If we use the output from an inverting amplifier as one output, we can send that output to a second inverting amplifier to invert the phase again for a second output. The problem is that the second amplifier has an amplitude that is larger by a factor equal to its gain. We can compensate for this by adding an attenuator to its input to reduce the signal amplitude by the same amount. This is the basic approach used by the paraphase inverter.
Voltage attenuation is applied to the input signal of the second amplifier through the use of a voltage divider formed by the resistors R1 and R2 shown below. Together these resistors also act as the grid resistor RG for the inverted phase. The paraphase uses two triodes and produces the same voltage gain as a single-triode preamp.
The calculator computes the values of R1 and R2 needed to attenuate the signal to the second phase input. Here we assume that the cathodes are fully bypassed for AC signals by large capacitors, which is usually the case.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Power Amps, (Seattle: Pentode Press, 2008).
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