This amp started life as a stereo single ended amp. When I added the Karlson to my system, I needed a mono amp that could handle low frequencies. Rather than start from scratch, I decided to convert an existing amp.
This amp represents the simplest possible conversion from a stereo single ended amp to a mono push-pull amp. The only parts I had to add were two resistors-one from the grid of each output tube to the grid of one of the input tubes, and a push-pull output transformer. I had to adjust the coupling capacitors to account for the lower grid resistance. I also removed the bypas capacitors from the cathodes of the input tubes and tied the cathodes together to improve the AC balance.
This circuit probably doesn't really need two 12AT7s. If I were to build it again, I would use only one, or perhaps even one 12AX7, with different cathode and plate resistors. I would also add some way to adjust the phase splitter network for perfect AC balance. This amp sounds pretty good though, and I don't plan to change anything.