S625 Marconi screen-grid tetrode by Captain Round circa 1928 with thoriated tungsten filament

Description

The S625 made by the Marconi-Osram is a screen-grid tetrode from 1928.

Basically the S625 is a screen-grid receiving valve having thoriated tungsten filament.

It was designed by Captain Round. You can clearly see electrodes are made of a set of discs which span from one side to the other of the valve. The filament is V shaped and the screen grid has a sort of metal mesh style that distinguishes Round’s design from many other valves of that era!