OK...
It's usually something pretty obvious when a Twin isn't
making any sound at all. First - are you *sure* that
there's *nothing*? Not even a little bit of hum when you
take the amp off standby? Does it make a soft "thump"
or other noise when you flick the standby switch to the
"play" position? If so, you're probably looking at a pre tube
failure, so pull all of the tube shields (the metal can-looking
thingies that start at the right-hand side of the amp, looking
from the rear) and eyeball the tubes. If you see one that has
white flaky-looking stuff inside the glass at the top, rather
than the silver or black-looking stuff the rest of them have,
that's your boy. The silver/black stuff is a compound which
absorbs oxygen, with an eye towards keeping the vacuum
inside the bottle as hard as possible. When a tube cracks,
this compound (called the "getter") turns white in a matter
of minutes. This means the tube is full of air, and it won't
work at all that way. If you find one that looks this way,
replace it. Next - are they all lit? Be aware that some
preamp tubes can show very little or even NO glow due
to the fact that the filament wires may not be sticking
out of either (or both) end(s) of the cathode sleeves; if you
see a preamp tube that appears not to be lit, grab it and
see if it's hot. If it is, it's probably working (if it hasn't
turned white). Just for shits and grins, remove and re-seat
all of the pre tubes. ONE AT A TIME, please, so they stay
in the correct order. And this is a good time to check to
make sure the tube types are in the designated positions
according to the tube chart - you should have 12AX7/7025s
in positions 1, 2, 4, and 5 (counting right to left) and 12AT7s
in positions 3 and 6.
Next up, check the speaker connections. First thing
to do here is to unplug the speaker wire from the
main output jack (keep the amp on standby) and run
a cable - any sort will do for this test, doesn't have
to be a "speaker" wire - from the speaker output jack
to another speaker box of some kind - PA, guitar,
home stereo, etc. - to see if you get sound when
you take the amp off standby. If you do, you can be
sure that you have an open or short circuit in the amp's
speaker wiring, and it will be in the plug or the twisted
wires that go down to the speakers. (If there was an
open in the wires that go from one speaker to the
other, you'd still get sound from one speaker; a short
here will also knock the output dead, but it's not likely
at all.)
Other things that can kill the amp's sound:
Bad preamp tube - you could have a failed
tube in position 2, 4, or 6 (counting from the
right, facing the amp from the rear); it's perfectly
possible (in fact, it's common) for a tube to light
up and still not pass any signal, so "glow" is not
a definitive test here.
Bad standby switch. Not likely, but not unheard of.
A wire has come loose from the eyelet board
or one of the pots.
A component has failed somewhere.
Transformer failure is *not* likely...but not impossible, either
Most of the time when a power transformer fails, it'll blow
fuses. (Not always.) If an output transformer fails, it depends
on the failure mode whether or not the fuse will blow, but it
won't blow until you take the amp off standby in any event.
A shorted secondary is the most common failure in output
transformers; you can usually still hear something, though,
if you have the amp cranked up. The output will be very
low, and heavily distorted.
Bad guitar cord. (Don't laugh...I get one or two a month
from people who didn't try another one before they decided
their amp needed to see the doctor.)
Nothing you can touch without taking the chassis out of the
wood is going to zap you. Only the #3 preamp tube gets hot
enough to make you say OUCH if you grab it too hard. The
power tubes will get *very* hot with the standby switch on;
if they don't (takes about 5 minutes) that's a pretty good
indication that you have no plate or screen voltage for
whatever reason. Tech time.
If you have further questions, give me a buzz:
303-778-1156
Lord Valve