I keep wanting to call you mercedes benz. Wonder why?

Perhaps this will (or maybe it won't :=)) be more intuitive....

say on a conventional series feed single ended output stage... you have an output trans with two windings... a primary and a secondary... the dc plate supply for the anode goes in the primary from one end and gets hooked to the anode of your 2A3 or 300B on the other end....

guess what you have.... you have in essence a plate choke feeding the anode of your tube... only now the ac signal is also in series with this same winding...

the primary of your output transformer produces a certain amount of L... it is this L (in para w\ the reflected load from the secondary) that governs or determines the magnitude of load impedance as seen by the tube.

so... in a sense your standard output transformer could be thought of as a plate choke with a secondary hung on it....

tell me if that helped or not...

msl