The 12AL5 miniature all glass envelope houses two separate signal diodes. The 6AL5 has a 6.3 Volt heater and the 3AL5 a 3.15 Volt heater in all three the total heater power is the same and electrically they are identical.
The two diodes are completely separate flat structures separated by a flat metal screen. This clearly shows the signal usage, where it is important to keep RF isolation.
These diodes were specifically designed for vision and sound detection in television receivers. Additionally they are recommended for limiting and DC restoration.
This valve was made with three variants of heater voltage. This example has a 3.15 Volt heater and a controlled warm-up time of 11 seconds.
The makers name. The central plate is the electrostatic screen.
One of the signal diodes seen face-on.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 36 mm tall.
Reference: Datasheet. Type 12AL5 was first introduced in 1947.