Foil Caps In Guitar Tone Controls Cause RF

Guitar Tone Caps - why you should NOT use foil capacitors!
When electric guitars were first introduced with ‘on-board’
volume and tone controls, the tone control was equipped with
a ‘wound foil’ capacitor. Because the capacitor is made using
a long length of coiled up aluminium foil, sandwiched inside a
thin polyester or Mylar membrane, the capacitor has a small
amount of undesired ‘inductance’ as well as the desired
Before you decide to rip out that ‘ceramic’ capacitor, think
carefully about what I have said! Save your money and do not
In the professional electronics world, it has long been known install ‘bumble bee’ or ‘orange drop’ capacitors. You could
that a ‘coil’ of induc#ve material like this can be very a$rac#ve find, depending on where you live or how close you are to
to radio frequency transmissions from far off shortwave radio radio transmi$ers, that installing them will make radio
sta#ons, taxis, aircra', etc. This is why most radio sets
interference more of a problem. Even the #me of the year
working on the old Long, Medium and Shortwave frequency
can make RF interference much more prevalent… so why
bands employ coils of wire inside them. They are used to help increase the risks?
tune in the sta#ons and could not work without them.
Ceramic capacitors are used extensively (or should be) inside
The guitar’s Tone control capacitor’s capacitance combined
guitar amplifiers too. They are used to decouple (short to
with this small amount of inductance forms what is known as ground) RF from the power supply rails. This job cannot
a ‘resonant tuned circuit’ - just like inside the radio set. That
(should not) be done with foil capacitors for the same reasons
tuned circuit’s resonant frequency is o'en coincident with the already given! CERAMIC CAPS ARE NOT NASTY EVIL THINGS
transmissions from radio sta#ons or other broadcasters.
When ‘ceramic’ capacitors came along, the industry quickly
changed to them… for two reasons. They were cheaper and,
more importantly, they are NOT inducve. Therefore,
switching to these meant that the risk of induced unwanted
interference from far off shortwave radio sta#ons was
prac#cally eliminated!
“Foil Caps Are Best, Everyone Says So!”
Sadly, there’s a lot of ‘opinion’ out there, but forums are the
best place for misinforma#on. They are where ‘semi-skilled
brain surgeons’ live! And in any case, because something is
widely believed, doesn’t make it right! People believed the
world was flat once… now, there was a surprise!
“Foil Capacitors Are A Must Have Tone Improvement!”
Take my word as an electronics professional, that’s simply not
true! “But we’re told that foil capacitors are essen#al for
great guitar tone.” Well, yes, they are be$er quality for good
high frequency applica#ons, but they are only brought into
use when the tone control is set to minimum treble. In which
case the treble control, through its capacitor, is shor#ng all
the high frequencies to ground. You don’t need a high quality
capacitor for this purpose. So you’ll never hear their superior
performance… ever!
At maximum treble, the capacitor is out of circuit en#rely but
can s#ll induce RF (radio frequencies) into the instrument.
To avoid RF interference… I urge you to save your money and
do not install ‘bumble bee’, ‘orange drop’, or like capacitors
into your guitar!
Stewart Ward - Guitar amp designer since 1967