Mullard designate the EF37A as a high gain, high impenance, pentode for use in AF pre-amplifier stages. The electrode system is of anti-microphonic construction, and the specially designed heater reduces hum to negligible proportions.
High quality amplifiers have used the EF37A for many decades. The ME1400 electrometer pentode is also rated for high quality audio use and is a direct replacement for the EF37A upon which it was based. The EF86 was its successor.
The data sheets also give parameters for use as a triode.
See also 6J7 almost the same valve in operation.
The classic envelope is 29 mm in diameter and, excluding the IO base pins, is 85 mm tall.
References: 1004 & 1040 Type EF37A was first introduced in 1942.