BF Princeton R reverb knob adds "gain".

Discussion in 'Amp Tech Center' started by BobbyZ, Jul 31, 2020 at 7:54 PM.

  1. BobbyZ

    BobbyZ Doctor of Teleocity

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    First up its a 1964, all new electrolytics 6-7 years ago, including the cap can.
    Haven't used it for maybe 6 months, was just playing it at like 2 or 3 and noticed it seemed kinda "ratty". Messing around I tried it with the reverb off and it seemed normal, that got me switching the driver tube, a 12at7 for a 12au7. Then I started turning up the reverb knob, which normally is on 2ish all the time, noticed the volume goes up with the reverb knob and adds distortion.
    Just thought before I tear into it, maybe one of you gents has run into this before.
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Does the reverb also function too as the knob is turned up? Something weird with the pan would be my first guess...but let us know what you come up with.
     
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  3. jondanger

    jondanger Poster Extraordinaire Ad Free Member

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    I had this problem with a Traynor Guitar Mate, and a Vibrolux Reverb. In both cases it had to do with a grounding issue in the cables/jacks to the pan but I can't remember which side of the pan it was on.
     
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  4. jondanger

    jondanger Poster Extraordinaire Ad Free Member

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  5. BobbyZ

    BobbyZ Doctor of Teleocity

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    I'll check that, thanks!

    @corliss1 Yeah the reverb works fine, other than the added gain thing.
     
  6. bwacke

    bwacke Tele-Meister Gold Supporter

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    BobbyZ, try cutting out the cathode bypass cap on the driver. If it's shorted, it'll distort like crazy. Check the value of the cathode resistor, too. They overheat and change significantly.
     
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  7. King Fan

    King Fan Poster Extraordinaire

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    Three excellent ideas here. Reverb pans and cables? Usually. Grounding? Always. 1964 bypass caps? Seriously.

    If none of those pan out (no pan pun intended) it seems *much* less likely but I might poke around V3a also. Still, seems like maybe it's before the pot, no?

    Better yet, if no joy, post some voltages and pics -- the pics just to give us PR freaks something to drool about. Homer Simpson voice: "Ooooh, sweeeet '64..." :)
     
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  8. BobbyZ

    BobbyZ Doctor of Teleocity

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    @bwacke The electrolytics were all replaced 6 or 7 years ago when I got it, did that as soon as the cap can came in the mail. Have you had issues with 25-25s that new? I used Sprague there, the can's a C&E and the bias cap an F&T.
    Going to mess with a little now before my day gets busy.
     
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  9. BobbyZ

    BobbyZ Doctor of Teleocity

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    Well that was easy to find @jondanger nailed it right off, reverb cable right at the output is bad.
    I've got some of those RCA ends just like Fender used back on the day, have to shorten the cable a bit and solder a new end on.
    Did I mention I HATE soldering those old style RCA plugs on that old cable. Lol
    Last time I had trouble at the jacks the shield on one wasn't making contact to ground and the DR I was messing with stated playing the Coffee Time program from KPRM 870 AM.
    Old Ampegs let out a squealing noise that'll clear a city block with a bad reverb cable.
    But I don't recall one doing this before, the reverb pot acting like a volume pot.
    Thanks for the help everyone !
     
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  10. BobbyZ

    BobbyZ Doctor of Teleocity

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    The original 1964 electrolytic caps are sealed in a Zip Lock bag somewhere, along with some untouched solder joints. :)
    This was one amp I hated to service because I was possibly the first one inside since it left Fullerton. But at the end of the day I want an amp to work the way it should.
     
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  11. King Fan

    King Fan Poster Extraordinaire

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    Good for you! And @jondanger !!!

    Still wouldn't offend us to see a few pics. Just sayin'... :):):)
     
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  12. bwacke

    bwacke Tele-Meister Gold Supporter

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    Well done, guys. I see so many old reverb system problems lately that I'm building a complete external outfit (driver, cables, spring unit, return amp) so I can troubleshoot in one, quick (hopefully) session. The 90's and 2000's Devilles with their soldered-in cables and op amp drivers and recoveries are still a giant pain.

    Speaking of pain...don't burn your fingers, BobbyZ. :>)
     
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  13. BobbyZ

    BobbyZ Doctor of Teleocity

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    No kidding ! A smarter person would just get new cables to run from the amp down to the pan.
    I'm not smart.
    New cables in a 1964 amp look wrong to me, plastic ends with a 90 degree bend built in? No that just makes way too much sense.
    Nope I'll find that zip lock bag of NOS RCA jacks I scored in a thrift store and make um look factory, burning the crap out of my fingers in the process. That way the cables can stick straight out the back, get bent and cause problems later on, just like Leo intended. :)
     
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