Introduced to market in 1946 by Mullard, the EL38 was designed as a television line output stage valve in the late 1940s. In the early days of CRT based television the tubes could have been either electrostatic or magnetic deflection. The EHT supplies were probably derived from special mains transformers and ran at about 10 - 14 kV.
Th EL38 had the capability to withstand an 8kV surge and an anode dissipation of 15W, allay this to a grid disippation of 8W and voila, you haver a pentode MORE CAPABLE than an EL37 and suddenly they are doing duty in many guitar amp applications where their ruggedness and smooth sound goes down a storm.
Having the Mullard family elongated ST envelope, the device diameter is approximately 41mm with height of approximately 120mm