This exhibit was an ebay find and came as an unknown valve. There are no visible markings on the glass or base and usual techniques did not reveal any information in spite of over an hour of inspection. At one angle an N with a tail appeared and this gave a clue. It was the N of Lissen I believed but further investigation in other lighting conditions failed to reveal any more. In desperation I tried a new lighting technique, the whole Lissen logo appeared as did the 'and' of England. Further work positioned the envelope until at one critical angle the PT with the very faint 225 below appeared.
In 1930 the PT225 battery output pentode was re-engineered onto a B5 base thus placing this exhibit at 1929.
The valve is a low power type with a bright anode and a single inverted V filament. On test the filament was 2.0 V at 0.27 A. The load impedance should be 20,000 Ohms and the output is 250 mW.
The box anode is wider than the front to back dimension. The side screw terminal can be seen.
The three wire grids are formed with the same box shape as the anode.
The results of the new lighting. The Lissen logo and part of the number.
The 225 is clearer and the PT can be seen.
The balloon envelope is 48 mm in diameter, and excluding the B4 base pins is 100 mm tall.
Reference: 1043. Type PT225 was first introduced in 1929.