The Cosmos SP42/U is a directly heated full wave rectifier.
The twin shortpath rectifier sections in the balloon envelope. The central rod holds the filament and is insulated from the anode by the two ceramic rings. The oxide coated filament is an inverted V.
The BVA logo etched into the side of the glass. At the top of the right hand rectifier section can be seen the hook support and the top loop of the filament.
The assembly at the top employs glass bead insulators. Perforated mica sheets came later.
Another view.
Here the shape of the anode is clear to see including the back flange providing extra heat dissipation.
The B4 base and a clearer view of the etched lettering. The lower word in parenthesis is (FULL) as can be seen the the advert referenced above.
The balloon envelope is 58 mm in diameter and, excluding the B4 base pins, is 140 mm tall.
References: Data, & 1043. Type SP42/U was first introduced in 1927. See also1927 adverts.
