CHANGING OVER

Evening Post, Volume CX, Issue 69, 18 September 1930, Page 23

CHANGING OVER

BATTERY TO MAINS

"Grid Bias" has been asked more than once whether an ordinary "battery" model receiver can be easily changed into an "all-electric," and whether the change is unduly expensive. The change is not difficult, the expense probably in most cases means economy, and the results should be quite satisfactory. Until quite recently the only way of getting rid of the A or filament battery was to install an "A eliminator," a costly piece of apparatus that did not always work well. This is no longer necessary, as it is far more economical to use alternating instead of aireet current, and to replace all or some of the valves with those suitable for alternating current. The change, though simple, is not a job that should be tackled without some experience on the practical work of set construction, and some theoretical knowledge. The first ease to consider is that in which a B eliminator is already in use.

The first step is to decide on the valves to be used. If a power valve is already in use, with a filament taking i ampere or more, it can be retained; it will work quite well with alternating current. Although the radiofrequeney valves and the first audiofrequency valve may be of the type using an ordinary filament, providing each takes i ampere or more, it is probably the best practice to reject them, as complications in design are liable to arise. A special valve, of the "indirect heated" type, is essential for the detector.

There is a fairly wide range of suitable valves to choose from. A comprehensive catalogue, giving the essential data for practically all types, is pnblished by the Philips Company, and is sufficiently wide in its information to serve as a guide to most of the popular makes. The first point to be decided is the voltage of the filament supply. This should be the same for all the valves except the power-valve, and may be either 2| volts or 4 volts. These valves have five pins, the fifth being for the cathode, which is the element heated by the filament. The "grid-return" lead, which in ordinary valves is connected with the filament, is connected to the cathode, suitable provision for grid-bias being of course interposed. Each pair of filament terminals is fed by a pair of twisted flex leads, so as to avoid induction that might .cause hum, and these aro joined to the source of a.c. supply. The original filament wiring, including the rheostats, switches, etc., is, of course, scrapped. The filament heating current is provided by a suitable transformer. This is already incorporated in some B eliminators, but in most cases it is not. Fortunately, a suitable transformer _is neither expensive nor difficult to build if it is desired to construct one, and design data can easily be got. There must be two secondary windings, one for the power valve and tho other for the other valves; and if one is specially built, care should be taken to see that the wire used is not skimped in size. Liberal windings give much better results, and tho difference in. cost is merely a matter of pence. The power valve winding should be bridged by a centre-tapped resistance of 50 or 100 ohms, to provide a voltage balance that avoids hum.

The filament transformer is connected to the mains in parallel with the eliminator, and is controlled by the same switch.

In cases where there is no B eliminator, the set-owner has to face a bigger expense, because he must, of course, provide that accessory as well as change his valves. But if he selects his eliminator well (he may, of course, decide to build one, saving little in cash, but having a lot of fun), he will see that it contains suitable filament heating windings. This article is not intended as directions for making the changes suggested, but merely as an answer to the questions referred to in the opening paragraph.