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unkown output impedance measurement?

Discussion in 'Electronic Repair' started by Henry Kolesnik, Nov 2, 2008.

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  1. What the easiest ways to measure the output impedance of a push pull
    tube amplifier?
    Thanks
    Hank
     
  2. Engineer

    Engineer Guest

    Henry, do you mean the true Thevenin O/P impedance of the unit as a
    "near voltage" source, i.e. very low indeed, less than, say, 0.5 ohm?
    Or do you mean what is the nominal speaker impedance that it is set up
    for, e.g. 4, 8 or 16 ohms?
    Cheers,
    Roger
     
  3. normal speaker impedance
    Henry, do you mean the true Thevenin O/P impedance of the unit as a
    "near voltage" source, i.e. very low indeed, less than, say, 0.5 ohm?
    Or do you mean what is the nominal speaker impedance that it is set up
    for, e.g. 4, 8 or 16 ohms?
    Cheers,
    Roger
     
  4. This is in an old military radio which I suspect is not 3.2 ohms but
    somewhat higher like maybe 600. It's kind of hard to get to the
    primary. I guess I should have stated that at the beginning. I kind of
    recall some way of measuring the open circuit voltage from a tone and
    then with a pot connected across the output, set the pot to where the
    voltage is 1/2 and then measure the pot. Do I have that correct?
    Hank
     
  5. N_Cook

    N_Cook Guest

    I would go for a set of watty resistors in series, tone source and a DVM on
    AC volts, with amp set at low output.
    If on some set of resistors of total value of R , the o/p voltage is greater
    than with value R+d and R-d , where d is about R/4 then R is the output
    impedance. If highest at R-d then go down, in steps, till it peaks, if R+d
    is highest then go higher for peak value
     
  6. Charles

    Charles Guest

    Load the amplifier with an adjustable resistor. When the loaded output
    voltage drops to 1/2 of the no-load voltage, the resistor can be removed and
    measured with an ohmmeter.
     
  7. Ian Jackson

    Ian Jackson Guest

    Just in case the amplifier is 'unhappy' with such a low load, it might
    be better to load the output with a resistor low enough to produce a
    less severe - but measurable - drop in output voltage (say by 10 or
    20%). Then do a simple 'pot-down' calculation to get the output
    impedance.
     
  8. It's a BC-794 but I think it's been modified and if not something else
    is kaput.
     
  9. Engineer

    Engineer Guest

    Sorry, not so. This will only give you what I originally mentioned,
    i.e. equivalent generator source impedance (see Thevenin), and it will
    be very low - no use in selecting speakers. You need the O/P
    transformer (OPT) ratio. Make sure the receiver/amplifier is OFF. Use
    a filament transformer run off a variac to energise the OPT secondary
    (use the speaker terminals) - keep it low, say 2 to 4 VAC. Measure
    the OPT primary and secondary voltages at a few levels. Calculate
    each ratio and average them. Figure out the correct plate load for
    the O/P tube (not covered here) - it will likely be in the range 4
    Kohms to 8 Kohms, call it Rp. Then the correct speaker load will be
    Rp/OPT ratio squared. Ex: Rp = 5000 ohms, OPT ratio found to be
    36:1. Then, speaker should be 5000/(36)^2 = 5000/1296 = 3.86 ohms.
    So use a 4 ohm speaker.
    Cheers,
    Roger
     
  10. Ian Jackson

    Ian Jackson Guest

    In message
    Surely this IS the output impedance?
    But what does the invariable negative feedback (from the OPT secondary
    to an earlier amplifier stage) do to the output impedance?
     
  11. Roger
    50 years ago I wound and rewound many a transformer with great success.
    But today I'm older, more patient and lazy and looking for an easy way
    out that may not be totally accurate.
    But I kind of recall reading an excellent article with a trick that I
    thought was pretty ingenious but I never saved it. It may have been a
    ham mag, or Howard Sams booklet or maybe Rufus Turner. Maybe someone
    will remember.
    Hank
    Sorry, not so. This will only give you what I originally mentioned,
    i.e. equivalent generator source impedance (see Thevenin), and it will
    be very low - no use in selecting speakers. You need the O/P
    transformer (OPT) ratio. Make sure the receiver/amplifier is OFF. Use
    a filament transformer run off a variac to energise the OPT secondary
    (use the speaker terminals) - keep it low, say 2 to 4 VAC. Measure
    the OPT primary and secondary voltages at a few levels. Calculate
    each ratio and average them. Figure out the correct plate load for
    the O/P tube (not covered here) - it will likely be in the range 4
    Kohms to 8 Kohms, call it Rp. Then the correct speaker load will be
    Rp/OPT ratio squared. Ex: Rp = 5000 ohms, OPT ratio found to be
    36:1. Then, speaker should be 5000/(36)^2 = 5000/1296 = 3.86 ohms.
    So use a 4 ohm speaker.
    Cheers,
    Roger
     
  12. I just recalled I have an old GR 583-A Output Power Meter and if it
    still works it'll tell me. It's been wasting gravity for years, Google
    it and you'll see one on Sphere. Thanks to all for the comments, and
    I'd like to know any tricks?
    Hank
     
  13. Engineer

    Engineer Guest

    Ian, it reduces the source output impedance of the amplifier as a
    voltage generator (increasing the damping factor) but does not affect
    the speaker impedance to be used. The speaker impedance determines
    the O/P tube plate (anode) load via the OPT ratio.
    Cheers,
    Roger
     
  14. Ian Jackson

    Ian Jackson Guest

    In message
    But the original question was "What the easiest ways to measure the
    output impedance of a push pull tube amplifier?" It didn't mention
    loudspeakers.
     
  15. Engineer

    Engineer Guest

     
  16. Neil Sutcliffe was kind enough to send me a manual on his GR 783A which
    is a more recent version of my GR 583A Output Impedance Meter. The o/p
    xfrmr on my Hmmarlund Super Pro 210X Type O is 10 ohms and I looked
    inside and saw no evidence that it was changed. I confirmed that the
    meter works by the same technique on my Ten Tec SP 325 which is 600 ohms
    and it measured 600 ohms. The GR 583A is a neat piece of gear, easy as
    pie to use.
     
  17. I'll see you on the third half of the show
     
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