Radio Broadcast (May 1928-Apr 1929)

Screen-Grid and Automatic Receivers WE HAVE already published some data obtained in the Laboratory on several of the a.c. screen-grid tubes which are appearing on the market. The curves shown in Figs, i and 2 are taken from the Arcturus Bulletin and give interesting facts an the Arcturus tube of this type. The electrical characteristics are given below: Heater potential . . . 15.0 volts Heater current .... 0.35 amperes Control-grid potential . . -1.0 volt Screen-grid potential . . 30.0 volts Plate potential .... 135 volts Plate current . . . . 1.0 mA Screen-grid current . . 0.50 mA Amplification factor . . 400 Plate resistance . . . 700.000 ohms Mutual conductance . 570 micromhos I The interesting fact, shown on Fig. 2, is the marked peak in the value of mutual conductance and amplification factor at minus i-volt bias on the control grid. From all appearances there will be few commercial receivers using screen-grid tubes available for some time. As usual the home constructor is still able to keep far ahead of the commercial receiver manufacturer; he still has better audio amplifiers than the majority of receivers, and he can build one of several kits now on the market using screen-grid tubes. It is interesting to note that English set manufacturers are ahead of the Americans in the use of the screen-grid tube. The Marconiphone 6i uses three screengrid tubes, each with a voltage amplification of 30 — which amounts to about 27,000 before the detector — and a two-stage resistance-coupled amplifier which brings up the total voltage amplification in the receiver to about three quarters of a million. The entire receiver is copper shielded, each stage being carefully isolated from the others except through the proper conductances. In a loop ten by twelve inches in size, it is possible, in London, to bring in wjz and wgy with loud speaker volume. Set Data Sheet No. 8, on page 278 of this issue of Radio Broadcast, contains more information on this interesting receiver. The Radio Exchange receiver — also sold in England — uses two screen-grid tubes, has six sets of circuits each of which is permanently tuned to a given station and which is connected with a switch on which is engraved the name of the station. At the Radio Trade Show in Chicago there were several screen-grid receivers, and at least one automatically tuned receiver, the Zenith. So far as we could see, there was little interest in the latter, although many people wanted to learn more about the screen-grid tube sets. We hope to present data on these receivers soon. The fact that few people seemed to care for automatically tuned receivers is but a commentary on the American's point of view — he wants to do his own tuning! The automatic tuning feature seems best suited to nickle-in-the-slot radios which will probably appear in pool rooms, cigar stores, ferry boats, chop suey joints, etc. a Strays 1 \ Laboratory E1200 600 31000 500 \ Ep ■135 En X Gm \ < 1,400.000 1.200,000 1,000.000 ' 800,000 ■ 600,000 j I 400,000 j 200,000 ' 30 40 50 60 70 80 SCREEN GRID VOLTAGE FIG. I THE following letter from Trailing "Power A. B. Chastian, Tulsa, OklaLeak" Interference homa, describes one source of what is usually called "power leak" interference, which may give many radio owners an opportunity to go through their own premises to see if occasional noises originate close to home in some unsuspected part of the house lighting system. To the Editor: You might be surprised to find your bothersome "power leak" interference very close at hand. Witness the following case: A noise that sounded like a leak from wind blown wires scraping on a wet roof had been reported from widely separated points in the city. Because of the similarity of descriptions the trouble was believed to emanate from one source, but my part of it was traced to a swinging light fixture which had served for several years as both light and plug-in for various household appliances. I did not suspect the lights for a long time, as the trouble would appear at any time, especially in wet weather, regardless of whether the lights were on or off. However, after much trouble shooting on an unusually damp day, in desperation I gave this fixture a gentle slap, and though it was turned off, the interference responded to the resulting motion of the fixture. The old crumbly rubber insulation on the fixture wires was found to be chafed away in places by the metal parts of the fixture, causing a contact of high resistance which responded to weather conditions and vibrations of the room, sometimes as a loud continuous hum and again as an intermittent hum or sharp crackling. Where interference sounds of this sort are heard, I would direct suspicion to any near-by fixtures, especially those having pull chain or key switches which jar the fixture when used. The joggling of the fixture chafes the insulation and causes these near-shorts, and in time will cause a real short that may prove dangerous. AFTER stating a SMore 'Radio number of complimen Hoaxes tary things about Ra dio Broadcast, Mr. Ernest G. Kroger, Radio Operator on the SS. J. L. Luckenback, says, " I am particularly interested in 'Strays from the Laboratory.' That was a hot expose of the output transformer in the July issue. I opened one I bought for $i .40 and found the same thing that you did. It carried the fancy name of 'Tone Filter.' Mr. A. H. Klingbeil's theory on fading due to street cars may be o. k. where he lives; maybe he can come forward now and give some explanation of fading on the high seas." It is apparently Mr. Klingbeil's move. While we are on the subject of radio hoaxes, we must congratulate Radio News on exposing the inner works and methods of sale of theGeppert " KleerTone" radio cure-all. We have already commented on devices that eliminated static, increased volume, increased d.x., and decreased the A and B-battery consumption. In July Radio News the Geppert wave trap — selling at $4 — is described, and as the editor states, better devices can be purchased for less than $\. Another thing we should like to see exposed is the freak antenna, and we should like to get at the truth of this underground antenna business. Has anyone any data — not qualitative statements, but definite quantitative data — on relative signal strengths obtained from a 50-foot wire strung out in the open and the same length of insulated wire buried in the ground? SEVERAL months ago we cAt Last— A Line announced that automatic 'Voltage Regulator voltage regulator devices were soon to be on the market. The idea is to place one of these devices between the a.c. line and one's a.c. receiver or power supply or other apparatus requiring a constant input voltage. When the line voltage goes down, this device boosts the voltage, and when the line voltage goes up, the device reduces the voltage to that required by the receiver, let us say, and absorbs the additional voltage within itself. The data below is the result of a test in the Laboratory on the Acme Apparatus Company's unit. Volts input from Percent over Output volts to 18 line voltage watt load 96 -12.8 110 100 -9.1 110 110 0.0 110 120 +9.1 110 126 +14.6 110 Volts from input Percent over Output volts to 75 line voltage watt load 94 -14.5 108 109 -1.0 109 123 +11.8 110 The device is designed to operate with appa I.MU> ICljUlllllg IIUL UVCI UU WdUS, WIIIL-I! WUUIU include all standard a.c. receivers getting fila ig J Gm / ■\ f 1 \ / *X \ Ep.135 Egs-30 \ f -* -6 -5 -4 -3 -2 -1 CONTROL GRID VOLTAGE FIG. 2 2.800,000 2,400,000°: 2,000,000 o z 1,600,000 £5 1,200.000 ^ 800,000 3 a. 400,000 0 259