The Mullard SP20 screened amplifier pentode was introduced in 1933. The SP13 can be seen as the AC mains version and it was introduced in 1934.
The data-sheet describes the design use in DC radios as HF amplifier or frequency changer in superhet receivers and even as the final detector.
The mesh anode connects to the top cap. Construction pre-dates the use of mica sheet supports and uses glass beads to hold and insulate the supports. The three grids can be seen between the anode and the top cap.
A closer view of the construction. The screen grid is closely wound at the same pitch as the control grid and both are almost aligned.
Here the oxide coated cathode tube can be seen.
The etched lettering on the glass.
The balloon envelope is 50 mm in diameter, and excluding the B5 base pins, is 126 mm tall.
References: Data-sheet, & 1043. Type SP20 was first introduced in 1933. See also1933 adverts.
