[Amps] Push pull amps

[Tom Osborne Sr.]

I recently purchased some QST's from '47, 49, and 50.  I notice a lot of
transmitters and amps back then used 2 tubes in push-pull circuits, instead
of parallel, like we mostly do now.

What was the idea behind push pull compared to parallel, and when did the
change take place?  Thanks and 73
Tom W7WHY