The PT225 is a low power audio Pentode with a bright anode and a single inverted V filament. The load impedance should be 20,000 Ohms and the output is 250 mW.
In 1930 the PT225 battery output pentode was re-engineered onto a B5 base thus placing this exhibit at 1929. The base is brown mottled Bakelite that seems to characterize Lissen valve bases of this time.
The side terminal is original. The ridge down the base indicates the anode pin.
The envelope contains no visible markings but the Type designation has been written on the pinch stem. Above the pinch can be seen a wire jumper that connects the control grid to the suppressor grid.
The oxide coated filament is an inverted V. The shapes of the three wire grids is clear to see within the box anode.
The balloon envelope is 48 mm in diameter, and excluding the B4 base pins is 96 mm tall.
Reference: 1043. Type PT225 was first introduced in 1929.