Type 6BN6 is a late 1940s 'gated beam' valve in which a carefully formed electron sheet or beam is deflected, by control elements on and off of the anode. This 'chops' the amplified signal, typically at a very high rate.
By using two gated beam valves signals from two different inputs can be selected, chopped, or interchanged at any desired rate. The data-sheet on 6BN6 & 6BN6 gives a detailed description of the valve and use and is in two parts. The data-sheet here gives the usual operating details.
The segmented electrode system. The cathode is on the left and the anode on the right.
Looking at the back of the anode box.
The insulated heater can be seen on the lower right.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 57 mm tall.