Conversion to AC/DC Universal Operation

THE PROBLEM

The introduction by Act of Parliament of a National Grid System ot electricity distribution in this country means that eventually all mains supplies will be A.C. (Alternating Current). In the meantime, whilst some supplies are still D.C. (Direct Current) and in indefinite expectation of being changed over to A.C., the buyer, possessor and maker of an electrically operated receiver is faced with a very great difficulty. The same problem has to be faced by the large number of people who lor one reason or another change their place of residence.

THE SOLUTION

TUNGSRAM at considerable expense and after extensive researches at their world-famous Radio Laboratories have come to the rescue with a wonderful range of UNIVERSAL. VALVES—-valves that will operate equally well on A.C. and D.C. mains. That disturbing hum which is usually associated with an all-mains receiver is practically eliminated and the valves are extremely economical in use—the consumption being only 35 watts.

THE METHOD

TUNGSRAM UNIVERSAL VALVES are so constructed that they all require only 0.18 ampere for the heater, as against 4 Volts at l.0 ampere for ordinary A.C. Valves. The heaters are connected in series so that the total current passing through is 0.18 ampere. The resistance in the filament circuit consists of the following sections :—
From tapping 260 volts to 240 volts - 120 Ohms.
From tapping 240 volts to 220 volts - 120 Ohms.
From tapping 220 volts to 200 volts - 120 Ohms.

 In order to obtain the value of the main part of the resistance : —
Subtract from 200 the sum of the valve healer voltages, then multiply by 6. Thus, for instance, if the valve heater voltages added together total 110, then : Main portion of resistance = (200 - 110) x 6 = 240 ohms. On the diagram a further small section of resistance is shown, value 60 ohms, which is intended as a shunt for the dial lamps so that should a dial lamp fail the receiving set will still continue to operate. 

THE CIRCUIT

The circuit shown in Fig. 1 indicates the basic system of wiring the filaments and provided this system is adhered to no difficulty or trouble should be experienced Any modern Receiver can be constructed to take UNIVERSAL. VALVES. Circuits described in the various wireless journals lor A.C. receivers can quite easily he adapted for UNIVERSAL operation. In constructing such a set it is only necessary to introduce the circuit alterations described below.

THE CONVERSION OF EXISTING RECEIVERS TO UNIVERSAL OPERATION.

1. If your present set is designed to operate from a D.C. supply and you desire to bring it up-to-date so that it will operate equally well from either D.C. or A.C. mains, the only alteration necessary is to dispense with the mains resistance now in your set and substitute another tapped resistance of the appropriate size, the value of which is shown in the diagram Then re-wire the rectifying circuit in accordance with Fig. 1 — an extremely simple operation presenting no difficulties. Having inserted the correct TUNGSRAM UNIVERSAL VALVES the set will continue to function from your present D.C. mains and when your supply is changed to A.C. or should you move to another district where A.C. is already the normal source of supply, your set will require no adjustment whatever, with the exception of the adjustment of the tappings on the resistance to your existing voltage.
2. If the receiver to be converted is an A.C. set, your first step is to do away with the existing mains transformer, and then to arrange the rectification and re-wire the filaments in accordance with Fig. 1. Insert the correct TUNGSRAM VALVES and you have an up-to-date UNIVERSAL Receiver. It's all so simple.

Circuit Diagram

TUNGSRAM ELECTRIC LAMP WORKS (G.B.) LTD., 72, OXFORD ST., LONDON. W.I