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Music Man 2100-65 (12AX7 driver) HT voltage doubler output reduction

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  • Music Man 2100-65 (12AX7 driver) HT voltage doubler output reduction

    This nice old Music man came in with a bodged nonstandard mains transformer fitted messily, not producing anything like the 725 volts these things like to run on their 6CA7s. I have fitted the TAD mains transformer which seem to be a compromise type to fit a variety of models, and took the PSU wiring back to stock with new parts.

    The HT supply is via a voltage doubler as you can see.

    My problem is the HT is now 840vDC. Everything works fine but the output tubes - JJ 6CA7s - don't like it. One shorted. Any ideas for reducing this to something more like 700vDC?

    Click image for larger version

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  • #2
    Is the heater supply at 6.3VAC, or is it high as well? If the latter, you could put a bucking transformer in series with the primary of the PT (if you can find a place for it).

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    • #3
      Is that PT for a 100W amp? I don't see what 50W model it could be designed for with that high of an output voltage?
      Can you link to the PT you used?
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


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      • #4
        Just checked their xxx-65 model, it claims 285 VAC secondary which translates to 403V straight or 806V doubled.Add the typical 250V plus UK mains and there you have your extra voltage.
        Which is deadly.

        Specs claim
        0V Black
        220V Yellow
        240V Brown
        300V White

        not sure whether you are using the 220V or the 240V tap.
        Juan Manuel Fahey

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        • #5
          You could use a FET dropper but you'd have to manage the heat. Or a bucking transformer on the secondary. The bottom like here is that the TAD transformer isn't really suitable for the advertised application. The secondary should be more like 250V to give 700V on the plates.
          Experience is something you get, just after you really needed it.

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