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Fender tweeds - why short the secondary...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by xk49w, Aug 16, 2008.

  1. xk49w

    xk49w Member

    Jun 10, 2008
    ...of the output transformer when the speaker is unplugged? This looks to be the case in the 5E5-A Pro, 5F4 Super, 5E7 Bandmaster.
  2. phsyconoodler

    phsyconoodler Member

    Jul 11, 2005
    calgary canada
    All Tweeds and all fender amps for that matter have a shorting jack on the output.If the speaker is disconnected the idea is that it doesn't go 'open' but instead goes to ground,which can save your output transformer.It works for quite a while,but don't test it unless you want a new OT.
  3. jay42

    jay42 Member

    Dec 20, 2006
    Sandy Eggo
    There are other approaches. Since Fender made so many combos, it's not that big a risk, but with heads I prefer either open jacks with a 250 ohm 5W (or greater resistor) across the output, or what Rivera does...at least on my Rake. There's a switch built into the primary output jack, which opens the output tube cathodes. It is effectively an extra Standby switch. That actually helps in the case where some dillweed plugs into the ext. spkr jack instead of the main one.
  4. hasserl

    hasserl Member

    Oct 24, 2004
    So Cal
    Doesn't help though in the case of a blown speaker (neither does the Fender shorting jack), The 250 ohm 5w will.
  5. mooreamps

    mooreamps Senior Member

    Nov 8, 2007
    Sunnyvale, Ca.
    I've wrote comments on this one before, because I believe it's worth it to mention.

    I've never liked either the shorting jacks, not the open jacks on the outputs. Both will cause the primary coil of the output transformer to spike. Since I've written my first comments on this, I've looked at this with an o'scope on one of my prototype builds. I can see when you plug into a shorting jack, it spikes the transformer when it goes from an electrical short, to a momentary open, and then to 8 ohms from the loud speaker, during the process of plugging in the speaker jack... I surmise this is the same process of how the points in an automotive ignition spikes an "ignition coil" to the sparks plugs !!!! As such, I will always wire in a 100 ohm load across one of output taps to keep the impedance jumps to a minimun.


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