Subject: Output Transformers
From: "Saved by Internet Explorer 11"
Date: Tue, 24 May 2016 15:46:31 -0700

Testing Output Transformers

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You can fairly quickly determine if a transformer is grossly bad. This does
not say anything about the subtleties of it's tone, only that it is functioning.
Tests of relative tonal "goodness" are also possible, but require a lot of
equipment and experience to do correctly. For the tests described here,
you'll need an ohmmeter for the simplest testing, and for more advanced tests,
a means of measuring AC voltage and current simultaneously, such as a pair of
VOMs or DMMs, and a 110/120 to 6.3VCT filament transformer, and either a variac
(variable transformer) or a light bulb socket in series with the primary of the
filament transformer to limit the power you put into the transformer under test.

If you do the advanced tests, be aware:
Both the filament transformer and
the transformer under test will have at least AC line voltage on them, an may
well have much higher voltage, several hundred volts on one or more windings.
You are therefore in danger of being KILLED if you are not both
knowledgeable and careful about how you do these tests. These tests are clearly
hazardous as defined in the warnings section of this page.

Simple Tests

Unplug the amp and drain the capacitors before you start. Do not remove the
transformer from the amplifier before you start. Remove the output and
rectifier tubes.

Identify which wires are which by circuit connection. The primary will have
two (single ended), three (push pull, or very rarely single ended with ultra
linear tap), or five (push pull with ultralinear taps) leads. Noting which
leads connect to the B+ line, measure the resistance from the B+ lead to each
plate of the output tube(s). Write the resistance reading down. Measure the
other side if it's puah pull. Plate windings are almost always in the tens to a
few hundred ohms of resistance. A reading much over 1K is pretty sure sign that
the winding is burned open. If it's open, the transformer is dead. [There are
ways to fix "slightly-dead" transformers, but this is extremely risky, and
probably doomed to failure. I'm not going to write down how. Don't waste your
time.]

While you're at it, measure the resistance of every lead to the chassis. This
reading should be very high, hundreds of K or preferably megohms. A low reading
here indicates a short to the transformer core - again, dead transformer.

Do a similar test on the secondaries - look for open windings, or shorts to
the core. Finally, test for the resistance from primary to secondary. A low
value, under a few K indicates an internal short, and a dead transformer.

If the transformer fails any of the simple test, it's dead. Replace it. If
it passes all of the simple tests, it may still have an internal short Sometimes
a primary will have enough resistance that the transformer does not burn up or
burn the wires in two , but will just bog down, have low power and sound bad. If
you suspect that is the case and you don't have the skills to do the advanced
tests, take it to a competent tech.

Advanced Tests

Do the shorts/opens tests above first.
Then:
Identify a secondary
winding, 8 ohm tap if it's available, and hook up one and only one winding to
either 1/2 of the 6.3VCT or to the variac. Make D^&%ED sure the variac is
all the way down if you're using one of those. Make sure that no other leads are
connected (or shorted together, or touching your screwdriver on your bench
or... well, you get the idea). Check for safety load resistors on the output
jacks or secondary windings. These can cause you to erroneously think it is bad.
There must be no loads on any winding. Put your voltmeter on the winding, and
the current meter to measure the AC current through it, hook up the 3.15 VAC
source, and turn on the AC - not to the amp, but to the AC feeding the winding
you're driving. The voltmeter should measure 3.15 (or close) volts AC, the
light bulb (if used) should NOT be lit brightly, and nothing should be humming
or smoking ;-). There should be only a little current going through the
winding. If the voltage is lower than 3 volts, or you are pulling amps of
current, then there is a load on the transformer, internally since you have
disconnected all the leads, meaning that there is an internal short. (Which
means that you ought to be sure that there is no other load on the transformer,
because you're going to throw it away if it has an internal short - be sure
you're right!)

If all is well, no smoke, flames, loud hum, poultergeists, or lightning,
measure the voltage that now appears on the other windings. The voltages will
be equal to the ratios of the voltages that will appear on these windings in
normal operations. For the primaries of output tubes, this can be up to a couple
of hundred volts, so don't think that you can relax your safety precautions - it
can still kill you, even driven with 3VAC on the secondary of the output
transformer. The half-primaries of the output transformer should have identical
voltages on them. Secondaries should have multiples of the voltage you're
putting on the tap you chose. If you used an 8 ohm tap, a 4 ohm tap will have
about 2.3VAC on it, a 2 ohm tap will have 1.6VAC on it, and a 16 ohm tap will
have about 4.5VAC on it.

If the transformer passes all these tests, it's almost certainly good.