The 6V6 family of beam tetrodes came in many constructions and were very popular as the final audio stage of broadcast receivers with very good reason due to the quality of their output.
We have the original American metal valve 6V6 produced in the mid 1930s and subsequently used for military designs etc. and a miniature glass tubular version the 6V6GT. The 6V6 shown here is of the large glass type and the inside of the envelope has a grey smoked finish.
The base also carries the old UK military stores code of 10E/349 and so this American valve was made for the UK Government.
The valves were capable of push pull use but the normal environment was as a single end output stage. In the 21st century with the recognition of the value of valve audio 10 Watt push-pull amplifiers still use this design. An example can be found here and on the museum office desk.
The A M and crown of the UK Air Ministry.
The manufacturers code.
The top mica with copper supports for the control grid a flattened oval cathode tube and the tops of the beam plates.
The classic envelope has a maximum diameter of 45 mm and a length excluding the IO base pins of 99 mm.
References: Data-sheet, 1040 & 1043. Type 6V6G was first introduced in 1937.