The literature indicates that Mullard made the HL13 in two versions. We have the Ct8 based type and the B7 base option. What we have here is a third version based on a B5 base.
Mullard describe this valve as an indirectly heated medium impedance triode for use in DC/AC mains receivers. The use for the valve would have been as an RF amplifier in radio receivers.
The classic envelope is 36 mm in diameter and, excluding the B5 base pins, is 103 mm tall.
References: Datasheet, 1040 & 1043. Type HL13 was first introduced in 1934.
Absolute Maximum Operating Conditions¶
Updated March 13, 2014.