The VP13A is a variable μ amplifier pentode, and features the Ct8 side contact base.
In use the variable μ pentode was designed for AGC controlled stages of superhet receivers, the RF and IF sections. The control grid is taken to the top cap for maximum isolation between grid and anode circuits.
The reverse is plain screening paint. Our other VP13A exhibits have the shouldered or classic envelope shape. This balloon example is an earlier production but has the top hat top cap rather than the screw top cap of the early 1930s. Clearly the mid 1930s was a time of change and maturity in valve construction.
There is a mica top disc but no mechanical connection to the glass.
The balloon envelope is 45 mm in diameter, and excluding the Ct8 base pins, is 101 mm tall.
References: Data-sheet & 1043. Type VP13A was first introduced in 1934. See also1934 adverts.