| Back to = the top=20 of the Amp Debugging Page | Back to the GEO Home=20 Page |
You can fairly quickly determine if a transformer is grossly bad. = This does=20 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,=20 you'll need an ohmmeter for the simplest testing, and for more advanced = tests, =20 a means of measuring AC voltage and current simultaneously, such as a = pair of=20 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=20 filament transformer to limit the power you put into the transformer = under test.=20
If you do the advanced tests, be aware:
Both the filament =
transformer and=20
the transformer under test will have at least AC line voltage on them, =
an may=20
well have much higher voltage, several hundred volts on one or more =
windings. =20
You are therefore in danger of being KILLED if you are =
not both=20
knowledgeable and careful about how you do these tests. These tests are =
clearly=20
hazardous as defined in the warnings section of this page.
Unplug the amp and drain the capacitors before you start. Do not = remove the=20 transformer from the amplifier before you start. Remove the output and=20 rectifier tubes.
Identify which wires are which by circuit connection. The primary = will have=20 two (single ended), three (push pull, or very rarely single ended with = ultra=20 linear tap), or five (push pull with ultralinear taps) leads. Noting = which=20 leads connect to the B+ line, measure the resistance from the B+ lead to = each=20 plate of the output tube(s). Write the resistance reading down. Measure = the=20 other side if it's puah pull. Plate windings are almost always in the = tens to a=20 few hundred ohms of resistance. A reading much over 1K is pretty sure = sign that=20 the winding is burned open. If it's open, the transformer is dead. = [There are=20 ways to fix "slightly-dead" transformers, but this is extremely risky, = and=20 probably doomed to failure. I'm not going to write down how. Don't = waste your=20 time.]
While you're at it, measure the resistance of every lead to the = chassis. This=20 reading should be very high, hundreds of K or preferably megohms. A low = reading=20 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=20 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=20 it passes all of the simple tests, it may still have an internal short = Sometimes=20 a primary will have enough resistance that the transformer does not burn = up or=20 burn the wires in two , but will just bog down, have low power and sound = bad. If=20 you suspect that is the case and you don't have the skills to do the = advanced=20 tests, take it to a competent tech.
Do the shorts/opens tests above first.
Then:
Identify a =
secondary=20
winding, 8 ohm tap if it's available, and hook up one and only one =
winding to=20
either 1/2 of the 6.3VCT or to the variac. Make D^&%ED sure the =
variac is=20
all the way down if you're using one of those. Make sure that no other =
leads are=20
connected (or shorted together, or touching your screwdriver on your =
bench=20
or... well, you get the idea). Check for safety load resistors on the =
output=20
jacks or secondary windings. These can cause you to erroneously think it =
is bad.=20
There must be no loads on any winding. Put your voltmeter on the =
winding, and=20
the current meter to measure the AC current through it, hook up the =
3.15 VAC=20
source, and turn on the AC - not to the amp, but to the AC feeding the =
winding=20
you're driving. The voltmeter should measure 3.15 (or close) volts AC, =
the=20
light bulb (if used) should NOT be lit brightly, and nothing should be =
humming=20
or smoking ;-). There should be only a little current going through =
the=20
winding. If the voltage is lower than 3 volts, or you are pulling amps =
of=20
current, then there is a load on the transformer, internally since you =
have=20
disconnected all the leads, meaning that there is an internal short. =
(Which=20
means that you ought to be sure that there is no other load on the =
transformer,=20
because you're going to throw it away if it has an internal short - be =
sure=20
you're right!)
If all is well, no smoke, flames, loud hum, poultergeists, or = lightning,=20 measure the voltage that now appears on the other windings. The = voltages will=20 be equal to the ratios of the voltages that will appear on these = windings in=20 normal operations. For the primaries of output tubes, this can be up to = a couple=20 of hundred volts, so don't think that you can relax your safety = precautions - it=20 can still kill you, even driven with 3VAC on the secondary of the = output=20 transformer. The half-primaries of the output transformer should have = identical=20 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=20 have about 4.5VAC on it.
If the transformer passes all these tests, it's almost certainly = good.=20