Mod 6:
Swapping Tone-Stack Resistors
Another way to alter your amp’s frequency
response is to swap the slope resistor for one
of another value. In this picture of our Twin,
it’s the one with brown, black, yellow, and
silver bands being gripped by one lead with
needle-nose pliers.The part of an amp’s circuit that governs
the ranges of its tone controls is known
as the tone stack. This part of the circuit is
most commonly a combination of three
potentiometers (for bass, mid, and treble
knobs), three capacitors, and a resistor
called the slope resistor. One simple mod
that will change the tonal character of your
amp is to experiment with the value of the
slope resistor, which controls how frequencies
are divided over each tone control.
Simply put, the slope resistor changes the
slope of the midrange dip if it were
charted on a frequency-response chart.
Typical slope-resistor values range
from 33 kΩ to 100 kΩ. A larger value
yields a sound with more of a midrange
scoop (i.e., where treble and bass
frequencies are louder than the mids).
Smaller values accentuate midrange. In
our Twin Reverb, the vibrato channel’s
slope resistor is the 100 kΩ one (with
brown, black, and yellow rings) attached
to a 100 kΩ resistor on one end and two
blue .1 μf coupling capacitors on the
other. To change the value of the slope
resistor, follow the previous instructions
on how to replace a resistor.
Mod 7:
Removing the Bright Cap to
Tame Harsh Treble
To tame treble response in a Marshall head, simply clip or desolder the bright cap on the volume pot.
In case you decide to reverse the mod in the future, make sure you leave as much of the capacitor’s
leads intact (if you decide to clip it) to facilitate easy reinstallation.
If your amp has a treble response that
feels too harsh to your ears—especially
at lower volumes—you can tame it by
removing the bright cap. In a Marshall
amplifier such as a Super Lead, you simply
remove the capacitor that lies across
two legs of the volume pot. This cap
allows the high frequencies in the guitar
signal to bypass being attenuated by the
taper of the volume pot, so removing this
cap eliminates the amp’s severe-sounding
highs at lower volumes.
To remove a bright cap, simply desolder
the leads or clip them at a point
near the lugs on the pot. Be sure to leave
enough lead on the cap so that, if you
later decide to reinstall it, there will be
enough length left to be able to solder it
back into place.
Mod 8:
Adding Shielded Wire to
Reduce Noise
If your amp has a lot of hiss and background noise, you may want to check and see if the wire connecting the input jack to the grid of the first preamp
tube are made with unshielded wire. If so, replacing it with shielded wire should decrease noise. Here, we’re stripping the shielding from one lead prior
to soldering the connection, then tinning the gathered shielding lead that we’ll solder to the input-jack side.
Our final project here is a mod that will
subdue hiss or unwanted background
noise in your amp. A lot of the time when
an amp is plagued with this malady, it’s
because it uses unshielded wiring in key sections
of the circuit. Strategically replacing
these lengths with shielded wire is a fast,
easy way to improve the amp’s noise floor.
Perhaps the best place to start adding
shielded wire is the section going from the
amp’s 1/4" input jack to the grid of the
first preamp tube. The grid in question for
a 12AX7/ECC83 or 12AT7/ECC81 tube
socket will be pin number 2. Any noise
picked up in this part of the signal path is
passed through each of the amp’s gain stages,
getting amplified each time, so adding a
shielded wire here should yield significant
noise reduction.
To perform this mod on an amp like our
Twin Reverb:
• Snip the lead or desolder the wire
where it attaches to the input jack.
(A standard soldering iron will work
for desoldering, but a solder sucker/
desoldering pump will create a cleaner
joint for the new connection by
removing excess solder.)
• Snip or desolder the other lead where
it attaches to the grid pin of the preamp
tube. The grid on a 12AX7 will
be pin 2 or 7
• Solder the two leads from a length
of new shielded wire to the newly
vacated spots.
Ground the new wire by soldering the
shielding on the input-jack side to the
ground on the input jack. On a vintage
Fender-style amp, this is the lug that is
making contact with the chassis. Only
ground this shield on one end.
Note:
It’s a good idea to tin the
leads of the wire you are
installing before attempting to solder it
into place. To do this, simply wick a small
amount of solder onto each bare end of
the new wire. Tinning the new wire before
installing it improves the quality of connection
it makes in the circuit.
Note:
Often the shielding on
shielded wire is braided
and needs to be unwound. I like to use a
pointed object to get between the braided
fibers to unravel them. Once you’ve unraveled
enough shielding on the end that will be
attached to the input jack, gently twist the fibers
together to create one uniform shieldedwire
lead—which you’ll then want to tin.
Go Forth and Mod
I hope you’ve found some modifications
here that seem like projects worth pursuing
on one of your amps. Although these
projects yield pretty significant and impressive
results considering how little work is
involved, I know it can be pretty daunting
to poke around inside a device with
significant safety risks for the first time. The
safety measures we’ve outlined should alleviate
any danger, however if you have any
doubts about your ability to pull these off,
it’s always better to be safe than sorry. But
even if you decide to have a qualified tech
execute these mods for you, at least this
information will give you a better understanding
of some of the nuances and possibilities
of guitar amp modifying.