Historically, most urban dwellers have not been interested in listening to radio stations outside the city they live in. As a result, radio manufacturers made a point of producing small 4-valve mantel receivers for the mass market. They were typically used in kitchens and bedrooms and were marketed alongside the more expensive 5-valve (or more) mantel and console receivers that were popular from the 1930s to the 1950s.
These little 4-valve sets and their larger siblings were mostly mains-operated and were usually quite simple in design. However, their performance was quite adequate for most city users, who tended to use them for background entertainment rather than for serious listening. It’s fair to say that the production of 4-valve sets, along with more ambitious receivers in classy cabinets, was the mainstay of radio manufacturing during the valve era.
Although such sets performed well in city areas, where there were lots of local stations, they were often unsuitable for use in remote rural locations. Many people at that time had no access to mains power, which meant that the sets had to be battery-powered or designed to run from 32V DC lighting plants. This in turn meant that power consumption had to minimised.
This is the view inside the old Airzone 612 console. A large loudspeaker and a decent baffle ensured good sound levels despite the output stage delivering a maximum output power of just 350mW.
Sets designed for use in rural areas also had to be more sensitive. AM radio stations back in the 1930s were not particularly powerful and were often even less so in country areas. That’s because commercial broadcast stations were allowed to operate with transmitter output powers of 5kW in capital cities but only 2kW in country areas.
As expected, country stations were located only in those areas where there was enough advertising revenue to make them viable. This meant that many areas had no stations within hundreds of kilometres. Where I lived as a youngster, the nearest station was 3WV which was 145km away, while the next nearest station was about 180km distant. And we didn’t live in the outback by any means!
As a result, for many people in rural areas, large outdoor antennas and sensitive receivers were needed to pick up a reasonable selection of radio stations. Fortunately, ABC national stations were allowed to broadcast at higher powers than the commercial stations and so many ABC country stations used 10kW transmitters.
Against this background, some manufacturers marketed sets that were specifically designed for rural listeners. One such set was the Airzone 612, a 6-valve battery-powered console receiver.
Fig.1: the Airzone 612 is a fairly conventional superhet design employing six valves. It's a dual-band set and was powered from a 2V lead-acid cell (for the valve filaments) and three 45V batteries (to derive a 135V HT supply).
The Airzone 612 6-valve set
The Airzone 612 console from 1938 was an impressive receiver, both as an attractive-looking console and as a sensitive battery-operated set for remote country areas. Airzone had been making good receivers right from the beginning of the 1930s and this model really performs well.
As mentioned earlier, receivers designed for the more remote regions of Australia needed to be quite sensitive. They also needed to have good selectivity and to use as little power as possible.