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Vintage Radio

DC-to-AC inverters from the valve era, Pt.2

By Rodney Champness, VK3UG

Many radios designed for rural areas ran off batteries during the valve era, since a mains supply was often not available. Often, this involved using separate batteries to power the filament, bias and high-tension (HT) circuits.

By contrast, those sets designed to work from a 6V wet battery used a vibrator power supply to derive a 90V or 135V HT rail for the plate supply. The filament wiring of the valves was arranged so that they could run from the 6V battery.

Other valve radios were designed to work off 12V, 32V, 50V, 110V and 240V DC supplies (eg, from home lighting supplies and small town power plants). In fact, 32V DC home lighting plants were common in farming communities and other areas remote from the reticulated 240V AC mains.

Unfortunately, not many appliances were designed to work off 32V. As a result, several manufacturers built 32V DC to 240V AC inverters to power mains devices, such as radiograms and later on, TV receivers. To my know-ledge, only Ferris radio produced a 32V DC-operated TV receiver, this being back in the valve black and white era.

Click for larger image
Fig.1: the AWA VB-32 used four germanium transistors to provide push-pull drive to an output transformer. Q1, ZD1 and their associated parts form a protection circuit which switches off the inverter if the input voltage exceeds 36V.

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