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 Mullard-Philips type EF37A, for many years the standard British low-noise audio pentode, is really a 6J7G under another name. The predecessor valve was the Type 57 and the characteristics are the same.
This excellent example of the 6J7 was donated by Alan Hensman M0NBR.
The lead for the top cap passes through a glass to metal seal before being attached to the cap and insulating disc.
The IO eight pin base was designed by RCA for the introduction of their range of metal envelope valves in 1935. The envelope is pinched onto the plastic, pin carrying, base. Inside the spigot is the pumping tube. The glass disc base is sealed into several metal rings before the diameter and expansion co-efficient matched the outer steel shell.
The thin metal tube envelope is 25 mm in diameter, and excluding the IO base pins, is 62 mm tall.
References: Datasheet, 1040 & 1043. Type 6J7 was first introduced in 1936.