All of the classic EL84 amps used push-pull output circuits for good efficiency and output so that is the way I decided to go. A push-pull amp requires a phase inverter to drive it so I looked at ways to do that. Nearly all the designs that I looked at either had serious compromises or required critical adjustments to make them work. It seemed the best way to go was a cathode coupled pair, but that requires a constant current sink in the cathodes. Back in the tube days, a large value resistor was used as a substitute, another compromise. I decided to use a solid state constant current sink instead. There is no way the tubes can tell the difference so long as the current is constant and it makes the cathode coupled phase inverter nearly ideal with no adjustments to make. The sink uses just a TL431 shunt regulator chip plus a 2N3904 and two resistors, keeping the design simple. The cathode current of V1A andB flows through Q1 and the 1K resistor to the -20V source. U1 has an internal 2.5V reference. When the cathode current of the combined tube sections reaches 2.5 mA, the voltage across the 1K reaches 2.5 V and U1 starts to steal current from Q1’s base, limiting the total cathode current to 2.5 mA. When the grid of V1B is at ground potential, the two halves of V1 share the limited cathode current nearly equally. When the input varies, the current shifts back and forth between V1A and V1B proportional to the signal amplitude resulting in precise phase inversion between the two outputs Vo+ and Vo-.