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.
The PT225 was initially produced in 1929 with the side terminal and B4 base, the B5 base and an increased output of 300 mW arrived in 1932.
This exhibit has the side contact for the screen grid and unlike our other PT225 exhibit this has the screw connector that holds the wire in place.
The gettering only obscures one side and so the construction becomes visible.
The three wire grids can be seen above the bright anode box.
Here at the top the filament ends join to the supports that supply the current. The centre tension spring is mounted above the pinch.
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. See also1929 adverts.
