Most pre WWII battery radio receivers used the minimum of valves - three or four - in the conventional way. Battery sets were always a problem because of their current consumption. Batteries - especially HT batteries -were expensive (and remained so up to their demise around 1973) and so ways were sought to improve gain and performance without increasing the drain on those precious batteries. One method devised was to use a very special valve: the circuit above is that of a double pentode but double triodes were also used and the circuit for these differs little in principle from the pentode one about to be described, although it should be noted that the triodes were used in 'Class B' push-pull circuit conditions. This is where there exists a standing, though very small, anode current under no-signal conditions ('quiescent' conditions) but as the triodes possessed high impedance the additional current drain was relatively slight.

Back to the pentodes, The normal situation for push pull stages is that as the gain of one valve falls, the gain of the other rises. With this battery system however, things are arranged slightly differently with the aim of minimising standing current. To achieve this, the two sections of the double pentode are over-biased in such a way as to make each valve amplify only when its grid becomes less negative. In a single ended output stage severe audio distortion would result from this over-bias condition but each valve makes up for the other, resulting in acceptable sound quality. This type of circuit arrangement is known as Quiescent push-pull or QPP for short. A QPP stage requires twice the input voltage that a standard push-pull arrangement requires for a given output level which is why the more sensitive pentodes are used rather than triodes. Although at peak levels of signal the power consumption is much as a normal push-pull stage, at low signal levels the anode current falls. The result is a worthwhile saving of power over a period.