Wiring 2 audio output transfomer secondaries in series or parallel
Unlike what others have said, this is NOT a push-pull stage.  Even though drawn as mirror images, the two tubes are driven identically.  They each drive a separate transformer.  The two transformers put out the same signal.

Note that each transformer is labeled as "4 &Omega;".  Wiring the outputs in parallel would yield 2 &Omega;.  Wiring them in series would yield twice the voltage, but at 8 &Omega;.  You get the same power capability either way, but in series they can deliver more power into a 8 &Omega; load.

However, there seems to be a mistake in this schematic.  As the two transformer secondaries are shown, they oppose each other's voltage.  With identically balanced components, the output would always be zero.  One of the primaries or secondaries should be flipped from what is shown.  Probably one of the 3,4 pin secondary markings was supposed to be flipped.

In case you're wondering, the 22 k&Omega; resistor at the top is likely the negative feedback path around the whole amp, although having the volume control inside the feedback loop doesn't make much sense.

Note that both output tubes are run open loop, with not even local negative feedback.  There are cathode resistors, but they are bypassed by capacitors.  The cathode resistors therefore are only for getting a predictable and stable DC operating point.

This stage is meant for one thing only, which is raw power.  The 6V6 tubes are beam power pentodes rated for 14 W plate dissipation.  Two output stages were used essentially in parallel almost certainly to get more power out.  This stage creates a large power boost at the expense of some distortion.  The negative feedback around multiple sections of the amp is intended in part to mitigate the distortion of this final stage.