Tube Tune-A-Lite ID37856

The following two photos show the I/V charactereristics of the main Anode/Cathode current for the Tune-a-lite 4 element neon tube used as signal strength meter in some Fada Radios as the Tune-o-graph. The 4 terminal Tuneon tube used in some Cossor radios works similarly to this tube. This first photo shows a low 130V trigger voltage for the main c
Tube Tune-A-Lite ID37856 - schematic

athode that is approximately the same as the operating the voltage. This low trigger voltage is facilitated by the 100uA current flow through the small pilot cathode. The pilot cathode is seen glowing to the lower right of the main cathode. This tube has some darkening, indicating use. After about one hour of use, the same 5mA maximum current was enough to cover the entire main cathode with glow. The terminal numbering in the photos refers to the convention used with 4 pin tubes, with a clock-wise count starting at the lower left thick pin, when the two thick pins are at the bottom. The small cathode eliminated the problem that the 2 terminal Tune-a-lite, and similar tubes, have of turning on suddenly, only when voltage is increased greatly after the glow is extinguished. The pilot cathode is present in all 3 terminal and 4 terminal tubes of this type. As the cruve traces below will show, when the cathode glow is below the top of the T, which is to say, with less than 1mA of cathode current, the T terminal behaves as an open circuit. When the glow reaches the top of the T, the terminal starts conducting suddenly. If the load to the T is just the 500kOhms to ground that is common in the application circuit, the Voltage at the T is about 15V when the glow is even with the top of the T, and increases to 40V at full glow with 5mA of cathode current. Some of the large variation in generated voltage at the T terminal can be due...

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