It’s in a guitarist’s nature, I believe,
that we can’t leave well enough alone.
Most of us have an ideal sound (or
sounds) in our heads, and we won’t rest
until our vision is realized. We can have
a perfectly fine guitar or amplifier, but
we still have an inherent urge to tinker
with it until it’s “just right” in feel or
tone. On this premise—as well as the
fact that many of us are on budgets that
don’t allow us to buy every amp that
strikes our fancy—the idea of modifying
an amp we already own strikes a
very appealing chord for many players.
Of course, before beginning any
sort of amp modification, you’ve got
to pinpoint exactly what you want
to accomplish. And you have to keep
in mind that an amp is full of many
parts that interact with and affect
one another, so even small changes
to any of these parts can yield major
differences in tone and performance.
However, this exponential effect that
small changes can have on tone means
there are many relatively easy ways in
which even inexperienced but adventurous
DIYers can mod their amp.
Here we present eight short projects
that pretty much anyone with rudimentary
soldering skills can tackle.
Even better, the mods we’re detailing
here are all reversible. So if they don’t
suit your fancy or you need to return
your amp to its stock circuitry (for
example, to sell it), you can do so
without much trouble.
No job can be done
well without the
proper tools—in fact,
attempting to do so
usually results in a
nightmare of frustration.
For the mods
we’re exploring here,
I recommend the
Phillips and/or flat
re-moving and securing
• Wire cutters/strippers
• Acid-free rosin
• Safety goggles
• Needle-nosed pliers
• A copy of your
Swap Preamp Tubes to Adjust Headroom
One of the most common things guitarists
request from us at our shop (schroederaudioinc.
com) is the ability to get more
or less headroom—either cleaner tones at
higher volumes or more overdrive or distortion
at lower volumes. Let’s begin by
looking at some simple ways to alter your
Left: You can alter your amp’s headroom
by swapping out the first preamp tube in its
first gain stage—typically the small tube furthest
from the power tubes. In this picture
of a Fender Twin Reverb amp chassis, the
power amp tubes are the two large glass
bottle-like things at far left, which means
the first preamp tube of the first gain stage
is the small valve at far right. The phase
inverter preamp tube is the third from left.
Right: A 12AX7 preamp tube (aka ECC83,
left) typically has a gain rating of 100
and yields more distortion, while a 12AT7
(ECC81) has a cleaner gain rating of 70.
The first preamp tube (aka “valve”)
in an amp’s circuit is used in its first
gain stage(s) of an amp. It’s usually a
12AX7 (aka an ECC83 in Europe and
abroad), and it’s the small tube located
farthest from the larger power tubes.
Typically, a 12AX7 has a gain rating of
100. One simply way to achieve more
headroom in your amp is to replace
this tube with a 12AT7 (aka ECC81),
which has a gain rating of about 70 and
will yield cleaner sounds than a 12AX7.
Conversely, players who have an amp
with a 12AT7 in the first gain stage can
get more gain and overdrive from their
amp by swapping it for a 12AX7.
Amp headroom can also be adjusted by swapping the resistor in a
negative-feedback circuit for a different value. Here, the resistor ringed
with gray, red, brown, and silver value marks is being desoldered, one
lead at a time, to make way for another.
You can further alter your amp’s headroom
by simply changing its phase inverter,
which is the preamp tube located right next
to the power tubes. It sends the signal from
the preamp into the power amp, and swapping
it with one that has a higher or lower
gain rating (i.e., a 12AX7 vs. a 12AT7) will
also adjust the amount of gain being sent to
the amp’s power tubes.
Left: Common tubeamp
Middle: Before touching anything
inside the chassis of a tube amp,
bleed off any lingering fatal voltages
being stored inside by attaching
one end of a 100 kΩ resistor
(inside the black shrink wrap in the
middle of the green wire) to ground
and touching the other end to the
positive side of each electrolytic
cap in the circuit (the blue
ones) for a full minute each.
Right: To confi rm that
voltage has been discharged,
measure each cap with a voltmeter
set to DC voltage and
make sure none is detected.
Touch the black lead to the
chassis, and the red lead to
the positive cap terminal.
All amplifiers contain lethal voltages.
If after reading through this
entire article you still feel unsure
of your capabilities, please refrain
from performing any modifi cation
to your amp. If you decide to
proceed, make certain the amp is
unplugged and that all tubes have
been removed before beginning.
Next, remove the amp chassis
from the box it is housed in and
turn it upside down so the circuitry
is exposed and easy to work on.
The most dangerous voltages
in an amp are stored in electrolytic
capacitors, even after the amp has
been unplugged from the wall. It’s
imperative that these capacitors
are discharged before proceeding
with any work on the amp.
The best way to do this is with
an alligator clip wire with a 100K
resistor in series to ground. Clip
one end of the wire to ground and
the other end to the positive side
of each electrolytic capacitor. This
will bleed off any voltage that may
be stored in the capacitor. To be
certain all voltage is discharged,
use a voltmeter set to DC voltage.
After about a minute, the capacitors
should be fully discharged. If
you are unsure of this procedure,
consult your local amp tech.