Type 12J7 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. 6Q7) made it a natural choice for audio.
This exhibit has a 12.6 volt heater but other wise is identical to the 6J7GT. The latter was for two decades the international standard valve for audio amplifier input stages and audio systems generally.
The Brimar (BRItish Made American Range) logo of STC.
A part view of the three wire grids.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 73 mm tall.