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.

--
Thanks & 73
Hank WD5JFR
"Henry Kolesnik" wrote in message
...
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
"Henry Kolesnik" wrote in message
news
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
"Engineer" wrote in message
...
On Nov 2, 5:47 pm, "Charles" wrote:
"Henry Kolesnik" wrote in message

...

What the easiest ways to measure the output impedance of a push
pull tube
amplifier?

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.

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