an experienced tech, because more
than likely the grid or screen-grid
resistor(s) have failed. If replacing the
output tubes has not alleviated the
problem, re-install the original tubes
and have the amp serviced by an
experienced tech.
An ‘Ove’ Glove or an everyday oven mitt is a
handy tool for dampening hot preamp and
power tubes to determine which is bad,
as well as for removing them once you’ve
isolated the offending valve.
Is the amp making rumbling or
glassy, high-pitched noises?
If so, narrow down the source of the noise
by turning down any internal effects such as
reverb or tremolo, as well as all volume con-
trols. With the amp in operating mode, hit
the top of the amp with your hand or fist
using moderate force. If this produces the
noise in question, read the steps below:
•;If the noise is of a rumbling nature:
It could indicate a faulty output tube.
For a quick fix, replace the amp’s
power tubes with a full set of matched
output tubes. If this alleviates the
noise, you’ve isolated and repaired
the problem. If the problem persists,
replace the original tubes and continue
troubleshooting.
•;If the noise is of a glassy or high-pitched nature: It is likely due to a
faulty preamp-type tube. Starting with
the preamp-type tubes closest to the
output tubes, tap each one with your
fingernail or the end of a pencil. If
one in particular produces the noise
For a quick fix,
replace the amp’s
power tubes
with a full set of
matched output
tubes. If this alle-
viates the noise,
you’ve isolated
and repaired
the problem.
•;A faulty rectifier tube. If your amp
uses tube rectification in its power
supply, a shorted rectifier tube can
cause a fuse to repeatedly blow. To see
if this is the case, locate the rectifier
tube in the amp—it’s usually the tube
closest to the mains (AC) transformer,
and it’s typically a 5U4, 5Y3, 5AR4,
GZ34, or 6X4. (Side note: It’s a good
idea to know what kind of rectifiers
are used in all your amps and to keep
one of each necessary type in your
spares kit.) After you’ve located the
rectifier, install a new fuse and a new
rectifier tube and try the amp again.
If it now functions properly, you’ve
found the cause of the failure. If not,
replace the original rectifier tube and
continue troubleshooting.
•;A faulty output tube. This is an
especially likely culprit if the fuse(s)
failing is an HT or output fuse. If you
suspect this is a problem, install the
new fuse(s) and replace the full set of
matched power tubes. If this corrects
the fuse-blowing issue, you’ve found
the cause of the failure. However, this
unfortunately does not mean the amp
has a clean bill of health. Some tube
failure can cause internal components
to fail. If you replace the power tubes
and notice lackluster performance,
an increase in hum, or one or more
tubes glowing exceptionally red,
the amp will need to be serviced by
in question, replace that tube. If this
alleviates the noise, you’ve found the
problem. If not, replace the original
and continue this process with the
remaining preamp tubes. If more than
one tube produces the noise during
the tapping process, a trick to nar-
rowing down the possible offender is
to attempt to dampen the adjoining
tubes while tapping. This can usually
be done by placing your hand on as
many of the tubes as possible except
the one you’re tapping on. If they’re
too hot to perform this barehanded,
If your amp is making loud, static-y sounds when you play, you may need to clean its tube
sockets with electronic contact cleaner.
