The EBL1 is one of very few double diode audio output pentode valves were designed. This exhibit came in the original box but the identification paint has rubbed off.
The EBL1 was released in 1938. The EBL31 is electrically the same valve but fitted with an IO base cap and was released in 1940.
The valve has a common cathode and was intended as a single valve detector and audio stage driving the output transformer (7,000 Ohm primary) and loudspeaker. It would have needed considerable drive at intermediate frequency and the output pentode is of high gain.
The main market for this type of valve was simple receivers for local station reception.
The control grid is wound on copper supports and fitted with a heat radiating fin. The cathode tube below the top mica broadens out to a wide active surface. The control grid is close th the cathode to achieve the high mutual conductance but this close proximity also increases the heat to be dissipated. The outer grids are wire wound on this true pentode.
A closer look reveals the mesh anode. It is possible that the two stout outer rods are the diode anodes.
The classic envelope is 44 mm in diameter and the length excluding the Ct8 base pins, is 125 mm.
References: Datasheet & 1043. Type EBL1 was first introduced in 1938.