The Long-Tail Circuit

Author: N.H. Crowhurst

Another variation puts the common resistor that carries the plate current of both tubes in the circuit between cathode and ground. The input voltage to the second tube is the audio voltage developed across this common resistor, because the second tube grid is connected to ground either directly or through a large capacitor.

It doesn't matter which way we measure a voltage, whether from grid to cathode or cathode to grid. In either case it will be the same voltage, different only in polarity or phase. Hence the grid voltage for the second tube will be due to the difference in the audio components of plate current in the common cathode resistor. This means that the audio plate current of the first tube must be a little higher than the corresponding component of the second tube. If identical plate resistors are used, the first tube will produce a slightly greater audio voltage than the second tube. This can again be overcome by using plate resistors of different values so that the audio voltages become equal.

The long-tail or cathode coupled paraphase amplifier