Type 6J7 was originally designed as a detector, however, 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 Outer screen is connected to the disc screen at the base. The anode cylinder can be seen just inside the outer cylindrical screen. The grids are just visible as is the white cathode and the four strands of insulated heater wire.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 95 mm tall.
References: Private communication & data-sheet. Type 6J7G was first introduced in 1936.