Quote:
Originally Posted by Jeffhs
The eye tube is painted on the dial? That's interesting. Since this radio has a ballast, however, I would guess the total filament voltage of the entire string must be well under the line voltage, with the ballast, of course, taking up the slack.
As I understand it, ballast tubes were the predecessors of resistance line cords and, later, wirewound filament dropping resistors. Ballast tubes were also used in early television power supplies, as I read in an old TV service book years ago; many a house fire was started when a ballast failed and the set owner carelessly jumpered across the defective tube.
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Ballast tubes had, IIRC, an iron filament inside a hydrogen gas atmosphere. It was designed to vary in resistance enough over a range of powerline voltages, to produce a constant current thru the tube heaters in the heater string. Somewhat fancier than just a power resistor, and the manufacturer got to claim that an extra tube was used in the radio. Think they came after resistance line cords were banned.
I noticed that the eye pattern on the dial looks to be translucent green plastic. Maybe whatever set this dial came from used a light bulb, which varied in brightness to indicate signal strength. Maybe a small light bulb in series with the plate current in the IF (or RF stages if it's a TRF) tube stage (the AVC line would vary the plate current, and that variation would vary the brightness of the bulb. It would have to be a small bulb, though. Maybe the kind used in small flashlights?