Although the output tubes are 6146s the circuitry of the driver is what is in question. The older 6SN7 could be replaced with the 5697 9 pin mini or at lower voltages the 12AU7s could work. The idea of the pahse splitter is to use a triode with exactly the same resistor for both plate and cathode load resistors. Use a small value such as 2 X 22K the at the grid of the push pull long tail driver use a very high resistor such as 470K each. The common cathode resistor of the long tail makes the circuit a semi differential amplifier which compensates of discrepancies in the phase splitter. These discrepancies include different output capacitances for plate and cathode and load handling capabilities. The differential amplifier corrects a lot of the differences making the outputs closer to a true phase 180 degree difference at a single moment and across a wider frequency responds. Of course if your only want a few watts of audio you can chose modulator tubes that will allow a larger grid resistor for AB1 operation with out going into thermal grid leakage run away. 6V6s, 6L6s, etc. with the smaller tubes and less heat you can use higher grid leak resistors and come right off of the phase splitter to the grids of the output tube with out using the differential amp in between.
73, John, WA5BXO
