WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Because in class A PP both tubes work at the same time into the same load sharing it, so for each tube it looks like a load of twice the impedance (two times 'lighter' load). In class B tubes work in alternate phases, one for each half-wave, so the load is not shared - each tube sees the full load for one half wave.