Phase Inverter Bass Response

The coupling capacitors C_G 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 below 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 R_GS, 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 cathodyne 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 Power Amps, (Seattle: Pentode Press, 2008).

²Morgan Jones, Valve Amplifiers, 2nd Ed., (Oxford: Newnes, 2003).

³Merlin Blencowe, Designing Valve Preamps for Guitar and Bass, (Raleigh: Lulu, 2009).