From Vintage Guitar Magazine.
"Capacitor upgrades for your Fender"
By Dan Torres
(The complete article is available from our literature page.)
Lots of you guys want to know how you can improve your amp without changing it. Kind of
impossible, you have to change something to make an improvement. But I know what you are talking
about, this is about upgrading the parts in the amp, staying with the original design, and improving the
Check out the large capacitors on your amp's circuit board. There are several different caps that Fender has
used over the years. Some are pretty good, some are great, and some are very lousy.
There are several factors determining the quality of a capacitor and it's performance.
"ESR" is "Equivalent Series Resistance" how much the capacitor acts like a resistor and cuts down
the audio signal in the amp.
"L" is "Parasitic inductance" again, how much the cap acts like an inductor (the opposite of a
capacitor) and degrades the audio signal. The larger the cap is the more "L" it will have.
"Insulation Resistance" Generally, now much the dielectric of the capacitor is insulated from the
outside world. Poor insulation will result in noise and can mask low frequency signals.
"Specified value" Not really a scientific property. Just how much the cap varies from what it is
supposed to be.
Heavy scientific stuff huh? You need to know that there are ways to determine what the cap is doing
to your guitar tone before you can fix it.
If you have a blackface fender, or one made close to 1965, you will find the real nice Fender tubular
blue foil caps. They have a white square area painted on them. It says "MOLDED" at the top and has the
specs written below it.
These are pretty good caps. Your amp will sound very good if it is all tubular blue foil caps. Their
insulation resistance is very high, no significant losses there. The L is medium. Pretty good but could be
better. ESR is fair - much better than some others Fender has used, but not the best. Very close to correct
specified value. They "deaden" the circuit a bit compared to modern high quality caps.
Late 60's, early 70's, and some Pre CBS Bassmen have the brown chocolate colored caps. These look
like a glob of melted chocolate. Not real well shaped and markings are real poor. Usually faded yellow
lettering. These are very poor caps. Very high ESR, very high L, terrible insulation resistance. An amp can
sound quite dead when these caps get old and go off of spec (all capacitors drift from specifications, some
more than others.)
Replace them if you can.
70's CBS amps, when Fender caught on "oh - you want the amps to be good?" May have the very
good dark blue mallory or sprague "drop" caps. Big and shiny, with white lettering on them.
("drop caps" means the capacitor looks like a drop of plastic, as if it dripped from a faucet and
hardened in mid-air, a term used to describe the appearance of the cap, not any of its qualities.)
These caps are fairly good stuff. Moderate ESR, medium L (they are fairly big and have some parasitic
inductance,) real good insulation resistance and fairly close to specified value. You could do worse than
So, how to improve them? Replacing some of the capacitors with better quality modern caps will give
your amp a tighter, richer tone - and in many cases far more volume and frequency response (better highs
Your kit includes our custom made Orange Drop capacitors - not the "industrial" electronic model found most commonly, but the audiophile and musicians favorite Orange Drop Series
All are made to our specs (very low tolerance, leads, etc.) and carry the Torres Brand.
If you are really into it, matching the two caps in the phase inverter circuit as close as possible will pay off in fantastic tone.
The signals passing through the caps should be as close to identical as possible in order for the push-pull
power tubes to work correctly. This can be as important, or more important than matched tubes! Fender didn't even think of matching caps. It is too labor intensive, and capacitor measuring
(included in the kit.)
I grab a handful of caps and begin measuring them with an accurate digital capacitance meter. I mark
them all as I am working and pick out a pare that match very closely.
They don't have to be exactly the specified value, but just both the same value. 20% caps can be off
either way. They could be .12 or .08 and still fall within the range or a "20% .1 cap."
If they aren't matched, you could easily have one of each, a .12 and a .08 with a difference of .04
between them. This will guarantee a completely unbalanced signal to the power tubes, and fairly lousy
Also in the circuit is a little 250 pico farad disk cap. As one of the audio guys has said, "disk caps don't
belong in audio circuits, they belong in trash cans." I'm not sure I agree, they can sound real good in some
amps, a bit of grainy breakup that adds to vintage tone.
But if you want a solid pure tone, replace it with a 500 volt silver mica cap. It will sound much
sweeter and perform better under strong distortion. The little disk caps do have very poor insulation
resistance and are microphonic when overdriven.