The EBL21 is one of very few double diode audio output pentode valves that were designed. This exhibit was made in Holland.
The valve has a common cathode and the diodes are for signal rectification for detector or AGC service. As an example of both use and date, this valve type was used in the 1947 Philips 462A radio for detection and audio output. It would have needed considerable drive at the intermediate frequency and the output pentode is of high gain to allow the use a a single stage from detector to output transformer.
Mullard describe the EBL21 as a high sensitivity output pentode, combined with twin diodes, for use in AC mains operated equipment.
The double diodes are formed by small plates mounted either side of the cathode and below the lower mica sheet.
The anode above and the screen below the lower mica. The screen shields the two signal diodes.
The wide glass tube envelope is 32 mm in diameter and, excluding the B8B base pins, is 76 mm tall.
References: Datasheet, 1040 & 3002. Type EBL21 was first introduced in 1947.