The EBL31 is a double diode and sensitive output pentode. The valve was designed for use in domestic superhet radio receivers that had access to strong local signals. These low cost receivers saved a first audio voltage amplifier triode by driving the output valve directly from the detector. The double diodes acted as both signal detection and AGC rectifier.
The EBL1 on a Ct8 base is electrically identical and was released in 1938.
The coating obscures the lower structure but the anode can be seen to be oval in shape.
The top of the anode showing the oval cathode and wire grids.
The classic envelope is 44 mm in diameter, and excluding the IO base pins is 117 mm tall.
References: Data-sheet & 1043. Type EBL31 was first introduced in 1940. See also1940 adverts.