Radio Broadcast (May 1928-Apr 1929)

Keeping r. f. current out of the audio The presence of radio frequency currents in the input of an audio amplifier is always to be avoided; they tend to make the amplifier overload more easily, promote troublesome hand capacity, and invite a lack of stability. In the February ist issue of the Wireless World, A. L. M. Sowerby discusses this problem and suggests the use of a resistance in the grid circuit of the first a.f. tube, as shown in Fig. i-a. and the equivalent circuit in Fig. i-b. Here the resistance, R, is the external series resistance, C is the effective input capacity of the tube, which under operating conditions is much greater than the capacity as measured when the tube is cold. At radio frequencies, without the external resistance, considerable voltage may be developed across the input capacity. With the resistance, however, this voltage is divided between that lost across the resistor and that appearing across the capacity. It is only the latter that is passed into the amplifier. If the impedance of R is greater than that of C, less voltage will appear across C. The following formula gives the ratio between the applied voltage and what actually gets to the input of the amplifier, and the following table gives the result of using an external resistance such that the product of R and C is 10, when R is in megohms and C in mmfd. Such a product reduces audio notes of 5000 cycles only 5 per cent., which is permissible. Frequency 1 500 kc. 1000 750 500 300 150 50 Per cent. Radio Frequency Remaining This shows that on the broadcast band, the r.f. currents can be cut down to a permissible figure without greatly decreasing the high audio notes at the same time. But in super-heterodynes operating at 50 kc. such discrimination is not sufficient; 30 per cent, of the r.f. remains and appears across the amplifier. Here a low-pass filter is necessary. What values of resistance and capacity should be used? Mr. Sowerby states that the effective input capacity of most tubes is about equal in mmfd. to eight times the amplification factor of the tube. This gives the following effective input capacities of American tubes, and using this figure we arrive at the values of resistance given. These resistors need not carry much current; in fact, if the amplifier is properly designed and operated the current that passes will be negligible. Tube type Amp. factor Effective ca 171 20 1 -A 210 Hi-mu Resistance pacity ohms 24 mmfd. 300,000 64 1 50,000 36 30,000 64 1 50,000 64 1 50,000 240 40,000 Many people seem to wonThe Short-Wave der at all the excitement Market about the allocation of fre quencies in the short-wave spectrum. This is probably because the uninitiated ones do not know that a license to operate a station below 100 meters is about as unique a franchise as has ever been granted. We often hear that there are no more lands to be developed, that nations must find some way to utilize the Arctic, that the nitrate beds are all doled out, that the oil interests have gobbled up all the available fields — and yet the entire surface and depth of the earth has not been explored, or populated. Other fields of oil or deposits of gold, or beds of gypsum may be discovered. It is not so in the realm of the short waves. All have been discovered — and unless the Radio Commission gets busy, all will be occupied by other nations, who are not at all altruistic about our getting our share. Once assigned, and with a station operating on a channel, say at 40 meters, the story is told. Any other station of equal power will interfere. Nothing that man has as yet discovered will alleviate the situation. In other words, nothing is so rare as a short wave; it is truly unique. It is like a rare Mauritius postage stamp — we know how many were issued in the year 1847, we know where these stamps are, and nothing can be done about it. No wonder the price is high. We listen in at infrequent intervals to the short-wave stations, hammering out ultra fast commercial traffic, facsimile transmissions, tele vision signals, shooting shafts of highly concentrated beams of electrical energy at Canada, South Africa, Australia. Nearly every time we listen we discover a new station; one of the blank spots on our dial has been filled. There are only a few left. h|iMhHh|iP FIG. I And yet the Radio Commission bewails the fact that in all the short-wave ether there are only places for 200 stations and they already have applications for 300 from this country alone. What is to be done? Not long ago we were called on the phone by a brokerage firm operating branch offices in all the larger cities of the South and Southwest. Their bill for wire lines was about $100,000 a year. Wouldn't short waves cut down that cost? They had heard that a short-wave station could be built for $1000; that the upkeep was small. Would we consider a consulting job of equipping this brokerage company with twenty such stations? 1 he Radio Commission has in its power the issuing of short-wave licenses, the most valuable pieces of paper in modern time. What are they going to do about it? It has been our in"Smkor Swim" tention for some time Tube Testing to mention the excell ent " Laboratory Broadcasts" which appear in the Hartford (Connecticut) Times, a column or two of radio ideas, gossip, popular explanations of what happens in your receiver, etc. L. W. Hatry, who has written for Radio Broadcast as well as other radio publications, is the author; he has had occasion to mention the Laboratory, and the writer, for which we thank him. He has recently discussed hard and soft tubes and states that a good test for a soft tube is to give it 100 or more volts — on the plate, we assume — and if it turns blue internally, it is soft. Which reminds us of the way we used to determine whether a small boy could swim or not. We threw him into the creek, and if, after counting up to a hundred, we pulled him off the bottom and found that he was blue in the face, couldn't tell his name, couldn't walk, couldn't even breathe — we assumed he couldn't swim. Can anyone suggest a better test? — we are now referring to Mr. Hatry's method of testing for soft tubes. In a recent release from Line Voltage the Radio Corporation of "Variations America, Doctor Alfred N. Goldsmith is quoted as stating that there is no serious line voltage problem. He is speaking of this non-existent problem — according to him — because many a.c. sets have caused no end of trouble due to tube failure, when excessive line voltage variations caused the voltage across the a.c. tubes to average too high a value for long tube life. Doctor Goldsmith's statement was based on a series of tests made in the New York metropolitan district, where the power companies spend much money to maintain their voltages constant at the consumer's home or plant. So we took an a.c. voltmeter home, plugged it into the socket and made readings at various times during the day and night. The average voltage was 110, the highest recorded was 113, and the lowest 105. Apparently, the people who generate and distribute power to this part of Long Island (Garden City) take as much care to maintain good lines and good voltage regulation as they do in the city. But we wonder what happens out in the smaller towns, at some distance from cities, say in places with about 35,000 population? How great is the voltage variation there? Is it within 201