B" Battery Eliminators for Small Sets

Radio Record, Volume I, Issue 37, 30 March 1928, Page 12

B" Battery Eliminators for Small Sets

SECOND INSTALMENT OF SIMPLIFIED CONSTRUCTION

-_--- PHS is a further simplified B eliminator to work from alternating current, the main point of simplification being the cutting-out of the necessity for constructing a doublewound transformer. This eliminator is only intended for use on small sets with two or three valves, the fixed condenser capacity having been cut down to the irreducible minimum, chiefly to keep down cost. By the addition of one, two, or more extra condensers of two microfarads each, a larger set could probably be supplied. The writer has made up, and actually used, this eliminator on a two-valve set, and knows that it can be made to function satisfactorily. In order to saye making a transformer with double windings, it is necessary to purchase a toy of bell-ringing step-down transformer with a primary winding, suited to the voltage of the mai#s, and a low-tension winding, giving a voltage suited to heating the filament of the rectifying valve. Al-

ternating current, in most places 230 volts, is then passed through the valve to be rectified, and afterwards smoothed by a low-frequency choke and condensers of large capacity. The lay-out and general construction of the panel, baseboard, and cover will be exactiv the same as described last week for the double-wound transformer type. The bell-ringing transformer will occupy the position of the transformer, and its primary winding connect to the fuses as before. suitable secondary voltage will nect to the rheostat on one side direct to the valve filament on will The conand the other. A connection also has to be made to the filament wiring, in order to draw off the rectified current, which always travels from plate to filament. In case of using an ordinary power | | valve as rectifier, the plate and grid are both connected together, so that the grid forms an addition to the emission surface of the plate. THE WIRING DIAGRAM, PLAN of the baseboard lay-out is given, showing the wiring connections clearly. ‘he inner ends of the fuses connect to the primary winding of the bell transformer, and also branch off, one to the R.F. choke and the other to one of the filament connections. The output of the transformer should be 4 or 6 volts to suit the filament of the rectifying valve, the voltage being regulated by the rheostat, which once set correctly is best left alone. ‘The plate and grid are now connected to the nega‘tive output lead. Other connections are as already described last week. ‘The two fixed resistances of 100,000 olims each as vhown will probably be required to cut voltage down for the detector, as there will only be the drop in the valve and two chokes, which will leave a voltage of perhaps 170, and 45 or 50 volts is ample for a detector. In the case of a two-valve amplifier one resistance would probably give a suitable voltage for the first valve, and the full voltage put on the second, though rather

than put too much on the second it might be a better plan in some cases to run both valves at the same plate voltage through the resistances with the means of regulating and finding the best amount. If a resistance has tu be placed in the "thigh" output circuit, it can easily be done, but an extra % mfd, condenser must be connected to the "high"? terminal and to the negative output, or common wire connecting ene side of all the condensers. It will be for each constructor to suit the eliminator to the exact needs of his set. Extra voltages ate provided by utilising an extra terminal for each, with fixed or variable resistance, and a 2 mfd condenser connected, as already ienticned. THE TRANSFORMER. SMALL transformers can be purchased at an electrician’s all ready for connecting to the mains for about 12s. 6d. Some of these have tapped

secondaries giving perhaps three voltages. ‘‘hree yolts would suit the Philips 273 or Mullard DV1C, or for a UX17], PM256, or PM6, a six-volt tap would be suitable. For ordinary valves, the PM4 or PM254 cre suitable, requiring 8.7 volts on the filament. AN IMPORTANT POINT. [Tt is important to note that when using an eliminator of this type without a double-wound transformer, no direct earth connection can be used on the set. ‘The way to get over this if an earth connection must be used is to insert a good-quality fixed condenser of at least 1 m.f.d. capacity in the earth lead. This makes no diminution in volume. ‘The best condenser for this purpose is one tested at 800 volts and guaranteed for operation at 500 volts. The condensers above menitioned cost about 11s. 6d. each in 1 mfd. and 18s. 6d. for 2 mfds. On no account must constructors omit to provide the tinfoil fuses as shown in the _ plan.