A plate bypass capacitor, like the 0.001uF value shown in the Soldano SLO overdrive channel's third-stage, is often used to attenuate treble in high-gain preamps. Our calculator computes the gain of the generalized circuit shown, where the coupling capacitor CG is assumed to act as a short circuit for all audio frequencies.
The Soldano circuit uses a 12AX7 triode, a plate load resistor RL = 100k, and an unbypassed cathode resistor RK = 39k. It drives two resistors and the output impedance of the SLO's low-gain channel, creating an effective load of 255k.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
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