In a tube the current runs from anode to cathode. The control grid determines the amount of current. When we wish to extract power from the tube, we should connect the output transformer to where the current runs, also to the anode or to the cathode or a combination of both.
In a single ended amplifier we use one power tube which draws a permanent quiescent current. This current will saturate the core of the OPT unless we gap the core, or we compensate this by means of an extra winding plus current source, or we let this quiescent current run through a choke while the OPT is connected with a capacitor to the tube.
In a push pull amplifier we use two power tubes. The two quiescent currents compensate each other in the primary winding, therefore the transformer core needs no gapping. The possible circuits can now be divided into basic push pull and paralleled push pull (like the Circlotron or the PPP circuit with two separate power supplies).
More news to follow soon!