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 screening cylinder and the view along the grid axis. Within can be seen the three wire grids.
The anode cylinder lies just inside the screen. The top and bottom screens are joined to the outer screen as can be seen in this image. The insulated heater passes four times through the cathode.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 96 mm tall.
References: Private communication & data-sheet. Type 6J7G was first introduced in 1936.