Dull Emitters

The first oxide coated valves often had warning labels that filament should not be visible in normal daylight. In the late 1920s valves (tubes) for mains use by using a coated cathode tube isolated from a heating filament (the heater) were introduced which have a visible orange glow at the ends of the cathode tube but not as much as the early Bright Emitters

All the types before 1938 used either mains powered filaments (usually indirectly heated cathodes from about 1928) or a lead Acid Accumulator. The problem with a mains derived Low Tension (LT) supply was that a rectified to DC  without “hum” and correct voltage was expensive (the “A” supply in USA). Creation of a suitable High Tension (HT or B+ supply in USA) was much easier and cheaper. If the AC at correct stepped down voltage is used directly the AC 60Hz or 50Hz is actually amplified. The solution was to coat a more powerful heating filament in porcelain and “fire” this. The heating element was then fitted inside an oxide coated Nickel tube wired as the cathode. This Indirect cathode and Cathode / Heater insulation not only allowed AC operation of the heater (though about x10 more power required) but also heaters of different voltages according to cathode required but the same current to allow series operation of  all the heaters direct from the mains with only a barretter  or resistor in series. So the cost and weight of a transformer could be avoided.

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