The EBL21 is one of very few double diode audio output pentode valves that were designed. This exhibit was made in Holland. Sylvania introduced the Loctal base in 1938 and this Philips valve was designed in 1941 and used in military equipment, but not used in commercial equipment until after WWII.
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 copper supports for the control grid have heat sinks above.
The double diodes are formed by the small rectangular plates placed either side of the lower part of the cathode. The screen is behind the diodes.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 75 mm tall.
References: Data-sheet, 1040 & 3002. Type EBL21 was first introduced in 1941. See also1941 adverts.