TECH VIEWS
Troubleshooting Tube Amps
Although I’ve never repaired amps for a living, I have repaired a lot of amps through
the years. I get a lot of questions e-mailed
to me from amp-repair novices about how
to fix amplifiers, and this was one reason
why I wrote the Tube Amplifier Debugging
page at:
http://geofex.com/ampdbug/
ampdebug.htm.
Through it, I hope to help the novices
out and give them a good start at how to
think about fixing amps. In addition, I have
advised a lot of people who do make a living repairing amps. These guys don’t ask
me about simple cases, so I get involved
with a high percentage of the odd or
tough cases, usually by email or telephone.
It doesn’t always work, but it’s a real kick
when I can help someone get to the bottom of an amp mystery.
Some of these are fun, like the time we
found an amp from a major amp maker
with intermittent crackling. It turned out
that the output tube sockets had never
been soldered. The connections to the
output tube sockets had been made with
the wires holding themselves in place in
anticipation of soldering. They functioned
for years that way, until finally vibrations
caused them to lose connection. Another
amp blew fuses when inside the cabinet,
but not outside the cabinet. None of the
usual suspects was a problem. It turned out
that the choke had a short to its grounded
core that opened up when the stress of the
mounting screws was removed.
I just ran into a new-to-me problem; this
may help you if you run into it. One of
our amps came in for warranty repair. The
owner needed it for a gig in two days,
and was very anxious. As you might guess,
it had failed suddenly on him about two
months earlier and only got to us when the
gig was two days away. Our repair tech
called me when the obvious fixes had no
effect at all. The tone stack on these amps
is similar to the traditional tone stack, with
bass, mid, and treble controls in the conventional arrangement. Turning the treble
knob down all the way made it very loud
and very bass-heavy. Turning the treble
knob up produced lower volume, but
almost no increase in treble until the sound
R.G. KEEN
suddenly went treble-y and thin most of
the way up.
This one is easy, right? New treble pot,
a couple of voltmeter checks and we’re
done. Not so. Replacing the treble pot had
no effect. Ah… one of the other pots, the
bass or mid, is bad. You might get that
behavior with an open on the bass pot
wafer. And while we’re at it, let’s check the
wires going from the PCB to the controls,
and measure all of the pots for total resistance and wiper remaining connected from
end to end of rotation. All of the pots and
wiring checked out. This was becoming
One of our amps came in
for warranty repair. The
owner needed it for a gig
in two days, and was very
anxious. As you might
guess, it had failed sud-
denly on him about two
months earlier and only
got to us when the gig was
two days away.
frustrating, even remotely by telephone.
Ah-ha! Shorted/open cap. Nope, no leaking
DC voltages, and all the parts looked to be
of the right value. Meanwhile, the owner is
standing there trying to look unconcerned,
and it’s coming up on closing time.
With no other option, I asked our repair
tech to start measuring resistances from
lead to lead on parts to be sure that there
was not a broken solder joint or some similar malady, even though he had previously
re-melted all of the solder joints on the
back of the board. All of the lead-to-lead
connections checked out. In desperation, I
asked for a check of all the resistances of
every single part in the tone stack. When
we got to the 100K slope resistor it measured zero ohms! Quickly removing it, we
measured the errant slope resistor and
found it measured—yes, you guessed, it,
100K. A quick measurement between the
pads on the board measured zero ohms!
Just to be sure I asked for one more measurement, and now it was open between
the pads. We were never able to get a
shorted measurement again.
Somehow, the glass-epoxy board had
worked perfectly for several months and
then developed an invisible short, which
vanished as we worked on it. Like other
shorts on circuit boards, this can be corrected with a few careful slices from an
X-Acto knife around one of the shorted
traces. I’ve seen open slope resistors, and
drifted (or drifting) slope resistors, but
never a shorted slope resistor. So in spite
of my kit-bag of experiences with amps,
I was a complete novice to this problem.
I’ve seen them shorted by other stuff, like
in the amp whose owner cleaned up the
panel with a little steel wool, or by beer
residue, but not by themselves in an otherwise clean amp.
In spite of our experiences, we are all novices when we hit that next funny kind of
problem that we’ve never seen before. To
solve these, you have to be open to dropping back to first principles, and testing for
things on a much simpler, less sophisticated level than you usually do. Going back to
basics is a very useful tool to keep handy in
your mental toolbox.
R.G. Keen
Chief Engineer
Visual Sound
visualsound.net
