The calculator computes the gain and output impedance of a common plate load mixer. Each input channel controls a separate grid, which affects the plate current through the common plate load resistor RL and cathode resistor RK. The optional cathode bypass capacitor CK increases gain by eliminating negative feedback. If a cathode bypass capacitor is used, the calculator assumes that it is large enough to short the cathode to ground at audio frequencies. (How large is large enough? See our Cathode Bypass Capacitor Calculator.)
As long as the triodes are not overdriven, the inputs are completely independent. Changes in control settings for one channel, for example, do not affect the signal characteristics of the other. Unlike a passive mixer, the circuit also provides substantial gain. Our online article The Gretsch Chet Atkins Active Mixer describes one of the ways that the circuit is used in a guitar amp.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Power Amps, (Seattle: Pentode Press, 2008).
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