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.
adjusting the value of the ResistoR
connected to the cathode of any of
the gain-stage pReamp tubes can
gReatly affect the oveRdRive
capabilities and headRoom.
MOD 2:
Swap Negative-Feedback Circuit
Resistors to Adjust Headroom
Another way to increase your amp’s headroom is to adjust the size of the negative-feedback 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 negative-feedback 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.
NOTE: Be careful not to leave the soldering iron on the solder joint for too
long as doing so could damage
the component.
•;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
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,
