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 bright box above the pinch is the pair of signal diodes, each anode being an L shape.
The control grid is connected to the top cap and the grids are all wire wound making this a true pentode. The grids can be seen clearly in our other EBL31.
The classic envelope is 44 mm in diameter, and excluding the IO base pins is 120 mm tall.
References: 1043. Type EBL31 was first introduced in 1940.