ASK AMP MAN
JEFF BOBER
Diagnosing a Distressed Tube Amp
Hi there, erally short internally and cause a fuse to blow,
I have a Fender Hot Rod Deluxe that is just or they can become mechanically noisy and
six months old and well looked after. It has exhibit a “rumble” sound through the speaker
recently developed a popping, crackling when vibrated. Preamp tubes, on the other
sound when warmed up—even when there hand, will generally either become microphonic
are no guitars or cables plugged in. What is and make high pitched ringing or squealing
wrong? Could the valves be on the way out? noises, or will emit noises such as crackling and
Is it something more serious? popping during idling. If the level of the noise
Please help! is controllable in any way by the setting of the
Ken Morton volume or tone controls, then there’s a good
chance that the symptom is caused by a preamp tube and not an output tube.
Hi Ken,
Thanks for your question. The
Fender Hot Rod series of amps
is a very popular one, possibly
their most popular. More than
a few have crossed my bench
over the years, some coming in
for a “crackling” issue. There are
many types of failures that cause
this symptom. I’ll go through
most of the typical ones here
and tell you which ones I suspect
might be the cause of the failure
in your amp.
5. General component failure.
Any electrical component can fail at any
time, causing anything from compromised
operation to a complete shut down. I did
a bit of research on this question to see if
there are any known failures of this particu-
lar amp that I had not personally encoun-
tered, and it turns out that there may well
be. According to the research, there may
have been a bad batch of resistors used as
plate resistors in the phase inverter circuit
of the amplifier that were responsible for
crackling noises in some ampli-
fiers—a very strong possibility
for the cause of your symptoms.
1. Input, footswitch and effects
loop jack connections.
Jacks are probably the most
physically used and abused parts
in the amp. They are soldered
directly to the circuit board, but
are also attached to the front
panel via the jack nut. This is usually not an issue unless the jack nut loosens
due to vibration. Once the nut loosens slightly,
the weight and motion of the guitar cable is
supported solely by the circuit board solder
connections. Over time, these connections can
be compromised and cause crackling and an
intermittent signal.
Fender Hot Rod Deluxe
2. Broken solder joints on large components.
Another vibration-caused failure, the solder
connections of larger-sized components, such
as large 5-watt resistors, can eventually be
compromised from the constant vibration of
playing, or simply a rough ride in your trunk or
the band truck.
4. Stressed component failure.
Some components become stressed over time
when the typical operating parameters of the
circuit are exceeded. This can easily happen
with the failure of an associated component.
Take for example an output tube. While a typical 2-watt resistor (used by many companies as
an output tube screen grid resistor) will function
fine under most normal operating conditions,
that resistor may be pushed past its capabilities by constantly playing the amp at very high
volumes or by an output tube shorting. This
may cause excessive screen grid current to
flow through the resistor, stressing it or causing
complete failure. A stressed screen grid resistor
can definitely cause crackling noises to occur.
(As a side note, this is one reason to overbuild
amps: to ensure the least possible chance of
failures—using a 1-watt resistor instead of
a half-watt resistor throughout the amp, for
example, or a 5-watt in place of a 2-watt, etc.)
Having described the most likely candidates for the cause of
your amp problem, I would say
that based on your description
of the amp needing to warm
up first, and no guitar needing
be connected, the most probable in your case is either 3 or
5. The easiest way for you to
do some basic troubleshooting
would be to first purchase one
replacement 12AX7 tube. Then,
one by one, replace each preamp tube and see if that alleviates the problem. If you still
have the symptom, you should
next replace the output tubes.
For the purposes of troubleshooting, you can install a new set without
re-biasing the amp, but if they are indeed
the cause of the noise then you should definitely take the amp in for biasing. If neither
of the replacement tubes cure the problem,
the amp will need to be taken in to your
local experienced tech for further troubleshooting—but at least you’ll have yourself
a couple of spare tubes for the amp, which
no self-respecting tube-amp-playing guitarist should be without!
‘Til next time…
3. Simple tube failure.
Although a tube may look good from the
outside, there’s no way to tell for certain if it’s
good or bad just by looking at it. Tubes can fail
in many different ways. Output tubes will gen-
Jeff Bober
Co-Founder and Senior Design Engineer
Budda Amplification
jeffb@budda.com or www.budda.com
©2008 Jeff Bober
