The EL41 is an all glass output pentode rated for 9 Watts anode dissipation, and intended for use in AC mains operated equipment. The EL41 should only be used with self-bias or semi-automatic bias.
Unlike the first B8A valves that had a metal base cover with locating spigot, this valve has a wider base to the envelope and a locating pip moulded into the glass. It indeed represents the transition point to the final all glass valve. The pins are still equi-spaced in the base and this soon gave way to the B9A and B7G styles that did not require a locating spigot or glass pip.
The reverse of the anode. Note the cooling fins on the top of the copper control grid supports.
This is a true pentode with three wire grids.
The thin glass tube envelope is 20 mm in diameter and, excluding the B8A base pins, is 68 mm tall.
References: Datasheet & 1040. Type EL41 was first introduced in 1947.