The 12BL6 is a vari-μ RF pentode for RF or IF stages of car radios. Typically the 12BL6 would be used with an HT of 12.6 Volts from the car supply.
These valves were used in hybrid receivers where the RF section was valve and the audio was transistor.
The notch in the anode reveals the three wire grids.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
Reference: Data-sheet. Type 12BL6 was first introduced in 1956.
Absolute Maximum Operating Conditions¶
Updated July 08, 2020. shape:thin.glass.tube construction:all.glass type:pentode age:1950.1960 base:b7g heater:12.6v pins:7 pin:1.g1 pin:2.g3.s pin:3.h pin:4.h pin:5.a pin:6.g2 pin:7.k