Separate bias winding wiring.

Discussion in 'Shock Brother's DIY Amps' started by 5F6Animal, Sep 14, 2021.

  1. 5F6Animal

    5F6Animal Tele-Meister

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    Hello again to everyone. It’s been a long while since I’ve posted.

    I’m looking at the Mercury FTBP-59 as a new transformer for a new replica build. I was never aware of any original #8087 Triad PT with a separate winding for bias in any original 5F6A. If there’s amps out there, that’s just an interesting tidbit.

    In the past I’ve only used the Classictone 40-18029 with the 355vac HT winding.

    How does one go about wiring a separate winding for bias? Do I ground one end with the usual center taps and the other end to a silicon rectifier as usual? Kinda like the older Tweeds without a heater center tap.

    Thanks!
     
  2. 2L man

    2L man Tele-Holic

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    Yes other coil end to CT/ground and diode connected making negative voltage AND electrolyt match the negative as well ;)

    Resistor values are smaller than on bias circuit which is dropped from HV. I usually search few schematics and pick values there and change to match what my PT Bias output voltage is.
     
    Last edited: Sep 15, 2021
    King Fan and Timbresmith1 like this.
  3. 5F6Animal

    5F6Animal Tele-Meister

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    I entirely forgot to add that the amp is a 1960 spec 5F6A. As far as I’m aware, there’s never been any Fender amp with an entirely separate bias winding in the power transformer. I’ve looked at just about every schematic/layout available for Fender amps. I’ve physically opened up ~20 58 to 60 Tweed Bassman amps and the bias tap has always come off the HT winding.

    If Mercury built these transformers with a separate tap as a factory original replacement, it must have existed.
     
  4. 2L man

    2L man Tele-Holic

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    I found that PT outputs 52V 50mA. I will help you calculate circuit resistors and potentiometer value!

    I have few PT which have separate bias output coils but used only one and I did use a bridge rectifier because it was small and easy to use.

    To me 380-0-380 seems grazy high for a bassman!

    Transformer should produce 380-0-380 when it outputs rated current which means when amp idles PT output voltage is higher. Using GZ34 380-0-380 might turn to 470...480VDC when amp is biased to 35mA and idles at about 75mA.

    What OT impedance have you planned to use?

    Try to find out what voltage that PT output few currents?
     
    Last edited: Sep 15, 2021
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