The 6BN4A is described as a medium μ triode. It was probably used in television reception. The twin connections to both the cathode and the control grid suggest an RF role.
The triode is on the right with a plane sheet of metal on the left. The latter seems to have no function other than to act as a spacer for the left hand side of the micas. The screening plate is connected to the cathode.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 46 mm tall.
References: Datasheet & 3002. Type 6BN4A was first introduced in 1960.
Absolute Maximum Operating Conditions¶
Updated February 08, 2013. shape:thin.glass.tube construction:all.glass type:triode age:1950.1960 base:b7g heater:6.3v pins:7 pin:1.k pin:2.g1 pin:3.h pin:4.h pin:5.a pin:6.k pin:7.g1