The EF37A is a high gain, high impedance, 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. This is achieved by a bifilar winding of the heater. Apart from the hum reducing construction the EF37A is the same as the EF36.
This valve was employed in the input stage of high quality audio amplifiers for many decades. The EF86 was the successor from Mullard.
The EF37A was also made with the classic shouldered shape of envelope.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 84 mm tall.
References: Data-sheet & 1043 Type EF37A was first introduced in 1942. See also 1942 adverts.