Type 6J7 is a contemporary of the Magic Three and almost as ubiquitous and here we have the glass tubular envelope version.
Although originally designed as a detector the 6J7's linear characteristic and good internal shielding (cf. 6Q7) 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 12J7GT is the same valve but fitted with a 12.6 Volt heater.
The solid screen obscures the inner structure.
The lower connections to the pinch. The control grid supports are not connected as the grid is coupled to the top cap.
The grid connection is made via a loop so there is no strain on the connections.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 72 mm tall.
References: Data-sheet, private communication & 1040. Type 6J7GT was first introduced in 1936.