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.
A diagram of the electron path in a 6BN6 gated beam valve.
Looking along the electron path, the heater and cathode is in the foreground.
Looking side-on to the electron path. The heater/cathode is at the bottom of the image.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 57 mm tall.