The A.C. Receiver

Radio Record, Volume III, Issue 12, 4 October 1929, Page 29

The A.C. Receiver

Valve Protection RHEZOSTATS are not used in alter-nating-current receivers, because the filaments of the special valves are not sensitive to slight variations in temperature. In other words, though the filament temperature may alter, the electron emission remains approximately the same. In the heated-cathode valves, this is due to the fact that the electrons are emitted by a small cylinder, which is indirectly heated by the filament. Naturally, once the cylinder has become hot, any slight temperature variation in the filament, unless constant, does not readily communicate itself to the cylinder. In the directly-heated filament A.C. valves, a very low voltage and high current are employed by the filament, and this also tends to keep a steady emission, even though the filament voltage fluctuates slightly. The life of any alternating current valve is materially reduced when the filament voltage is increased over the rated value, and, for this reason, it is beneficial to place a rheostat in each filament circuit, so that the applied voltage can be controlled, even though the line voltage fluctuates. In this way, by operating the filaments at a slightly lower value than that specified, the life of the valve will be greatly increased. This has a distinct advantage when an automatic line voltage control is not provided, especially if an A.C. meter be placed in parallel with at least the 12.5 remaining, or that supplying the greatest number of valves. The. pilot lights employed in an A.C. set can be connected to any of the filament circuits with equal results. However, since the bulbs supplied with these lights are usually desiged for a five-volt supply, it is considered desirable to connect the dial lights to the power valve, as this is usually operated from. a five-volt filament supply. Alternatively, it is an easv matter to change the bulb to one which will operate from the lower voltages usually supplied to A.C. valves. :