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 identification and branding is printed onto the glass.
The twin diodes are formed by two bright L sections formed into a box around the common cathode.
The envelope is clearer than our other EBL31 and the output pentode electrodes can be clearly seen.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 119 mm tall.
References: 1043. Type EBL31 was first introduced in 1940.