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Signal cap Polarity in Blkface Fenders?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Frostbite Slim, Jun 16, 2006.

  1. Frostbite Slim

    Frostbite Slim Member

    Sep 10, 2005
    Ft. Bend, TX
    Could someone please explain this as it applies to blkface Fender amps? I was told by a reliable source that the outside foil goes to the LOWER DC POINT.

    For instance coming off a plate the outside should NOT be connected to the plate as it's the higher-potential.

    As I understand it, this would mean that nearly ALL the caps (tone stack and coupling) in an AB763 circuit would be orientated such that the outside foils all point toward the pots (the less potential DC point).

    Is this correct or am I missing something? If you look at pics of old Fender chassis, you see that they have the outside foil connected TO the plates (pointing toward the pre-amp tubes).

    I'm really confused and would like a simple clarification.
  2. Dave C

    Dave C Gold Supporting Member

    Jun 10, 2005
    Waterford Ct
  3. billdurham

    billdurham Member

    Dec 29, 2005
    Man.... I would think you would need to do some double blind tests before I would accept that conclusion. Tone is so subjective that it is just very hard to say that something like this sounds better. Now if you fed the amp some pink noise and compared the output with a sprectrum analyser... and you saw a difference in the response curve...then I think you could honestly say it sounded "different"...but not necessarily better.

  4. ekp

    ekp Member

    Sep 2, 2003
    Berkeley Springs, WV
    The outer foil should go to the lower impedance place to minimize the effects of pickup of stray fields. Hooking it to the lower DC point is a safety issue that isprobably nothing to worry about since the cap encapsulation should handle it.
  5. donnyjaguar

    donnyjaguar Member

    Nov 4, 2005
    Yes, its to do with stray fields. Generally if the cap is attached to the plate of one voltage amplifier tube it should be outside foil as this impedance should be lower than the following circuit input impedance.

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