The ECC31 is a medium μ double triode with a common cathode. The CV1285 (formerly NR73) was based on the ECC31. The 6N7G has the same base connections but a much higher anode current rating and was designed for use as a Quiescent Push-Pull output stage.
The copper grid supports carry heat radiating fins at the top. The control grid winding can be seen above the lower mica. Also visible is the insulated heater wire wound round a ceramic tube as a double helix.
A closer view.
Face view.
The top mica showing the radiating fins on the grid support rods. The cathode tubes are reduced in diameter at the closed top end and pass through the mica disc in a hole the size of the control grid supports.
The classic envelope is 44 mm in diameter, and excluding the IO base pins is 85 mm tall.
References: Data-sheet, 1040 & 1043. Type ECC31 was first introduced in 1942. See also1942 adverts.
