The PT225 is a directly heated battery pentode with an oxide coated filament. The design purpose was as the audio output stage and the output power was up to 250 mW.This early example has a side contact for the screen grid. Later examples (from 1932) were made with a B5 base. Where the bulb is clear the pinch and bright anode can be seen. The nickel sheet anode has ribs pressed into it to add mechanical rigidity. The top of the anode showing the three wire grids and the oxide coated filament strands. All supports are held in place by a mica strip. Note the width of the anode is much greater than the front to back dimension. In this image of the top of the envelope the feint etched lettering is visible. This reads Made in England and PT225. The balloon envelope is 42 mm in diameter, and excluding the B4 base pins is 97 mm tall.Reference: 1043. Type PT225 was first introduced in 1929.