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.