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Tube Amplifier Crackling Noises – What to Look For

Here
are some things to look for if your tube amp is making intermittent crackling
type noises. These pesky noises can come and go or be triggered by certain
notes. There are a few things that can make an amp “crackle” and I will try to
give you guys some things to look at so you can try to diagnose the issue and
possibly fix it yourself.

On a vintage amp intermittent crackling can be caused by a few different
things. The first and most obvious is the tubes. You should always have a good
set of tubes in your amp. This reduces the chance for noise and crackling as
well as provides the best sound and reliability. In a vintage Fender amp “tweed
disease” can cause this unwanted noise to spoil your tone. There is a video
explaining this and the remedy and you can view it here:

Also remember to clean your tube sockets regularly. I like to use Caig
DeOxit. And while you’re at it clean the pots and jacks as well.

Here is a video showing how to clean a jack:

Another
thing to look at are the plate load resistors. These are usually a 100k value
and are the resistors that connect the DC supply to the plates of the preamp
tubes located on pins 1 and 6 of a 12AX7 or similar tube in most amps. Same
thing applies for older 6SL7 tubes or similar. More often than not the plate
load resistors back in the day were a carbon composition type and actually have
an inherent hiss type noise all their own, brand new out of the box. As the
Plate load resistors age (particularly the carbon composition type) they tend to
make more noise and can sound like frying bacon in the background. If you change
these with modern carbon composition equivalents the crackling will go away. Or
you can use carbon film or metal film resistors and never have to really worry
about the crackling or hiss again. The tone and to a certain extent the feel of
your amp WILL change. This is the subject of much debate among techs and
tweakers and I stand by my statement because I hear a change and I prefer the
carbon composition resistors most every time.

Some
modern amps can exhibit this sporadic crackling from the lower quality tube
sockets. I have seen this more than a few times with brand new amplifiers making
all kinds of weird noises. It is a laborious job that requires steady hands and
patience but by changing the sockets with higher quality phenolic or better yet
ceramic, you will be sure to greatly reduce this variable.

Another
source of crackling to look at, especially in combo amps, is the aluminum foil
on the amps back panel. This foil can drive you crazy if you don’t know to look
for the problem there. Some younger techs are unaware of it and trust me it can
be maddening. For example say you have the amp on your bench with the back panel
off and the amp plays fine. Sounds great, plays great and no noise. You figure
the owner is crazy or it was a fluke and the tube socket cleaning you just did
cleared it up. You mount the back panel on the amp and fire it up again. A few
minutes later just as you’re ready to turn the amp off and call the owner……its
back! This foil is supposed to shield RF and other extraneous noise. I find that
all it does is cause headaches and a piece of metal screen you get from a
hardware store is much better. Metal screen material was used on vintage amps,
particularly Fenders starting in the 1960’s. Anyway the problem is that the foil
does not make good contact with the chassis and will cause the crackling that
can come and go every minute or every month. To remedy this replace the foil
with the metal screen or put tape or thin weather stripping around the open edge
of the chassis where it comes in contact with the foil on the back panel.

Loose
components can also cause some crackling or even the signal dropping out that
sounds like crackling. Some components to look at are pots, jacks, transformers
and mechanically connected grounds. Make sure all these components are secure
and the mounting nuts are tight.  On the Fender blackface and silverface
amps there are mechanical ground lugs mounted to the chassis on the power
transformer studs with locking nuts. These can come loose form years of
vibration making an intermittent connection. The ground lug connections are
inside so you have to open up the amp to check them.

One of the last things I’ll mention is to look for bad solder joints. Over
the years a solder joint can oxidize and the connection will be either bad or
intermittent. I had a solder joint that was intermittent on a 1967 blackface
Vibrolux Reverb. It drove me a little crazy until I re-flowed almost every
solder joint on all the tube sockets. You could not see it was bad and I could
not identify the EXACT joint so I touched up every single one. Problem
solved.

I’m sure that there are some other case scenarios for crackling noises but
these are just some of the culprits you should look for. Always try different
guitars and cables before you start to take your amp apart or tear your hair
out. Sometimes it’s the simple, obvious things we look at last and then wonder
why we didn’t look at them from the start. I hope this helps some of you with
any crackling noise related issues. Please feel free to contact me if you have
any questions.

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