The EL86 is an output pentode with an anode dissipation of 12 Watts. The EL86 is designed for a higher anode current and lower anode voltage than the EL84. This would suit transformerless designs of the type called single ended push pull see data-sheet. It was also specified for use as a series regulator.
This exhibit has yellow paint and was made either in the diamond jubilee year of 1969 or in 1975 when more yellow paint was found at Blackburn. Either way this valve has had a hard life as can be seen from the discolouration of the glass.
The gap in the end wall reveals the suppressor grid support rod.
The control grid has cooling fins on the top of the copper support rods.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 70 mm tall.
References: Datasheet & 1040. Type EL86 was first introduced in 1956.