Type American service Type VT-91 (commercially Type 6J7) is a contemporary of the 'Magic Three' and the design almost as ubiquitous. Although originally made as a detector its linear characteristic and good internal shielding (cf. 6Q7GT) made it a natural choice for audio.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.
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 VT-91 was first introduced in 1936.