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. 6Q7G) 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.
The classic envelope is 36 mm in diameter and, excluding the IO base pins, is 99 mm tall.
References: Private communication & data-sheet. Type 6J7G was first introduced in 1936. See also 1936 adverts.