ton of amps. I just stuck it in a princeton to see if it would clean up the inverter some. I yapped about it, and my name got stuck to it for whatever reason. It's been a bit of a drag because I've had a ton of guys contact me to bust my chops about it being an old circuit when I always said it was!

    There's another way I like to set that up with that I ganked from a leslie that has to do with DC coupling the driver into the inverter. Torres talked about doing that in an old VG article - that might be what you're thinking of.

    Hey Mark - shoot me your number!

    Later, PaulC
    Tim & timmy pedals
     
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  17. hasserl

    hasserl Member

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    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.

    But that's just my hunch. This thread has me wanting to pull it out of the pile and hook it up to a load and crank it up and measure the temp of the power trans under a heavily loaded condition.

    Heck, I've already changed out the output trans, I guess I shouldn't worry so much about changing out the power trans. Maybe I should do that before it fails, set it aside with the OT in case I ever want to return it to stock (can't really see doing that though).
     
  18. trumpus

    trumpus Supporting Member

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    Just to bump this thread - just got a '68 Princeton NR and was considering these mods.

    If I understand correctly (which I may not, as I know a lot less than everyone here), would it make sense to swap out the OT for a bigger/more powerful one if consider the Stokes mod?

    Also, is it possible to do the PaulC (sorry Paul, I know you don't like the moniker) but not the Stokes mod? I thought I read somewhere that it doesn't make sense to do one without the other, but I can't recall which was which!

    Advise please!!

    Brian
     
  19. direwolf

    direwolf Supporting Member

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  20. hasserl

    hasserl Member

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    Yes, you could do either one of these mods without doing the other. I think MArk Norwine mentioned on the first page (doing the PaulC mod and not the Stokes mod).

    Changing out the OT may be a good idea regardless of whether you do the Stokes mod or not. I think it helps the amp a lot, but other folks like the stock OT. I think a bigger OT helps open the sound up for a more full sound, especially when coupled with a 12" speaker.

    But back to the concerns about the Stokes mod, it was the Power Trans that may be at risk due to the Stokes mod, not the output trans.
     

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