K4 Thorpe bi-grid Bower Electric Company valves top pip metal base – STC AR12 4020A POVT73A 3A/108A

Description

Introduced in 1924 the Thorpe K4 is basically a four electrode valve mainly used on Unidyne receivers as shown on cartons. Looking at this 5-pins valves we can see they gave two grids one into the other. The second grid is a charge grid in order to  reduce plate voltage. Both have top pips, metal base.

Pair STC AR12 telephone repeater triodes also used in audio applications, valve dated 1937 with slanted plate 45 degrees  made by STC . The Post Office type was POVT73A or GPO No.73A.