Re: When did Valve heaters change from 4v to 6.3v
In GEC's Osram valve catalogue for 1938-9, all the valves mentioned in #7 are in the "Replacement types, available for servicing" list. There are approximately equal numbers of 2V and 4V types, and a somewhat smaller number of Octal-based valves .
Most of the Octal types have 6.3V heaters, but various ones have 26V (KT32, U31), 26V/13V (KT33), and 5V (U50, U52) heaters. The data sheet for the KT33 says "By usingthe heater centre tap, the valve becomes suitable for operation from a 12 - 13 volt Low Tension supply, such as a Car Battery." This is the only reference the catalogue has to car use.
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