Re: unkown output impedance measurement?

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" <junk2007@xxxxxxxxxx> wrote in message news:32b2cc1a-3eee-47a1-ba86-eb96907cd648@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Nov 2, 5:47 pm, "Charles" <charlesschu...@xxxxxxxxxxx> wrote:
"Henry Kolesnik" <koles...@xxxxxxxxxxxxxxxxxxx> wrote in message

news:Kj7Pk.6569$be.3512@xxxxxxxxxxxxxxxxxxxxxxx

> 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

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