The DW30 is a directly heated full wave rectifier from Mullard and dates to 1929.
The carbonised anode boxes are ribbed to maintain dimensional stability and wide to accommodate the filament strands.
The identification is etched into the envelope dome. The filament passes through the anode three times and the top fixings are insulated by being held in glass rods.
The filaments look to be coiled but could just be ribbed to hold the oxide coating.
The balloon envelope is 58 mm in diameter, and excluding the B4 base pins is 132 mm tall.
References: 1043. Type DW30 was first introduced in 1929. See also1929 adverts.
