Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST screen-grid tube by putting 180 volts on the plate and 1.0 volt C bias on its control grid may be somewhat startling and beyond the expectations of the hopeful user, but it is true, too, that he should have no come-back on the manufacturer when his tube fails prematurely. Servicemen will do a favor for both the tube manufacturer and the user of tubes if they will insist on proper voltage values. A tube that gives fine initial performance and then fails long before its expected 1000 hours are obtained cannot but make the user skeptical of other products of the same tube manufacturer. Within the last year we have had two manuscripts in the office in which the writers advised the use of values of plate voltage and C bias on screen-grid tubes that would limit their life to several months instead of a year or more. In one case the values of voltage were so excessive that the filament actually changed its brilliancy when the plate voltage was turned on. How Long Will the Engineers' Job Last? RADIO engineers often wonder how long it will be before their jobs are jobs of the past, with the progress now being made toward perfection. As an example let us look at the curves in Fig. 5 which are taken from The Wireless Magazine (England) of July 1928. They show the progress that has been made in the Columbia (English) phonograph records and reproducers since 1920. What will the engineers do next? There is no object in extending the frequency scale? Will they invent some new frequencies? Our suggestion is to drag out into the open Cello & Human Voice Bassoon 128 256 512 1024 VIBRATIONS PER SECOND 4096 8192 Fig. 5 Frequency characteristics of old and new-type phonographs. this year's manufactured receivers use tubes of this type. Is it possible that receiver manufacturers are waiting until the kit people have developed circuits to the point where set manufacturers can use them without any development cost? Impedance of Standard Loud Speakers MANY readers have inquired about the impedance of loud speakers. On this page is a curve made on the W.E. 540-AW loud speaker made by Frank C. Jones, of California, who has written several articles for Radio Broadcast. It shows that this loud speaker, long the standard laboratory loud speaker, has an impedance of 4000 ohms at 100 cycles, 13,050 ohms at 1000 cycles, etc. Here, too, is the impedance curve on the R.C.A. 104-A loud speaker — ■ a moving-coil loud speaker. A Test for Screen-Grid r.f. Tubes A READER asks how to check his cx-322 tubes. The Cunningham engineering department states that a good emission test is to tie the grid, screen grid, and plate together and to apply 50 volts to these elements. Next, place 3.3 volts across the filament and measure the plate current. If the current is above 12 mA. the tube is good, Has every engineer, every serviceman, and every laboratician a copy of the Cunningham Tube Data Book? It has in it just this kind of data. 100 200 500 1000 2000 5000 10.000 FREQUENCY IN CYCLES Fig. 4 — Impedance curve of W.E. 540-AW loud speaker those machines which will play for an hour or so without the bother of shifting records. How Useful is a Tube? + 80 +70 <• 60 + 50 + 40 ' }0 + 20 + 10 -10 Impedance Curves of an RCA 104-A Loud Speaker ( MovingCoi! only) Diaphragm free I // v / Z Calculated / K Measured i i i I i i i 1 1 1 1 1 1 + 3 +2 100 200 Fig. 3 500 1000 2000 FREQUENCY IN CYCLES -Characteristic Curves of R.C.A. 104A loud speaker WE HAVE yet to find any figure in the radio industry who does not know George Lewis, vice-president of the Arcturus Radio Company. In a statement made recently Mr. Lewis suggests that a vacuum tube's usefulness has, in the past, increased with the square of the number of its elements. Thus the addition of the grid to Fleming's original two-element tube increased the tube's utility four times, then the isolated cathode, the heater, made possible the operation of tubes from raw a.c, and iinally the word of Schottky and Hull in putting the second grid in the tube has again increased the value of the tube. Mr. Lewis believes that the a.c. screen-grid tube is the most satisfactory tube in the world. We would agree with this belief except that no one seems to know how to make use of the tube's evident possibilities. Note that few of 1 < -2 5000 10,000 Accuracy of Variable Condensers THE Hammarlund Manufacturing Company states that individual units of a Hammarlund three-gang condenser are accurate to within one quarter of one per cent. It is interesting to see what difference a discrepancy of this amount from the rated capacity will make in a tuned circuit. Suppose one condenser of the three is one quarter of one per cent, higher in capacity than the other two. By now many cycles at 1500 kc. will the circuit which this condenser tunes be out of resonance? The capacity of the condenser compared with the others is C x 1.0025 and the frequency to which this circuit tunes, compared with the resonant frequency of the other circuits will be one half of one per cent, lower, or at 1500 kc, the circuit will actually tune to 1500 — 7.5 = 1492.5 kc. This is probably more accurate than commercial coils can be made — and so The Hammarlund Manufacturing Company has something to talk about in the accuracy of their production condensers. New High Voltage Recti fier Tube THE Raytheon company have announced a new rectifier, apparently a modern "S" tube whose demise was regretted by every transmitting amateur. This new tube will rectify 300 milliamperes of current at 3000 volts, and the voltage drop is only 10 or 12 volts. This is a lot of power as anyone who multiplies I = .3 by E = 3000 will find out. (It amounts to about one kilowatt). We are hoping to get some of these tubes in the Laboratory for our short-wave' transmitters, w2gy and w2ej. Incidentally, the Raytheon Company plans to manufacture all types of receiving and amplifying tubes of the filament type. New Regulation of Radio Commission GENERAL Order No. 55 of the Federal Radio Commission contains this paragraph, "No license shall be granted to any applicant for a fixed station, coastal station, or aeronautical station who is unable to satisfy the Commission that he can maintain the assigned station frequency with an accuracy of 99.05 per cent, or better at all times." At 6000 kc. this amounts to 3000 cycles. How is the applicant to satisfy the Commission on this point? — Keith Henney. february, 1929 . page 240 •