Unlike transistors which have NPN and PNP models of different polarities that can be easily used in a push-pull circuit, vacuum tube triodes and pentodes can only conduct in one direction.  Therefore, a phase splitter is necessary to produce two almost equivalent signals that are 180 degrees out of phase from one another.  This part of the amplifier also utilizes a 12AX7-A tube.

The phase splitter appears complicated at first, but is almost ingenious.  Here, the first triode functions similarly to the pre-amplifier triode in that it produces a signal that is 180 degrees out of phase with the input signal.  However, the circuit functions so that a signal that is in phase with the input is output by the second triode.

The first capacitor is used in a passive high-pass filter to get rid of the massive DC offset created by the previous stage.  However, as you can see, the resistor paired with it is not directly connected to ground, so the signal that goes into the first triode's grid has a DC offset that is set by resistor R_K2.

At the input of the second triode is a passive low-pass filter.  This is used to get rid of the residual signal from the high-pass filter in order to give the grid of the second triode a DC voltage (again, set by R_K2).  While the cathode voltage was held constant previously as the grid voltage changed, here the triode's grid is held constant as its cathode voltage fluctuates.

Pre-Amplifier            Power-Amplifier