The EF86 is the standard early stage audio pre-amplifier valve. This exhibit was made by Mullard who designed the EF86. Mullard describe the EF86 as a low noise pentode intended for use as a RC coupled AF voltage amplifier, particularly in the early stages of high gain audio amplifiers, microphone pre-amplifiers and magnetic tape recorders.
This exhibit has a plain bright outer screen whereas most EF86 examples have a perforated electrostatic outer screen.
The hole in the screen reveals the grid axis and all three wire grids.
The bright screen
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
Reference: Datasheet & 1040. Type EF86 was first introduced in 1953.