More Princeton Reverb mod questions -- NFB

Discussion in 'Amp Central Station' started by King Fan, May 15, 2016.

  1. King Fan

    King Fan Friend of Leo's

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    OK I'm using this weekend to do a bit more planning. Noting the enthusiasm about NFB mods for the PR at Fenderguru, I've ID'd the NFB resistor, 2700K across the top of the schematic, attached, and I figure I could simply test 'NFB off' with alligator clip bypass.

    Fender AA1164 PR schematic copy.png

    If it matters, my initial build is (on purpose) keeping the wimpy stock PT and OT, but (likely) incorporating a mids or mids / raw (tone stack suppressing) pot.

    Questions (answer any or all as you see fit):
    1. Is NFB a worthwhile mod, in your experience? Some folks suggest an NFB mod ain't all that much on a PR, or makes their amp too noisy.
    2. Would you use a switch (say 3-way, Normal / Less (bigger R) / Off) versus pot?
    3. Can you suggest pot (or resistor) size / values?
    4. Experience or thoughts about Fenderguru's .01uF or other value cap in series with the NFB resistor?
     
    Last edited: May 15, 2016
  2. Wally

    Wally Telefied Ad Free Member

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    The NFB loop has two resistors there, as I see. The effect of the loop is determined by the ratio of that 2700ohm 'top' resistor and the 47ohm resistor at the bottom. That ratio is already fairly high. It might be interesting to install am5K linear pot there is series with the 2.7kOhm resistor to see what goes on. I like pots rather than switches to another static resistance or to lift the loop. Note: I have never experimented with the Prin Rev NFB loop.
     
  3. vegie1

    vegie1 Tele-Meister

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    I have a custom vibroverb which came with no nfb and was incredibly noisy until it was molded to add one.
     
  4. King Fan

    King Fan Friend of Leo's

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    Thanks, Wally. Those are excellent points. I forgot about the ratio deal. And I agree -- a pot gives you more ability to nudge and tweak. If you always end up on one optional resistance, then you can simplify down to a switch.

    I hear ya, for sure, one reason I'm not just going for an on-off option.
     
  5. King Fan

    King Fan Friend of Leo's

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    Anyone found this NFB mod useful on a PR? Or useless? Any more thoughts / experience with bigger R values or pot size?
     
  6. jhundt

    jhundt Doctor of Teleocity

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    I tried it, but did not find it useful for my amp. And I started to question why I was trying to modify the amp anyway, since it sounded pretty good to begin with.
     
  7. King Fan

    King Fan Friend of Leo's

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    Thanks, sir. You may well be right. I sense a lack of excitement for this mod across the Googlesphere. Relatively few discussions, very few raves. (Maybe as Wally says cuz the NFB is already pretty small, cuz 2700/47 is pretty high). Why did I even think about it? I was interested because I found in my 5f2a (a completely different breed of Princeton, of course) just a *little* less NFB lets it hit the sweet spot early and often.

    And I hear ya about 'why mod a good thing.' (See my sig, below). One of my first ideas on this build is "WWLD" -- what would Leo do? Start with the stock OT and PT, tube, cap and resistor values, etc. Build it stock, and *maybe* later think about OT/PT/mids/NFB etc. I'm gonna build the NFB stock, so for now I'm just thinking through layout and hole issues if I decided to mod the NFB later. The mids I'm building in cuz I can always dial it to 0, and it'd be easier not to retrofit it if I want to try it.

    Having honored the first idea, I gotta admit my second idea is to mix it up a bit for just a hint of fun, safe, modern, or *mine* -- bias trim and test points, a sprinkling of metal film resistors, a personalized blend of cap types...
     
  8. dan40

    dan40 Tele-Afflicted

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    I added the nfb mod to a brown Princeton clone that I recently built. Removing the nfb does make the amp a touch brighter with some extra grit to the tone. It did raise the noise floor slightly but nothing to awful.
     
  9. King Fan

    King Fan Friend of Leo's

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    Thanks, Dan. I'm gonna go study that schematic too.
     
  10. Telenator

    Telenator Doctor of Teleocity Vendor Member

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    Not to get too far off topic here, but I think the best mod for a Princeton is a high quality 12" speaker. Brings the sound up to the next plateau. Helps it graduate from squawk box to real amplifier. I love the sound of a Princeton through a 12". The bottom fills out much better.
     
  11. King Fan

    King Fan Friend of Leo's

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    No worries. *That* is a mod that does get a lot of love. If and when I get my PR built, I can plug it into my 12A125 cab and compare!
     
  12. Telenator

    Telenator Doctor of Teleocity Vendor Member

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    Love the Weber 12A125. Sounds like you've got it covered!
     
  13. Mhat

    Mhat Tele-Meister

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    I just have a switch on the 2.7k NFB resistor. Stock the amp is a little more polite, lifted out the amp is a bit louder and hotter. This hiss increases but you'd never notice playing with a band. I like the option to add it in or out.
     
  14. King Fan

    King Fan Friend of Leo's

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    Good to know, thanks. I definitely can test *that* option with alligator clips once the amp is done.
     
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