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 visible cylinder is a screen with the anode within. The top cap is connected to the control grid.
The screen extends above and below the main electrodes and obscures (shields) the three wire grids.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 98 mm tall.
References: Private communication & data-sheet. Type 6J7G was first introduced in 1936.