The PA3 is the smallest of the 1923-4 range of Mullard audio output triodes. The PA1 was designated for 'high' power applications such as the final output stage of a multi valve set. The PA2 was a medium power output valve and the PA3 was the smallest of the set.
The PA3 was designed to substitute for the ORA or RA when extra power was required or if a lower distortion was to be achieved.
Thanks to Jim Austin for sending the images.
The PA3 from the rear.
The type one BBC stamp used from November 1, 1922 to September, 1924.
The anode, grid, filament strand and glass pinch.
The label on the front of the original PA3 two part paste board box.
The balloon envelope is 55 mm in diameter, and excluding the B4 base pins is 100 mm tall.
Reference: 1003. Type PA3 was first introduced in 1924.