The 6SN7GT is a small glass tubular audio double triode, the design dates to the 1940's, this exhibit is the Tungsram implementation and was made in the UK. This was a classic valve with two separate 6J5 triodes in the one envelope. The 6SN7 remains the most popular Octal based double triode valve for audio applications. The valve is especially well suited to phase splitter stages.
The 12SN7GT is identical operationally but has a 12.6 Volt heater.
The two small triodes held in place by the mica discs.
Face view of one triode. The thin sheet anode is ribbed for stability. The control grid is wound on copper supports.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 64 mm tall.
References: Datasheet, 1040 & 1043. Type 6SN7GT was first introduced in 1941.