The Cossor New Process 220HPT is a battery audio output pentode. The B4 and side terminal was released in 1932 with the B5 based version following the following year.
The anode carries the Cossor C that also strengthens the bright thin sheet anode.
The box anode and the three wire grids within.
A close-up of the grids showing the notching of the support rods.
Seen looking down onto the dome the top mica and the serrated edges can be seen. These serrations allow the mica to adapt to the minor variations of the internal diameter of the envelope and still form a rigid support.
The load resistance is 10,000 ohms and this is greater than high impedance headphones, so clearly this valve would have been used with an output transformer to transform the impedance to that required by a loudspeaker.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 105 mm tall.
References: Data-sheet & 1043. Type 220HPT was first introduced in 1932. See also1932 adverts.
