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Longer Life for Dial Lamps

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New rectifier for American midgets.

Midget receivers with the dial light directly in series with the heaters of the various valves are subject to the difficulty that the set ceases to function if the lamp fails. On the other hand, if the dial lamp is in parallel with one of the heaters the total drain of the filament system is greater. As our New York correspondent points out, the new RCA half-wave rectifier tube, Type 35Z5GT, with a (tapped heater element, provides an escape from this second difficulty. As shown by the accompanying diagram, the pilot lamp is connected in parallel with a short section of the heater and at the same time is in series with the anode circuit of the tube. The rectified current through the anode circuit is, therefore, added to the current available for heating the dial lamp and the short section of the rectifier heater across which it is shunted. The dial light receives about two-thirds of its rated current and delivers about 50 per cent. of its full candle-power and, if left on continuously, its life is increased about five times. Damage due to current surge at the moment of switching on is reduced by the fact that the rectifier does not conduct until the cathode becomes hot, accordingly, the momentary current surge is reduced by 50 mA.

When the 35Z5GT is used without a dial light across the short heater section, the heater wattage is some-what higher and, therefore, the output is greater. Without the dial light, the voltage across the heater is 35, the short section with which the lamp, is in parallel being responsible for 7.5 Volts of this. The heater current is then 150 mA and the rectified current has a maximum value of 100 mA. With the dial light shunted across part of the heater, the distribution of current and voltage is, of course, changed. The dial-light filament takes approximately 110 mA, the section of heater filament in shunt with it taking 90 mA. The voltage drop across the dial light and its section of heater is 5 Volts, there being 31 Volts across the entire heater. The rectifier current has then a value of 50 mA.

The new valve has a glass bulb and is fitted with an octal base.

Showing how the pilot lamp is connected across one-half of the heater element of the new American rectifying valve.
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