The Mullard EL38 was designed, in the late 1940s, as a television line output stage valve. The E3375 is the experimental Type designation for the M-OV equivalent but we have not found a specific data-sheet for this Type.
In the early days of CRT based television the tubes could have been either electrostatic or magnetic deflection. The EHT supplies were mainly derived from special mains transformers and ran at about 10 - 14 kV. Thus the line output valve was not as heavily stressed as later types had to be.
The EL38 was designed to withstand a positive anode surge of 8,000 Volts and in addition to the 25 Watt anode dissipation the screen grid was wound to withstand a dissipation of 8 Watts.
The GEC label to the right - the GEC brand was used for professional valves whereas Marconi and Osram were the consumer brands of M-OV. The construction is with a pressed glass foot rather than a pinch and is probably post war. The control grid supports can be seen to be copper.
The folded anode is rectangular in section with side flanges that fix to the support rods.
The wide glass tube envelope is 32 mm in diameter and, excluding the IO base pins, is 103 mm tall.
Reference: Data-sheet. Type E3375 was first introduced in 1946.