The PD220 is a double triode audio output valve. It was released in 1933 and the original envelope shape was the balloon as seen in this exhibit. Later production PD220 adopted the shouldered envelope shape where the top mica was an interference fit to the glass and thus held the electrodes firmly.
It was designed to work in class B2 push pull. These double valves were a way to achieve greater audio power at reasonable cost from a dry battery radio receiver. Crossover distortion was seen as an acceptable price to pay for longer HT battery life.
With a grid to grid peak to peak input voltage of 58 Volts, the rated 2.85 Watts output would be developed across an anode load of 11,500 Ohms. The input resistance of the push pull pair is 3,300 Ohms.
The twin triodes held only by the connections to the pinch. This example shows a slight lean to the left - possible the result of a knock.
The identification and the Mazda brand name are etched on the dome of the envelope. The single filament tension springs indicate a single inverted V filament in each triode. The finely spaced grid wire can be seen through the hole in the top mica.
The balloon envelope is 45 mm in diameter and, excluding the B7 base pins, is 92 mm tall.
References: Datasheet & 1043. Type PD220 was first introduced in 1933.