The QP21 is directly heated and for battery HT sets. The pair of pentodes were designed to operate in Class B1 and so draw very little standing current. An important fact when the cost of HT batteries was significant. The screen grid is also common to both sections.
The output power was of the order of 0.5 Watt and the anode to anode load was quoted at 25,000 Ω.
The Marconi brand label. The two central rods are connected to the anodes at the top as the small space between the suppressor grid and the anode would make connecting at the lower end difficult.
A close-up of one pentode. The filaments can be seen to be oxide coated.
The classic envelope is 39 mm in diameter, and excluding the B7 base pins, is 94 mm tall.
References: Datasheet & 1043. Type QP21 was first introduced in 1934.