Radio Broadcast (May 1929-Apr 1930)

Record Details:

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PROFESSIONALLY ! I'l ? I j5 , |6 , ,7 ■ £ 9 J4 i ' M 5 ' .' & 1 i . 8 '' ?ihl 1 1 1 1 1 1 1 ti 1 1 I !"l •! 2 , : | , : , . ■13 1 i* i5 is ' ' 7 I S 2 ,th™««»ib»t-ti ■ iiiiiii ■inn It is said that there is a potential marNeeded — a Simple ket in this country for nine-million radio Battery Set for receivers for homes in which there are the Farmer no power wires. These are rural homes, mn. , mi and homes far enough away from power lines to make it uneconomical to extend wires to them. It is in these nine-million homes that we believe a manufacturer with a good three or four-tube receiver operating entirely from batteries and doing it economically would do not only a remarkable piece of business, but a great public service too. Using one screen-grid tube of the 222 type as r.f. amplifier, another screen-grid tube as detector (which will be about five times as sensitive as the 201a), and a power tube of the 112 type, the total plate current will be of the order of 15 milliamperes and the filament current required would be from 0.4 to'0.5 amperes at 5 volts. The difficulty is that people have been educated to want greater power output than the 112 tube can deliver. The solution, which has been found in England and on the Continent, is the pentode tube, a tube with five elements instead of three or four as is common practice at present. It is an efficient power tube, consuming less filament power than most of our battery-operated tubes, requiring only moderate plate current at moderate plate voltages, and needing only a small input signal in order to deliver a moderate power output. Elsewhere in this issue (page 154) will be found some technical data on how much power one can obtain from a tube of this type. These curves and independent measurements in the Laboratory indicate that a pentode will supply a power output equivalent to that obtainable from a 171-type tube at a plate potential of 180 volts — which until a year ago satisfied nearly every one. Furthermore, the plate current drain of a pentode is not over 12 milliamperes. Now let us see what this means. If the pentode were connected to the detector through a 2:1 transformer the detector would have to put out from 2.5 to 8 volts (depending upon the tube) to load it up. A fully modulated signal of 0.25 volts (r.f.) will produce an audio voltage of 0.5 volts across 50,000 ohms in the plate circuit of a 301a acting as a grid leak and condenser detector. If the 322 is five times as sensitive it will deliver, on the same input, an output of 2.5 volts and will require about 0.75 volts to deliver the required 16 volts. In a single stage of screen-grid amplification a gain of 50 can be obtained, and if the antenna is series tuned and coupled to the screen-grid amplifier another voltage gain of 15 can be obtained, making the voltage step-up from antenna to grid of detector 50X15 or 750, so that a field strength of approximately 80 microvolts per meter into a 4-meter antenna would deliver 700 milliwatts into the loud speaker. This is the sensitivity of receiving sets of a year ago. Such a receiver would deliver signals of almost as good fidelity as receivers of a year ago. It would be somewhat deficient in low frequencies, and would not have particularly good discrimination against unwanted stations. It would have, Uv 3 4 ■ '! 2 9 8 7 6 5, SPEAKING perhaps, 30-kc. selectivity. The entire receiver, including loud speaker — as is conventional practice now — would be provided by the same manufacturer and so the farmer, or dweller far from power fines, could get his economical radio all in one package. the fact that the screen-grid tube is the The General-Pur tube par excellence for radio-frequency pose Tube of the sockets, and that it can be used as a Future— the 224 detector, coupled with the general use of • iminiimiimiiiii i single-stage a.f. amplifiers leads us to wonder if the a.c. screen-grid tube will not become the generalpurpose tube of the future. As a detector of the C-bias type, the 322-type tube is about three times as sensitive as the 327-type tube and the 324 is about ten times as sensitive as the familiar heater-type threeelemen' tubes now employed in the detector socket of all a.c. receivers. An article is being prepared for Radio Broadcast which will show the characteristics of the new a.c. screen-grid tubes when used as detectors, and data now at hand indicates that an audio-frequency output of sufficient voltage to load up a 171 or 245-type tube may be obtained with an r.f. input of from 2 to 3 volts. Now let us look at this rather critically. Such a tube will have to be coupled to the grid of the following tube either through a resistance or a choke because of its own very high plate resistance. A 500-henry choke or a 200,000-ohm resistor could be used, but the latter would make it necessary to supply more voltage to the ground end of the coupling resistor than is used in the power tubes — which we shall assume will be the 245 type, for it appears to be the power tube for nearly all home receivers. It looks, then, as though the screen-grid detector must be coupled to the following power tube through a fairly lowresistance high-inductance choke of low distributed capacity. If shunted by a 250,000-ohm resistor and by a capacity of not over 100 mmfd., a 500-henry choke would provide a characteristic as good as many of the best a.f. amplifiers now being put into commercial radio receivers. The disadvantage of resistance or choke coupling would be the fact that a push-pull amplifier could not be used, and it seems to be the tendency at the present tune to make the final stage push pull. If, then, another a.f. stage were inserted between the screen-grid detector and the power tube, a lot of a.f. gain would have to be thrown away in order to enjoy the advantage of push pull, namely freedom from hum on a.c. operation, cheapness of output transformer, and freedom from overloading difficulties. With one or two screen-grid tubes as r.f. amplifiers, a screengrid detector, and a power tube, with pre-selecting ahead of t he r.f. amplifier, we ought to have a pretty economical set to build, operate, and keep in order. And there you are. Will the screen-grid tube become the general-purpose tube of the future? Our guess is as good as any one's. 144 • • JULY • 1929 •