ASK AMP MAN
JEFF BOBER
Tube Amp Troubleshooting 101
Hi Jeff,
I don’t have much experience troubleshooting amps, as I’ve had very good luck
with all of mine, until just recently. I own
a Fender Blues Junior, which is a great
little amplifier. Recently, though, it died
at a rehearsal. It made a crackling sound,
then no sound. The power indicator lamp
on the control panel lights, preamp and
power tubes all glow, but no sound at
all. I’ve tried unplugging and quickly re-plugging the instrument input and speaker
cable, turning all knobs, and de-pressing
and releasing the fat switch. Still no sound
at all. With this fairly limited information,
do you have any thoughts as to what the
issue could be? I don’t abuse the amp, but
I do set the master volume at 4 or so ( 11
o’clock) to get a fairly loud clean sound.
in all of the tubes. An inoperative filament in
even one tube can cause the whole amp to
fail. If there are multiple tubes with filament
failures, substitute a new tube, one position
at a time, as some amplifiers place multiple
filaments in series, and a failure in one can
cause filament outages in multiple.
The next item to check, should the amplifier
have one, is a series effects loop. In most
series loops, if the switching contacts in the
jacks become dirty or oxidized, their ability
to mechanically close properly becomes compromised and signal will not flow properly
through them. This happens mostly in amps
where the loop is not frequently used, as the
jacks are not exercised and the switching
contacts don’t have the ability to self-clean.
Thanks for any help or suggestions
you can provide.
Keep smilin’,
Bill Hruz
Hello Bill,
Thanks for your question. I’m going to
use it as the foundation for a generalized,
amplifier troubleshooting 101, applicable
to most tube amps (and yours as well) and
executable by most players.
Since the power indicator lamp is lit, we can
be pretty sure the AC mains fuse is not blown.
Most AC mains fuses are placed in the circuitry
before power is applied to any part of the
circuit, so if something is lighting up when you
engage the power switch, it should be good.
Since there are numerous power indication
schemes, and numerous ways to illuminate
them, this doesn’t tell us much else. If there
is no indication of power in any way, the fuse
should be the first item checked.
Take any guitar cable (short effects jumpers work best) and “jump” the loop—plug
one end into the send jack and one into the
return jack. If the amp begins to operate,
then the problem could be dirty contacts
in one of the jacks. Spray a bit of electronic
contact cleaner into each jack (I prefer with
lubricant, if possible). Cycle a plug in and
out 8–10 times so that the contacts open
and close, and then leave the plugs inserted
in the jacks so that the contacts can dry
properly (anywhere between an hour to
overnight should be fine). Test the amp
again. If there are still intermittent problems,
the jacks may need replacing.
If you have a multimeter, check the resistance across the speaker(s). In a combo amp,
measure the resistance across the speaker
terminals (making sure that the speaker is
disconnected from the output of the amp). A
reading of 3 to 14 Ohms should be fine. For
an extension cabinet, attach a known-good
speaker cable to the cabinet and measure the
resistance between the tip and sleeve. Again,
if it is between 3 and 14 Ohms, the cabinet
should be functioning.
Next, we’ll move on to the speaker and
speaker connections. With a “no sound”
symptom, the amplifier electronics could be
working fine, but the sound has nowhere to
go. This can damage a tube amp, so you’ll
want to stay away from diagnosing the
problem by playing at high volume while
you bang on the top of the amp! A rousing rendition of “Smoke on the Backline”
can often follow. If possible, unplug the
speaker(s) connected to the amp and connect an alternate speaker using an alternate
speaker cable. If you now have output from
the amp, the cause of the failure is either
an open speaker (or speaker cabinet) or
open speaker cable. You’re actually as likely
to find a faulty speaker cable in a combo
as you are in a head/cabinet configuration.
I’ve seen plenty of “molded plug” cable
assemblies fail in combo amps.
If you’ve done all of the above and still
haven’t found the source of the problem,
the only other user-serviceable possibility is
to substitute a known-good preamp tube in
each preamp tube location, one at a time,
even if all the filaments are lit. Just because
the filament is lit does not mean the tube is
functioning. I wouldn’t suspect a faulty output
tube, as those will generally continue to blow
the fuse, but a faulty preamp tube can shut
down the audio path.
Next, you mentioned that the preamp and
power tubes all glow. This is an indication
that the filament voltage is being applied
to the tubes. This voltage, in some form, is
developed by one of the secondary windings of the mains (power) transformer. This
indicates two things: one, that AC power is
being applied to the primary of the transformer; and two, at least one of the transformer secondary windings is functioning. Be
sure, however, that the filaments are lighting
Those are the basic tube amplifier troubleshooting steps that every player can use.
Anything more than that needs the immediate attention of a qualified service techni-cian… (just wish I knew one!)
Play on, my friends…
Jeff Bober
Co-Founder and Senior Design Engineer
Budda Amplification
jeffb@budda.com or www.budda.com
©2008 Jeff Bober
