HINTS AND WRINKLES

Radio Record, Volume I, Issue 25, 6 January 1928, Page 12

HINTS AND WRINKLES

A AND B ELIMINATION | There is a method of running an ordinary set of four or five valves by obtaining both A and B current from an ordinary B eliminator. But certain provisos have to be observed. ‘Ihe vélve filaments must all be wired in series, and that is the only alteration to be made. ‘The output of an ordinary eliminator is limited, and for this purpose not less than an 85 milliamp tube can be used, end unless a ‘850 milliamp tube is used in the eliminator it is nevessary to discard six-volt valves and refit the set with 199’s, which only require 60 m.a. on the filament and 20 m.a plate cutrent, whereas ‘fA’? tubes draw 250 m.a. on the filament and 50 or 60 on the plate. Few would care to actually discard ‘**A"’ valves to give place to 199’s, and this idea is not put forward as a serious proposition, but just to show what is possible with very little alteration. The mere placing of a 850 m.a. tube in an ordinary eliminator would not ensure success, as other components would be asked to carry much more current thanthey were designed for. ‘This remark | applies very much to the variable re-| sistance delivering, the filament current. DISTANT RECEPTION. Detector valves will not work effectively cn weak distant signals when the grid charge is excessive, as too much energy is being fed back from the plate and the grid becomes too high charged. The feed-back should be decreased and filaments lowered or plate voltage reduced. CHEMICAL CHARGERS. When constructing a chemical battery charger, it is just as well to be particular about the metal plates employed, as they are th@ essential part of the rectifying process. Both the lead and aluminium should be the purest obtainable, especially the aluminium, but these should be kept small in size. No. Ik battery shonli be charged at a greater rate than }-amp., and .this rate is regulated by the amount of plate surface exposed or immersed in the liquid, and the distance separating the two electrodes, which should not be too small, not less than half an inch on 230-volt circuits A ‘saturated solution of ammonium phosphate is the usual liquid, but borax is sometimes used. It is important to "form" the plates before attempting to charge a battery This is done by connecting up a resistance in place of the battery and passing current through the rectifier for two or three hours. If the lead has formed it will show a dark chocolate colour like the positive plates of an accumulator. This "forming’? does not last long when the charger is not used for a few days, and it is this fact that causes a good deal of trouble with such rectifiers. It is a good plan to pass a forming current through for a few minutes if the charger has been standing for some time. Another point is to use fair-sized jars, as the larger amount of liquid contained will keep at a low temperature longer than a smaller quantity. Resistances in the circuit, usually lamps, cut down both volts and amperes. AERIALS AND REGENERATION. Any and all regenerative receivers work most efficiently on long aerials. When the aerial is too short it causes the set to oscillate too easily. The remedy is to reduce the number of turns on the tickler coil, or reduce the bypass condenser. capacity in the plate circuit of the detector, or do both. USE: OF BY-PASS AND CHOKES. Both by-pass condensers and R.I’. chokes greatly increase the efficiency of any receiver when properly used. The condenser is used to pass the R.I. currents around some circuit or component, and keep the hattery currents out. The choke coil has an opposite effect. It permits the battery currents to flow through, but chokes back or prevents the R.I’. currents from going through. IMPROVING RESISTANCE COUPLING. Readers who are using resistancecoupled audio amplification will be able to effect an improvement in reception by doing a little experimenting in the way of utilising newer valves than those now employed. The high amplification valves recently introduced, such as the UN 210, have so increased the ainplification per stage for resistance coupling as to compare favourably with the usual stage of transformer coupling. Thus a_ two-stage resistance-coupled amplifier, using one of these latest valves for the first stage and a powcr yalye for the second, will be found to deliver ample loudspeaker volume, while the B-battery drain will be found surprisingly low, as compared with the use of the standard "A" tuhe on the same cirenit. The power valve would be of the 171 or 112; tvpe, with a coupler made up of a 250,000-ohm plate resister and a 100,900-ohm grid leak, with a .05 mfd. coupling. condenser. The first stage wonld hé 'coupled in the same wav. but with a-different grid leak valve, 2 megohms. ~ WATCH YOUR FILAMENTS. There is a critical temperature for every yacunum tube, especially dull emitters with a thoriated tungsten filament. In these tubes a délicate balance imust be struck hetween the heat necessiry to drive out ithe required

quantity of thorium to coat the surface, and the excessive heat which results. in burning off the thorium coating, so that veritable bald spots are formed with the paralysing of the tube to shortlv follow. In most cases filaments are operated at from 10 to 80 per cent. excessive voltage, reducing the tube life bv hundreds of hours, not to mention the lowered efficiency of the receiver through serious deactivation. In other cases operators, afraid of harming their tubes, will operate at as much as 40 per cent. below the proper temperature, which is insufficient to preserve the cotrect crvstalline construction of the filament, and, at the same time, introduce distortion, due to the insufficient power of emission. ‘Megohm" recommends every constructor and experimenter to purchase a reliable voltmeter, which is the most reliable way of checking filarhent current. It is placed across valve socket filament terminals to test voltage, and not across battery terminals, each time the receiver is used. adjustment being made with the rheostats. This will ensure correct operation, long life to valves, and good tone reproduction, whilst the first cost of the meter will soon he saved by less frequent valve renewals