New Points For Listeners and Dealers

Radio Record, Volume I, Issue 41, 27 April 1928, Page 3

New Points For Listeners and Dealers

By

Meter

The aim of this section is to give listeners information of new and interesting devices and sets on the local market. It is free of advertising intent or influence and to the best of our ability will convey only absolutely reliable statements. Names, prices andl sources of supply are mentioned for the benefit of readers and to save individual nquiry-

HE name of the Radio Corporation ot America has always veeu associated with the highest standards in radio wares, and this tradition’ holds good in respect to the conipany’s 100-1 loudspeaker, which I have subjected to a fortnight’s testing. ‘This loudspeaker has proved itself in evety way .a monarch among its competitors in the astomshing faithiulness of its reproduction on all audible toes, and in its sensitivity. With it instrumental aud vocal music is a gew delight. The bass notes are superbly rich and clear, and the treble leave nothing ic be desired. Under the tertific. stain of band music broadcast from 2¥A, Wellington, I found it impossible to make the loudspeaker ‘‘chatter,’’ althougi: the music was passed through six valves at relatively close range to the big station. The 100-A R.C.A. loudspeaker is of the mantel-piece clock desigu, "housed itl a metal container. It embodies a new type of corrugated cone, wliich is completely weather-proof Its pole pieces and armature ate extra heavy, pertitting great volume without saturation. A large-size permancnt magnet of special alloy stcel gives greater sensitivity and volume throughout the whole musical scale. It is claimed that this magnetic strength will not decline with age, resuiting in ‘lecreased sensitiveness, as is Ue case with some loudspeakers. An electsical filter is also provided, which removes distortion produced by high-frequency harmonics and summation tones. ‘the inetlal case acts as a baffle-plate, and preserves the deep. full tones, whil: a light felt Jining destroys obnoxious resonant cfiects and insures uniform response throvghout its frequency range. This sneaker is retailed aj £12 10s. It was throeuch the courtesy of the National Electrical and Engineering Co.. Ltd., 42 Customhouse Ouay, Wellineton. that T have been able to test the R.C.A. 100-A loudspeaker. COMMERCIAL CONDENSERS, qs many instances the size of variable condensers is given in microfarads, and ii still more cases it is given in number of plates. ‘The number of plates is no gauge to the exact capacity of the condenser, as the size of plates diflers with different makes as well as the spacing between plates. As_ a tule, however, the requirements for variable condensers are not exacting as to size, the size specified being the maximuin capacity when the plates are all in. In general the relation between nenibves of plates and capacity is as folows *--~

The mere fact that a condenser has metal end plates does not mean that it is a better condenser or a "low loss’’ condeuser. There are just as many poor coidensers. with metal end plates as with insulation end plates. ‘There. are just as good or better condensers with insulation end plates as with metai eid plates. The name of the manufacturer is your best guarantee. The plates should be thick and stiff, preferably of aluminium or brass, equally spaced, and the frame construction rugged. A vernier dial such as ‘velvet grip" or ‘‘easy tutie’ is better than a separate vernier plate, as a rule. Usually small plates mean a better made conenser. A pig tail connection is not necessarily better than sliding contact, but do not oil the bearings on a condenser which has no pig tail. With so many good condensers to choose from you should pay more at4

