The DFA4 is a medium impedance triode designed as an audio voltage amplifier.
The Mullard logo, the DFA4 Type and the words Registered Trade Mark.
The reverse carries the type three BBC logo. This dates the valve to between 1925 and 1927. The wording is Protected by Patents and the maker code is C35.
The gettering obscures almost all of the bulb. Here we get a glimpse of the vertical box anode and close wound grid. There are just two filament supports and thus the filament is an inverted V.
A sightly better view of the pinch and lower structure.
The balloon envelope is 52 mm in diameter, and excluding the B4 base pins is 100 mm tall.
Reference: Data-sheet, Data & 1003. Type DFA4 was first introduced in 1924. See also1924 adverts.
