The EF42 miniature high slope RF pentode is of an all glass construction and dates from 1947 and was first seen at the Olympia show that year. Mullard specify this device for use as an RF or an IF amplifier.
The UF42 has identical characteristics but is fitted with a 100 mA heater for use in series heater chains.
The pins are equi-spaced and the external metal skirt has a locating pip, seen on the right in the picture. Our other EF42 has an entirely glass envelope.
The thin glass tube envelope is 20 mm in diameter and, excluding the B8A base pins, is 52 mm tall.
References: Data-sheet & 1040. Type EF42 was first introduced in 1947.