The coupling capacitors CG in the schematic below block the high DC voltage from the phase inverter while allowing the inverted and non-inverter audio signals to drive the power amplifier. Unless bass attenuation is desired the capacitors need to be large enough to induce very little signal attenuation at bass frequencies. (82Hz is the lowest note on a guitar with standard tuning.) They shouldn't be too large, however, or unwanted effects like blocking and motorboating may ensue, particularly if the phase inverter includes negative feedback from the output transformer.
The calculator computes the voltage gain measured from the phase inverter input to the grid of a power tube. The calculations assume that attenuation due to the grid-stopper resistors RGS, if used, is only significant above audio frequencies. The voltage gain for the long-tailed pair is an approximate value for both phases and assumes perfect balance. Calculations for the paraphase also assume perfect balance between phases. The concertina response is only valid when the power amp is not overdriven.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
2Morgan Jones, Valve Amplifiers, 2nd Ed., (Oxford: Newnes, 2003).
3Merlin Blencowe, Designing Valve Preamps for Guitar and Bass, (Raleigh: Lulu, 2009).
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