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B-15N crippled inside - tube amp advice needed

Discussion in 'Amps and Cabs [BG]' started by morebass!, Feb 9, 2015.


  1. morebass!

    morebass! I'm all ears Supporting Member

    May 31, 2002
    Madison WI
    Tonight, after playing for about 15 minutes at low volume my sweet '64 B-15N made a fairly loud click and completely lost volume, nothing left but a buzz. While checking to make sure the cords were ok I noticed a faint wiff of something burning. Oh crap. The power tubes were so hot that I felt the heat coming through the tube cage. So I turned it off and took off the cage and realized that everything was too hot to touch. This amp has been trouble-free for about 15 years when the transformer blew. It's probably been that long since I changed the tubes too but I don't play it much, maybe an hour a week. Does this sound like a transformer problem? tubes? Something else? Any help appreciated. Thanks.
     
  2. RoadRanger

    RoadRanger Supporting Member

    Feb 18, 2004
    NE CT
    Maybe something made the bias go away? In any case nothing you can fix yourself - off to the amp hospital I'm afraid :( .
     
  3. B-string

    B-string Supporting Member

    Transformer do pretty much two things; 1) Blow fuses. 2) Smoke pretty heavy if no fuse exists.
    Could be a diode, resistor, cap or tube. Were all the power supply caps changed when the transformer was replaced? If not you have some very old caps in there that need to be replaced at any rate.
    Can you post some pics of the underside?

    Calling @beans-on-toast
     
    Last edited: Feb 9, 2015
    Vintage-Blue likes this.
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Yes post some pics.

    A 1964 is a cathode biased model. There is a 250 ohm 10W resistor and 50uF 50V capacitor connected in parallel to ground on one side and the power tubes pin-8 on the other. Check that capacitor. Look for signs of black charing or an obviously blown power supply capacitor.

    Could be a resistor, cap or tubes. The last thing to consider is the output transformer.
     
    Vintage-Blue and B-string like this.
  5. Primary

    Primary TB Assistant

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