The K77A Ever Ready quiescent push-pull double pentode is in fact a Mullard QP22A as Mullard supplied all of Ever Ready's valves in the mid 1930s.
Quiescent Push-Pull or Class B1 had advantages for battery operated sets in that the standing current was low as the valves were biased almost at cut-off, thus saving on HT batteries, and the output power could be greater than that obtained from a single output valve.
The two sections were matched as closely as possible.
One section seen from the pinch. The oxide coated filament and the three wire grids can be seen.
Gold paint on the dome of the envelope carries the identification.
The K77A is just visible.
The classic envelope is 40 mm in diameter, and excluding the B9 base pins is 93 mm tall.
Reference: 1043. Type K77A was first introduced in 1935.