Eliminating the Power Transformer

Author: N.H. Crowhurst

Another system of supply was developed for use where the power source might be either a-c or d-c. (There are parts of the country where d-c power is still provided.) This method has the advantage that a power transformer is not required, which results in some saving in cost. (For this reason, similar circuits are also applied where there is no intention of using the equipment on d-c. In this arrangement, the heaters are wired in series. The tubes are designed to take the same heater current instead of the same heater voltage. (Typical heater current lines operate at 0.1 ampere or 0.15 ampere.) If the total voltage drop across all of the heaters in series adds up to, say, 84 volts, a series resistor will be used to give the required 117-volt drop total.

Heater are connected in series

In this case, the plate supply is usually half-wave rectified, using very large capacitors to keep the voltage up. In addition, special tubes are used that operate satisfactorily at relatively low plate voltages, such as 150 volts.

Plate supply may use large capacitors to keep the voltage up