Maximum isolation between oscillator and signal was the design objective of the 6L7 to provide a mixer which was efficient at VHF where stable oscillation would otherwise require a separate VHF oscillator triode.
For lower frequency applications where a self-oscillating mixer (i.e. pentagrid converter) could provide adequate performance there were other Octal types specifically designed for this purpose.
The oscillator injection needed to be 18 Volts peak to peak. The signal feed to the top cap is typical of the period to improve signal isolation.
The reverse is plain painted steel.
The complication for metal envelope valves was the provision of a second metal to glass seal at the top. At the base was a hard glass disc containing the lead-out wires to the pins.
The IO base cap with locating key on the central spigot.
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 63 mm tall.
References: Data-sheet, 1040 & 1043. Type 6L7 was first introduced in 1936. See also1936 adverts.
