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Ampeg VT-60 bias issue

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  • Ampeg VT-60 bias issue

    Hello folks,
    I am wondering if anyone has had issues with the bias adjustment on this 1x12 Ampeg combo.

    It appears it is like the HiWatts where they were originally designed for tubes that naturally have lower idle bias current, but with the newer tubes no longer produce enough neg bias voltage to properly bias the tubes. IOW, the bias current is too high at the highest bias voltage setting.

    The secondary tap is only about -40vac which by the time you rectify it & filter it & such, is about -35V or so. Lifting the divider resistor to ground & or jumping the series resistor in the bias supply section only just barely get the tubes to bias at 35ma. Then the supply is no longer 'stiff' & too unstable to be safe that way.

    In lieu of building a doubler circuit in there I got lucky & found that Lord Valve (Fat Willie @ NBS Electronics in town here in Denver) had a set of 6L6 JJ's that had a naturally low idle bias reading that he couldn't get rid of (8ma on his tester). Got them cheap & all was fine. Of course the next time will be an issue.

    Just wondering if anyone has built a doubler for this amp and what that looked like. Thanx, glen

  • #2
    If you have a bias winding of 40VAC, why do you wind up with only 35VDC? You are not using a 5U4 to make bias, are you? 40VAC ought to net you about 56vDC.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      hey Enzo,
      I don't recall the exact figures, but there definitely wasn't enough voltage after rectification to do the job. It is diode rectified of course. thanx, glen

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      • #4
        Oh I easily accept a lack of voltage, i was just tryimg hard to accept your numbers.

        And no, I have not done that to one of those.
        Education is what you're left with after you have forgotten what you have learned.

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