The Type 57 is a pentode designed for use as a biased detector. It features the original American specification for indirectly heated valves - a 2.5 Volt heater.
The 6J7 was a later implementation of the design and RCA say the characteristic curves are the same for both.
The Ken-Rad name on the Bakelite base cap.
Through the glass at the top the tops of the electrodes are visible. Note that the screen grid supports are made of copper and not the control grid.
The classic envelope is 38 mm in diameter, and excluding the base pins is 105 mm tall.
Reference: Data-sheet. Type 57 was first introduced in 1932. See also1932 adverts.
