Type 6J7 is a contemporary of the 'Magic Three' and almost as ubiquitous. Although originally designed as a detector its linear characteristic and good internal shielding (cf. 6Q7GT) made it a natural choice for audio. It was for two decades the international standard valve for audio amplifier input stages and audio systems.
The 6K8G is the classic triode hexode frequency changer or mixer for broadcast applications. The exhibit has a clear glass bulb and would rely on external shielding where other mixers had external zinc paint sprayed on to provide screening. For isolation of the signal input and output circuits the hexode input grid is taken to the top cap. The original...