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 Type designation is faintly etched into the glass. The outer words are Made in USA.
Top view showing the control grid connection to the top cap. Although distorted by the glass the grids can be seen to be made of wire - not a beam tetrode.
The lower view also distorted by the glass.
The classic envelope is 38 mm in diameter, and excluding the base pins is 108 mm tall.
Reference: Data-sheet. Type 57 was first introduced in 1932. See also1932 adverts.
