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 generally. The Mullard-Philips type EF37A, for many years the standard British low-noise audio pentode, is really a 6J7G under another name. In this exhibit we see the Mullard 6J7. The central spigot of the IO base has been broken off.
The thin metal tube envelope is 25 mm in diameter, and excluding the IO base pins, is 60 mm tall.
References: Datasheet, 1040 & 1043. Type 6J7 was first introduced in 1936.