The 220B is a double triode audio output valve from the early 1930s. Like similar valves this was designed to offer reasonable volume at minimum consumption from dry cell HT batteries by being operated as a Class B push-pull stage. Pure Class B suffers from cross-over distortion as the initial conduction region of the valves operating slope is non linear.
Our other Cossor 220B has lettering on the glass rather than a paper label.
The top of the valve. The filament tensions springs sit on the top mica. The anode is jointed in the centre of the working face where the flange connecting to the support will offer maximum heat conduction. The control grid is plated with copper or gold.
The classic envelope is 44 mm in diameter, and excluding the B7 base pins, is 91 mm tall.
References: Data-sheet & 1043. Type 220B was first introduced in 1933.