tention to mechanical construction and strength, especially in the bearings where sluckness soon develops, CORRECT RHEOSTATS. .if problem of determining the cor-. rect resistance of rheostats often confronts a buyer who likes to design and build his own equipment. ‘The proper resistance to use depends on the voltage of the "A'’ battery and the normal current that the valve operates on most satisfactorily. , For WD-11 and WD-12 tubes on 15) volts a 6-ohm rheostat is satisfactory. | ¥or single C-30l-a or UX-20l-a tubes or any ¢ ampere tubes on 6 volts a 30ohm rheostat is proper. Tor two C-80l-a or UX-201-a tubés in parallel a 20-ohm rleostat will suflice. For four of these tubes in parallel a_ 6-ohm rheostat wili be sufficient. For UX-199 or C-299 tubes on 44 volts use a 30-ohm rheostat. For UX-112 and UX-171 tubes use a 15-ohm rheostat on 6 yolts. lfor UX-112-a and UX-I7l-a tubes use a 30-ohm rheostat, ‘hese are the new quarter-ampere valves. ; | | | R20 service men should interest their clients in the protection of | their loudspeaker, Due to the in- | creased clarity of signals, as well as_ to the additional power which may be obtained, many owners of receiving sets are incorporating power valves in sets which were not originally built to accommodate them. Tt has been the sad experience of many of these | fans to find out that after a shox: period the loudspeaker would go ou a "strike" due to the windings of the electro-magnet coils burning out. This condition arises from the fact that the additional current necessary to operate these power valves is really more than the windings of the speaker can stand, with the result that a powerful surge induced by a loud burst of music will canse the fine wires to fuse. Damage to the loudspeaker can he prevented in a very simple manner by the use of an output cirenit, one type of which is described herewith. It is not necessary to place this appara- | tus in the cabinet, as it may easily | be mounted right on the base of the loudspeaker. | The apparatus necessary is a choke | coil and two Imfd. fixed condensers, | These are connected in the following manner: First, they are mounted, with the two condensers in parallel. From one side of the output going to tlic speaker, connect a wire to one side of this condenser bank. From the other side of this condenser bank a wire goes direct to one side of the choke coil, and continues from there to one terminal of the set output, | Irom the other output terminal of the set connect a wire to the other side of this choke coil and to terminal for the output to the loudspeaker. "NEW" CIRCUITS, HE home constructor is confronted with "new" circuits in nearly every radio magazine, and he is often tempted to tear down his set and test tlre latest circuit, only to find it is tie better than the last. An American radio writer says:"When a radio wave coming through the air at the rate of 186,000 miles per second comes in contact with the wire of an aerial it sets up in that wire a current which, after a great deal of amplification, emerges as sound energy from the loudspeaker, Whether this sound is a faithful reproduction — of what took place in the broadcast | studio is merely a matter of engineer-_ ing, and as we generally assume that there is no distortion between the studio and the receiving aerial, the main troubles are encountered most often in the receiving sets* themselves,

fr 6*fA5 hhas been intimated, the problem of distortion is one that has been engaging the attention of engineers ever since the start of broadcasting. Sets of all types, circuits, number of valves, etce., have been tried with the idea in mind that distortion was to be reduced to a minimum or eliminated ettirely. New circuits were brought out, which upon inspection proved to be nothing more than old hook-ups dressed up in new clothes, and many of these newcomers were little better than their forerunners, "So many of these so-called ‘new’ circuits have been foisted upon the radio public that every time something really worth while is put on the market the thinking portion of the radio enthusiasts look at it askauce. It is seldom that anything radically new is presented to the radio constructor as something that is actually new in every sense of the word." TO FIT A VOLTMETER. RAvdIo traders who build high-class sets should equip them with the best voltmeters. ‘hey add greatly to the finish of a set. A voltmeter is a very handy instrument to have ou the panel cf a set using five or more valves, for with it you can tell the condition of both A and B batteries at any time. Some set builders have not fitted a voltmeter on accourt of the apparent difficulty of cutting the large Lole necessary to take the instrnment. If this has proved a difliculty to ycu, the following instructions will show you how to make any size of hole in the panel. Mark off panel with a pair of dividers the exact size of the hole desired, then mark out a second circle 4 inch itiside. Now mark off the inside circle in +4-inch sectious all round, then take a bradawl or the point of a sharp file and spot each hole. The next operation is to drill a hole at each f-inch mark with a }-inch drill. wy take a f-inch drill, and drill into each of the }-inch holes. If you have marked off your }-inch sections correctly, you will find that when you have finished with the f-inch drill that the piece of panel in the centre will Fall out. f Finish off the hole with a half-round le.

