The 6L6 is an audio output beam tetrode. The 6L6 is both highly sensitive and capable of as much as 60 Watts output as a pair used in push-pull.
The GE Logo and white lettering on the black painted steel envelope.
The IO base and flange.
See also 6L6 for historic details.
The wide metal tube envelope is 31 mm in diameter and excluding the IO base pins, is 92 mm tall.
References: Datasheet. Type 6L6 was first introduced in 1936.