The 7K7 is a double diode triode typical of the late 1940s in the UK. The diodes would be employed as signal detector and ACG rectifier whilst the triode would be the first audio voltage amplifier. Unlike earlier designs such as the 6Q7 the control grid is not taken to the top cap. The shorter internal pathways making it easier to avoid instability.
The valve came complete with purpose designed metal screening can.
Seen from both sides it can be seen that the valve has two identical anode cavities. In one the grid support wires form part of the control grid of the triode. In the other cavity, one notched support rod is one of the anodes with the anode proper as the second diode anode. The second notched support rod is left unconnected.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 55 mm tall.
References: Data-sheet & 1040. Type 7K7 was first introduced in 1938.