amp sounding too muddy when pushed
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
naturally, the idea when modifying
coupling capacitors is to get the great
bass response you desire without
causing the amp to sound too boomy.
NOTE: Cathode capacitors are of- ten 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.
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.
Swapping the Coupling
Capacitors to Adjust Bass
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.
Swapping Tone-Stack Resistors
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
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).