The Lissen BB220A is a directly heated battery class B audio double triode. With a load of 10 kΩ the output is given as 2.5 Watts. With a lower heating requirement than the BB240 and BB240A valves the BB220A would suit sets used in smaller rooms.
The view up from the base: the filaments are run in parallel.
The dome carries the identification and maker's name. The filament tension spring holds the top of the inverted V filament. The wire is white indicating the expected oxide coating.
The top mica with gripping fingers around the circumference. In the foreground the fingers are pressed downwards showing that the shouldered envelope is gripping the electrode assembly.
Thanks to Louis Vermond for donating this valve.
The classic envelope is 39 mm in diameter and, excluding the B7 base pins, is 98 mm tall.
References: 1043. Type BB220A was first introduced in 1934.