The 6K8GT was purpose designed for radio as a mixer or frequency changer and here we have the Tungsram implementation in the GT format and we also have the Tungsram implementation in the original 6K8 metal version. The valve has a common cathode and the triode and hexode control grids are internally connected.
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 outer screen obscures the inner details.
The top mica components reveal little about the construction. On the right is the triode anode with the common cathode and common g1 supports. The rest of the hexode is to the left. See6K8 for a diagram of the original RCA design.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 71 mm tall.
References: Datasheet & 1040. Type 6K8GT was first introduced in 1938.