Marconi 'Osram' DE4

Marconi 'Osram' DE4 dull emitter (thoriated tungsten) filament triode, probably from before 1925. A 1925 example at the National Valve Museum has a bakelite base. The valve bears a 'BBC' mark, (second picture above) of the style used from 1924 to 1927 and the serial number 'W1512'.