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Help me yoink the NFB out of my Silverface Princeton non-verb?

Discussion in 'Glowing Bottle Tube Amp Forum' started by Willie Johnson, Oct 7, 2020.

  1. Willie Johnson

    Willie Johnson Tele-Meister

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    This old gal needs a facelift, but don't have the funds/gumption for taking it into a tech for a full-on going-over. Can you guys help me locate this? I did this on my Champion 600 and it sounds kickass.
     
  2. King Fan

    King Fan Poster Extraordinaire

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    While somebody looks up the schematic/layout and tells you which wire to cut ("cut the red wire! no, wait, cut the blue wire!"...) I'd also ask if folks think it's a good idea. NFB is a small deal in most Champs, and adding a little more raw+noise can be Champ-y. I'm not up on the non-verb Princeton, but I wonder if NFB does more there, and if the effect of a total cut would be less popular...
     
  3. teek_s

    teek_s TDPRI Member

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    I think I'm gonna have to agree with King Fan. Maybe if you really want to see what your Princeton would sound like with NFB, you could add one that's switchable.
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    A wire and a resistor:

    https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Princeton-AA964-Schematic.pdf

    If you can't tell *which* wire and resistor from that schematic, you need to do some homework before cracking open the chassis of the amp. <Insert usual "high voltage please don't die" message here>

    I'd also make it switchable so you can take it in and out. The non-reverb princetons are pretty clean even when wound up loud, so this may be a noticeable change.

    If anything, the amp needs the full going over. I wouldn't bother playing it much/at all before it has a full cap job.
     
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  5. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    For a quick test, unsolder the wire going from the extension speaker jack to the 2700 ohm resistor and you should be good to go. If you don’t like it, simply resolder.

    I agree that a switchable NFB loop is the best permanent solution. If the amp already has a 3 prong power cord and the death cap has been removed, the old ground switch would be the perfect candidate. If it hasn’t had those things done, get ‘em done.
     
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  6. Willie Johnson

    Willie Johnson Tele-Meister

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    Point well taken; I think it's the 27k at the top left of the layout, but can't say for sure. I've got a Fender amp book somewhere down in the basement, so thanks for the schematic.

    When I got this thing 25+ years ago, it had EL34s in it and a 12at7 in v1. It sounded awesome, but always felt kind of blow-y upp-ey and hot, so when those started to go, I tubed it to spec. Interestingly, my tube chart says a 5u4gb for the rectifier; I've got one of the plug-in diodes in there. Anyway, thanks for the (negative?) feedback.
     
  7. drf64

    drf64 Poster Extraordinaire

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    yoink?
     
  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yup, the 2.7K.

    I would also take some voltage readings (if you're able) as that solid state rectifier is likely to boost the voltage in the amp, ESPECIALLY if it still has the original caps/cap can. You could easily exceed the voltage rating during startup until the tubes start to draw some current.

    With all those changes it should get a real good evaluation before any tinkering happens.
     
  9. wabashslim

    wabashslim Tele-Afflicted

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    NO NO NO! That's the bias resistor. You're looking for a 2700 ohm in the approximate center of the board, wired to the ext. speaker jack. But just maybe this is a job for someone else...

    "With all those changes it should get a real good evaluation before any tinkering happens."

    Totally agree.
     
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  10. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Ah, he says layout, not schematic.

    Please take the amp to a tech :D
     
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  11. Wally

    Wally Telefied Ad Free Member

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    A year and a half ago a fellow with a 1964 Princeton Reverb did not want to expend the funds to get his amp in proper working order. A couple of months ao, he spent a much larger amount having me bring it back to life with not just the recapping as I had earlier advised but also it’s a new power transformer.
    +1 on having some idea as to what is going on with a tube amp circuit when contemplating going inside to make changes.
    That said, it is your amp and your life.
     
    Last edited: Oct 8, 2020
  12. wabashslim

    wabashslim Tele-Afflicted

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    "That said, it is your amp and your life."

    That is the exact closing sentence I deleted from my previous post. Great minds think alike.
     
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  13. 72_Custom

    72_Custom TDPRI Member Silver Supporter

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    Not to put too fine of a point on it, but if one is not careful, it could spell the end of both of those things.
     
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  14. wabashslim

    wabashslim Tele-Afflicted

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    Ummm........
     
  15. TwoBear

    TwoBear Tele-Meister

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    If there's no hurry for the change, may I suggest jumping onto the internet and reading up on schematics and comparing them to layouts and just a little bit of background before you hop in there. It's super easy to read schematics, with sites that will teach you all the way in depth to different types of zener diodes and whatever... You might get bitten and start to want to make effects also. You might ramp up to the amp by first building a dim bulb tester if you don't have one. And an amp cradle, so when it shocks you it doesn't go flying off the table just kidding!!! But seriously, as others have noted the negative feedback is right there at the extension Jack-snip... But taking it from there and making it switchable, rather than just electrical taping or wire nutting it, take you closer to the filter caps and a good or bad shock. Just simple things like dry socks and insulating shoes and what you're standing on all that stuff helps. The last good shock I got like super strong was from my fingertip to my forearm-either a bead of sweat or a tiny hair because I'm not very hairy and my arm didn't touch Haha...meh... That being said we all start somewhere and it's great to know that people are there to help you and don't even putz around in there if you're unsure, answers come quick either here or El 34 world or wherever.
     
  16. Paul G.

    Paul G. Friend of Leo's

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    Honestly, I'm not seeing an awful lot of NFB there -- 2.7k resistor connected to a 1.5k bypassed cathode resistor with a 47R tail. I think pulling that would just add some hiss and that's about it.

    There is really no purpose to modifying an amplifier that isn't working properly in the first place. Changing your spark plugs won't help if you've got no compression. Put the amp away until you can afford to have it checked out.
     
  17. avspecialist

    avspecialist Tele-Meister

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    I would look at the Uncle Doug video on youtube on his idea of molding the negative feedback, either switchable using the ground switch or variable using a pot. I’ve had good luck changing the negative feedback resistor to 4700 ohms in several Fender amps. Usually it’s 820 ohms.
     
  18. ale.istotle

    ale.istotle Tele-Holic Silver Supporter

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    Really not sure if OP wants to yoink or yeet.
     
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  19. craigs63

    craigs63 Tele-Holic

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    NFB is a TLA I don't know. I have my own assumption what the "F" stands for.
     
  20. Engine Swap

    Engine Swap Tele-Holic

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    NFB = Nocturnal Flying Baloney
     
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