Various brands 6L7 Heptode

The whole point of the 6L7 is to ensure maximum isolation between oscillator and signal and to provide a mixer that is efficient at VHF where stable oscillation would require a separate VHF oscillator triode.

For applications where a self-oscillating mixer (ie 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 thin metal tube envelope is 23 mm in diameter and, excluding the IO base pins, is 81 mm tall.

The 6L7 was first introduced in 1936.