The EL91 is an audio output pentode valve for radio receiver use. In single ended configuration it will produce 1.4 Watts, enough for a receiver.
If used in push pull an output of 4 Watts is possible, but this valve was not expected to be used for hi-fi, however, it performs the task of quality music reproduction very well and when used as a stereo push-pull amplifier it compares favourable with a Quad 33-303 combination for small room use.
The two inner grids are wound on copper supports for good heat transference.
This valve was new old stock (NOS) and the box is delightful.
The thin glass tube envelope is 18 mm in diameter and excluding the B7G base pins, is 47 mm long.
References: Datasheet & 1040. Type EL91 was first introduced in 1949.