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.
Essential Tools
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
following tools:
• Standard-size
Phillips and/or flat
screwdrivers (for
re-moving and securing
the chassis)
• Wire cutters/strippers
• 25–40-watt
soldering iron
• Acid-free rosin
core solder
• Safety goggles
• Needle-nosed pliers
• A copy of your
amp’s circuitry
schematic
Mod 1:
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
amp’s headroom.
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.
Warning!
Left: Common tubeamp
capacitor types.
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.
Mod 2:
Swap Negative-Feedback Circuit
Resistors to Adjust Headroom
Note:
Be careful not to
leave the soldering
iron on the solder joint for too
long as doing so could damage
the component.
Another way to increase your amp’s headroom
is to adjust the size of the negativefeedback
resistor. Because the earliest
tube guitar amps from the 1950s weren’t
intended to overdrive (though it wouldn’t
be long before rock ’n’ roll pioneers harnessed
the glorious sound), the negativefeedback
circuit was implemented as a way
to reduce distortion. It does so by taking
a very small signal from the amp’s output
and injecting it back into the gain stage—
only it’s out of phase with the output.
This causes phase cancellation and affects
the amp’s overall gain character.
The negative feedback resistor located off
of the amplifier’s output jack. Decreasing its
value will increase your amp’s overall headroom.
In the photo above, the feedback
resistor is located between the top two blue
coupling capacitors—it’s the component
with (left to right) gray, red, brown, and silver
bands on it, and one of its leads is being
gripped by needle-nose pliers. (For complete
information on how to read resistor
color codes, visit wikipedia.org and search
for the “Electronic color code” entry.)
To remove the current resistor and
install a new one:
• If you have a soldering iron that
lets you set exact temperature, set
it for between 700 and 800 degrees
Fahrenheit.
• Heat the solder joint on one end of
the feedback resistor and gently lift it
out of the circuit, then do the other.
• Bend the tips of the new resistor’s
leads to fit neatly in the two vacated
solder joints.
• Snip off excess length on the leads of
the new resistor.
• Heat one of the solder joints and put
one end of the resistor in place, and
then proceed to the other solder joint.
• Add a bit of solder to the new solder
joints so that there’s a solid connection.
• Repeat the steps above with different
value resistors until you are satisfied with
the increase or decrease in headroom.
Mod 3:
Swap the Cathode Resistor
to Adjust Headroom
Shown here is our Fender Twin Reverb. Its 1.5k Ω cathode resistor is marked by the brown, green, red, and silver bands.
Adjusting the value of the resistor connected
to the cathode (the main filament-like
part that forms the core of a vacuum tube)
of any of the gain-stage preamp tubes can
greatly affect the overdrive capabilities and
headroom. The bias of a preamp tube—
how much voltage is running through it—
occurs in the tube’s cathode.
Not all amps have a cathode resistor, but
when they do, it’s wired in parallel with
a cathode capacitor—which can also be
swapped out for one with a different value
to increase or decrease headroom (see Mod
4, below, for more on this).
Generally, the range of values for the
cathode resistor is 820 ohms (Ω) to 10
kΩ, but the most common value is 1.5
kΩ. Decreasing the value causes the tube
to bias hotter, which in turn causes the
tube to overdrive quicker, yielding a hairier
tone due to the increase in gain. It follows
that increasing the value of the cathode
resistor causes the tube to bias cooler,
lowering the gain of the tube and thus
increasing clean headroom. To change
the value of the cathode resistor, refer to
the steps in the Mod 2: Swap Negative-
Feedback Circuit Resistors to Adjust
Headroom section.
Mod 4:
Swap the Cathode Capacitor
to Adjust Headroom
To increase or decrease gain, you can swap
out the cathode capacitor (here, it’s the black
component with green writing) with one of a
different value—a lower value for more gain,
higher for more dirt.As mentioned above, the cathode capacitor
also has a significant effect on an
amp’s available gain. The larger the value
of the cathode capacitor, the more low
end is accentuated in that gain stage. The
smaller the value of the cathode cap, the
more high end is accentuated. The typical
range of cathode capacitor values is
anywhere from .68 μf to 250 μf. A typical
cathode cap value in lower-gain amps
(including the Fender Twin we’ve been
working on here) is 25 μf. In higher-gain
amps such as a Marshall Super Lead,
you would expect to see a cap value of
.68 μf. The reason higher gain amps
use cathode caps with such small values
(especially in the early gain stages) is to
tame the potential for too much bass to
be amplified—which could result in the
amp sounding too muddy when pushed
into overdrive.
Some amplifiers—including old Supros
and Magnatones—do not have cathode
caps on the first gain stage(s). You can
increase the gain of these amps by adding a
cathode capacitor in parallel with the cathode
resistor of that gain stage. To change
the value of this cathode capacitor, follow
the rules for changing a resistor in the two
previous sections.
Note:
Cathode capacitors are often
electrolytic—meaning,
they store electrical charges and therefore
have + and – poles that must be installed in
the proper direction. It’s therefore imperative
that you pay special attention to where
the existing capacitor’s + and – poles are
oriented before removing it. The negative
side must be attached to ground, and the
positive side of most electrolytic caps is the
side with a lip near the end. The negative
side will not have such a lip and will be flat.
Mod 5:
Swapping the Coupling
Capacitors to Adjust Bass
Response
To alter bass response, you can swap coupling caps for different values. In our Twin Reverb example, the coupling caps are the two blue cylinders at the
end of the circuit board (closest to the power tubes).
The second most common request we get
at our shop is to change the overall tonal
character of an amplifier. As with changing
an amp’s gain, small changes in the circuit
can greatly affect the tone.
If you’re looking to get more (or less) bass
out of your amp, its coupling caps—which
act as frequency filters—are great candidates
for modification. Coupling capacitors typically
have values from .022 μf to .1 μf. The
purpose of coupling caps is to block DC voltage
and can be found in several places in the
circuit. The specific ones that we’ll be dealing
with are situated between the phase inverter
plates and the power-tube grids. Smaller values
such as .022 μf attenuate the bass in the
preamp, preventing it from being passed into
the power amp section. Larger values such as
.1 μf allow more bass to pass through. In a
bass amp, you may see up to .47 μf.
Naturally, the idea when modifying
coupling capacitors is to get the great bass
response you desire without causing the amp
to sound too boomy. High-gain amps typically
have a smaller value than clean amps
for this reason.
Coupling caps are rarely electrolytic and
will therefore function without regard to
polarity. That said, certain types of coupling
caps—including film and paper-in-oil
varieties—may yield small sonic differences
depending on the direction of travel.
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.
