Fig.1: the Airzone Model 520/550 is a typical 5-valve superhet from the mid-1930s. The 6A7 functions as a frequency converter and is followed by a 6D6 IF amplifier, a 75 detector/audio preamplifier and a 42 audio output stage.
By the mid-1930s, many manufacturers were building well-designed superheterodyne receivers. This was made possible by the development of new components. In particular, valves specifically designed for the task of frequency conversion had become common and these considerably improved the circuitry around the converter/mixer stages.
Airzone, like many other manufacturers, was quick to embrace this step forward in valve design, as they made it easy to design receivers with greatly improved performance. Airzone was established in Sydney in 1931 and quickly established itself as a manufacturer of good-quality domestic receivers.
The company’s 520/550 5-valve superhet receiver was released in the mid 1930s and built on that well-earned reputation. It is a well-made set with distinctive styling and is easy to service and align.
The Airzone 520/550
Airzone’s method of identifying their sets was different from that used by other manufacturers. Basically, they gave the chassis one number and the cabinet that housed it another number and combined the two together. This enabled them to quickly identify both the chassis and the cabinet.
In the case of the Airzone 520/550, the number indicated a model 520 chassis fitted into a model 550 cabinet. By contrast, other manufacturers gave sets with slight cabinet variations different model numbers, even though they used the same chassis. This meant that a string of sets with different model numbers could, in fact, be electrically identical.
As shown in the photos, an attractive, tall, brown bakelite cabinet was used to house the Airzone 520/550. This featured an ivory-coloured escutcheon around the dial and speaker cloth. However, the speaker cloth in this particular receiver looks as though it has been replaced so I cannot be sure what the original looked like.
The controls along the lower edge of the cabinet are white bakelite and from left to right they are for volume, tuning and tone. As was typical of the mid to late 1930s, the power was switched on and off at the wall socket.
For a mantel receiver, the 520/550 is quite tall and would also have needed quite a wide mantelpiece to safely support it. In fact, it could have just as easily been called a “table” set rather than a “mantel” set.
The layout inside the set is straightforward, with the chassis sitting on the bottom of the cabinet. However, as shown in the photos, the speaker, speaker transformer and filter capacitors are all fitted to an elevated bracket assembly that’s attached to the chassis, with the parts sitting just behind the loudspeaker. As a result, the set cannot be rested upside down for service and instead must be laid on its side.