conductors of current. In fact, nearly all oxides are insulators. So proper contact must be assured if the tiny one part of a set to another. THE VALUE OF SOLDERING. "THE radio service man, professional and amateur builder, should solder connections in a set wherever possible. One cannot receive unless the energy is transmitted through proper contact from metal to metal. An invisible coating or film. of air surrounds every metal, no matter how tight it may be pressed against another metal. Or worse, this film of air coats the metal with an invisible oxtde. All oxides are notoriously bad surges of current are to be led from With poor contact a surge of current cannot build up so as to produce oscillation. One must remember that a complete oscillation is a cycle of current running O degrees minimuin to then to 180 degrees, where a reversal 90 degrees, the maximum in strength, f polarity occurs, and on to a similar change with opposite polarity. Now, noor contact is hed, the current canvot build up from 0 degrees to 90 degrees, consequently no current will flow and no signals will be heard. So the wise radio builder will see that good contact is made and kept by soldering every place in his set where current flows from one metal part to another. ‘The soldering must be properly done, the joint first cleaned by scraping and sand-papering, then the parts tinned, using a small amount of | not-corrosive rosin flux, and finally the "arts soldered together, using just enough solder to make a good electrical connection which has mechanical strength. Every joint should he mechanically. i strong, as ‘well as electrically conductive. If one inserts his hand into the |

set for any reason, and brushes against ‘ie wire, the joint, if not mechanically strong, may break. GOOD EARTH ESSENTIAL. SERVICE men who receiyed complaints about poor recepti 1 sliouid never fail to examine thei clients’ earth connections. It generally requires a few months to educate the average radio novice ou the necessity for providing an efficient earth connection. The directions given in the radio bovks, magazines, etc., read ‘‘attach the giound wire to a water pipe, hot-water cadiator, ¢tc."’? In the hurry ft get the set into operation the new enthusiast makes a very quick job of the earth connection, then wonders why his set will not operate properly. The groutd wire does not have to be covered with insulation, although it would be better if it were, tut it should be made of a fairly heavy gauge pure copper wire of about 12 or 14 gauge. Secure an earth as near yvour set as possible, as the shorter the distance from set to the carth the better will be your results. A cold water pipe makes the best place to connect the eaith wire. Kirst scrape or file about 1 incl: round the pipe, then with 2 picce of sandPaper brighten the copper wire. Wrap from 10 to 15 turns of the wire round the scraped part of the pipe, pulling it un tight with @ pair of pliers. The wire should be soldered to the pipe, and then wrap the joint with Wack friction tape or adiiesive plaster. This will keen out the air, and prevent the bright part of the pipe and wire fromm oxidising and so reducing the efliciency of the earth in a short time. WEIGHT OF COPPER WIRE. HE following table showing the number of feet per pound of copper wire, should prove handy to dealers :- | | | } | WIRE TABLE OF FEET PER POUND. | B&SSHinele Double Single Double

Number of Capacity of plates, Microfarads, 1 0.00025 25 0.0005 43 0.001 65 0.0015

gauge cotlon cetton eilk silk Enamel 311 298 319 312 320 389 370 403 389 404 488 461 503 493 509 1 612 584 636 631 642 762 745 800 779 810 957 903 1 ,005 966 1,019 1 ,192 1,118 1 265 1 302 1 286 1,488 1,422 1,590 1,543 2822 ,852 1,759 1,972 1917 '042 1 1858 2,207 2,570 2,485 2,570 3,868 2768 3,145 2,969 3,240 800 768 3,943 3,683 4,082 4,375 3 737 4 950 4 654 5,132 5 390 4,679 6.180 5,869 6,445 6 KE0 6 '168 7,740 7,111 8,093 8,050 6,737 9 600 8,534 10,197 1 9,820 7,877 12,000 10,039 12,813 1 1,860 9,309 15,000 10,666 16,110 14 300 10,636 18,660 14,222 20,274 39 17,130 11, '907 23,150 16,516 25,519 40 21,590 14,222 23,7c0 21,333 32,107