The 6K8GT was purpose designed for radio as a mixer or frequency changer. 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.
Our other 6K8GT has a solid screen whereas this exhibit has a perforated screen surrounding the triode hexode.
The electrodes can be seen below the screen. On the left is the hexode anode. The square section in the centre is an internal shield and the common cathode is on the right with the triode anode on the extreme right.
A diagram of the construction of the 6K8 can be found on the page of the original metal valve.
The perforated screen and lower mica.
The triode anode is the main part visible, beyond that is the common cathode and hexode grids. The triode grid is common to both triode and hexode and thus can modulate the hexode directly.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 72 mm tall.
References: Data-sheet & 1040. Type 6K8GT was first introduced in 1938.