The EL42 is an audio output pentode of an early all glass design. In single ended Class A an output of 2.5 Watts was to be expected. This was sufficient for most car radio receivers, but a pair in push-pull would generate a respectable 5.5 watts for larger receivers and audio equipment.
The EL42 took over from the EL2 and EL32 and was in turn replaced by the EL95 but the EL85 is electrically identical to the EL42.
This valve envelope is the final form of the B8A based valves with an all glass construction including a locating pip on the 'skirt'.
Opposite the holes in the anode the glass is coated to reduce the release of gas from the glass through the action of electron bombardment.
The slots in the anode seen through the coating.
The thin glass tube envelope is 20 mm in diameter, and excluding the B8A base pins is 52 mm tall.
References: Data-sheet, 3002 & 1040. Type EL42 was first introduced in 1947.