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Build The "Aussie-3" Valve AM Radio

So you thought valve technology was dead! Well it is - but we have exhumed enough of it to produce a 3-valve radio which has quite a respectable performance. It is a superheterodyne circuit, based entirely on readily available components. It is suitable for moderately-experienced constructors - even those who've never touched a valve in their lives!

by Keith Walters

Why would anyone want to build a valve radio, one that doesn’t even pick up FM stations? If nothing else, to get a feel and understanding for old-fashioned technology.

There are lots of people who are attracted to valve amplifiers (particularly musicians) and lots of people busily restoring vintage radios, television sets and all manner of thermionic technology. So why not build a valve radio from scratch? Despite the relatively few parts the radio uses, this is certainly not a toy and it illustrates how much performance you can get out of just a few valves.

As far as its lack of FM reception is concerned, there were no FM radio stations in Australia during the valve era! (While experimental broadcasts started back in 1948, the first FM radio stations, 2MBS and 3MBS, did not start transmitting until 1975).

The "cabinet"

The prototype radio is housed in a whimsical gothic cabinet which pays homage to some of the "cathedral style" radio cabinets of yesteryear. Some people will hate it and others will like it. If you’re in the first category, then build a more conventional cabinet.

Why "Aussie Three"?

Click for larger image
Here's the front view of the Aussie Three removed from its Gothic-style cathedral case. We're willing to bet that the vast majority of Aussie Threes built will remain in this state!

Well that’s a dig at the "All-American Five" concept that emerged in the USA in the 1930s. As an alternative to the grandiose (and expensive) timber cabinet radios that are the delight of collectors now, some manufacturers started marketing the virtues of a basic, no-nonsense but perfectly serviceable superheterodyne that the "regular guy" could afford; the "Model T" of radios if you like. There was no RF stage (which wasn’t really necessary in urban locations anyway) but any lack of sensitivity could be overcome by connecting a decent aerial and earth.

The valve line-up was the now-classic rectifier, mixer/oscillator, IF amplifier, detector/audio preamplifier and a pentode audio power output stage.

Our Aussie Three uses three triode-pentode valves, deletes the valve rectifier in favour of semiconductor diodes and adds a ferrite rod antenna to come up with quite a respectable performance.

To any non-technical user, it’s just a radio: you turn it on and it works! Despite its tiny PVC tuning capacitor, there’s surprisingly little frequency drift, even right up at the top of the AM band. From my home in the outer suburbs of northwest Sydney, it picks up all the Sydney stations with just its ferrite rod antenna, all at about the same volume.

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