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How to tell safe output impedance?

Discussion in 'Amps and Cabs [BG]' started by ihixulu, Feb 28, 2008.

  1. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    South Shore MA
    How would one tell what the operating impedance for an amp is if you don't know its make and model?

    Specifically, I have a couple of small amp chassis, tube and SS, and I am wondering how to measure (if that's even possible) the impedance of the OT on the tube amp and the safe low impedance for the SS?

    No names on the boxes, I'm guessing they might have been kits...?

    Any help would be appreciated.
  2. Hi, ihixulu.

    If those are bare chassis 19" or 21", good chances are that they're 70V or 100V line amps.

    If they are for speaker loads then:

    For SS (no OT) I'd just test it with a scope and a signal generator (and a dummy load), 8 Ohm is a safe starting point. On really old perhaps 16 Ohm.

    For tube OT (or SS OT) You'll first have to find the taps for different impedances. That can usually be determined with a multimeter. Then off with the tubes obviously, and cabs bled etc. etc.

    Then You measure the voltage ratios of the OT with a signal generator. 10:1 voltage ratio equals 1000:1 impedance ratio for example. Then You find out the plate to plate impedance of the output tubes and do some additional easier math. If the plate to plate impedance is 4000 Ohms for example, the tap you were measuring was the 4 Ohm tap.

    I've read about this in the GT tube amp book, not the best book I've heard but it was all that I had in the 80's.

    That's the "gearing" method. Some amp manufacturers didn't design their circuits for "correct" impedance gearing between the load and the tubes for various resons.


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