The 6K8 triode hexode valve was purpose designed as a radio frequency changer and the American Government purchased it as the VT-167 and later in the glass version as the VT-167A.
The valve has a common cathode and is an excellent design in that the triode and hexode control grids are internally connected in a way that saves space and still allows for a separate triode anode. See 6K8 for a structural diagram.
The local oscillator would produce 7.5 Volts peak to peak signal from the triode section. The input signal would be coupled to the frequency changer via the top cap connection to grid 3.
The VT-167A and USA on the base cap as well as the RCA logo. On the glass can be seen the octagon border with 6K8G inside.
The gettering hides the pinch but the lower leads are visible.
Here the printed 6K8G is easier to see. Some of the ink rubbed off in cleaning.
The top view with the offset cathode on the left and triode anode on the far left. The innermost grid is both triode control grid and oscillator injector to the mixer.
The classic envelope is 38 mm in diameter and excluding the IO base pins, is 96 mm tall.
References: Data-sheet & 1040. Type VT-167A was first introduced in 1938. See also1938 adverts.
