Radio Broadcast (May 1928-Apr 1929)

110 RADIO BROADCAST ADVERTISER These Men Tell How They Make ^350 toHSOa/nontfh $350 a Month "I (eel proud of my success in Radio to date. My profit during the last two months amounts to £700. I am making pood and I have not finished my N.R.I, course yet. I am grateful for your training and cooperation to date and look forward to still bigger success when I graduate." Clarence Heffelfinger, Temple, Penna. $450 a Month "I want to say a few words in appreciation of the benefits received from your training. In addition to my regular work in what I believe to be the largest and best equipped Radio Shop in the Southwest, I am now operating KGFI. I am proud of the fact that I installed and put KGFI on the air without help of anyone except N.R.I. I am averaging #450 per month." Frank M. Jones, 922 Guadalupe St. S.in Angelo, Texas. I'll Help You Become a Radio Expert Trained men are needed for this new live-wire industry. Big and little fortunes are coming out of Radio every year. 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I am so sure that my training will put you ahead that I agree to give your money back if you are not satisfied upon completion. Investigate Without Cost What others have done and are doing is proof of what you can do if you try. Do you want more money ? Sure you do, enough to get the good things of life. Radio will give it to you. This 64-Page Book FREE Send the coupon for a copy. Men from 16 to 50 without Radio experience or full grammar school ducatron have found success in Radio through this book. I will send you a copy without the slightest obligation. J E Smith, President Dept. 6-0, NATIONAL RADIO INSTITUTE Washington, 0. C. J. E. SMITH, President Dept. 6-0, National Radio Institute Washington, D. C. Dear Mr. Smith: Send me a copy of your book "Rich Rewards in Radio." I am interested in getting ahead and want to know all about Radio money-making opportunities and about your practical way of teaching with six big outfits of Radio parts. I understand this request does not obligate me in the least. Name Age Address City State Occupation , SHEETS THE Radio Broadcast Laboratory Information Sheets are a regular feature of this magazine and have appeared since our June, 1926, issue. They cover a wide range of information of value to the experimenter and to the technical radio man. It is not our purpose always to include new information but to present concise and accurate facts in the most convenient form. The sheets are arranged so that they may be cut from the magazine and preserved for constant reference, and we suggest that each sheet be cut out with a razor blade and pasted on 4" x 6" filing cards, or in a notebook. The cards should be arranged in numerical order. In July, 1927, an index to all Sheets appearing up to that time was printed. Last month we printed an index covering the sheets published from August, 1927, to May, 1928, inclusive. All of the 1926 issues of Radio Broadcast are out of print. A complete set of Sheets, Nos. 1 to 88, can be secured from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Orders for the next set following can also be sent. Some readers have asked what provision is made to rectify possible errors in these Sheets. In the unfortunate event that any serious errors do occur, a new Laboratory Sheet with the old number will appear. — The Editor. No. 193 Radio Broadcast Laboratory Information Sheet "Motorboating" June, 1928 HOW IT CAN BE PREVENTED AyTANY amplifiers at times show a tendency to "motorboat" due generally to interaction coupling between stages, due to common coupling in the plate-supply unit. This effect can generally be eliminated by using the circuit shown on this Laboratory Sheet. This circuit was suggested in a recent bulletin from the E. T. Cunningham Company. The anti-motorboating circuit consists of a network of condensers and resistances connected between the power unit and the B-plus detector terminal on the radio receiver. The effect of this circuit apparently is to eliminate coupling effects at the low frequencies at which such effects are most troublesome. The circuit has been used with good results in the Laboratory, in connection with resistance-coupled amplifiers which generally show the strongest tendency to motorboat, but the circuit may be satisfactorily used with any type of amplifier. It is not difficult to add this circuit to any existing receiver installation. To do this it is simply necessary to connect the resistance R in series with the lead connecting between the B-plus detector terminal on the receiver and the B-plus detector Radio Receiver B 9* 0<l terminal on the power unit. One 2.0-mfd. condenser Ci must then be connected between the B-plus terminal and the B-minus on the receiver and another condenser C2 connected between the B-plus detector and minus B terminal on the power unit. It is preferable to locate t'he resistance at a point close to the receiver rather than near the power unit. The value of the resistance depends to some extent upon the characteristics of the receiver and the power unit. With some amplifiers we have found a value of 10,000 ohms to be satisfactory, and with other amplifiers, a resistance of 50,000 to 100,000 ohms was required to prevent motorboating. A value of about 50,000 ohms seems to be satisfactory in most cases. No. 194 Radio Broadcast Laboratory Information Sheet Push-Pull Amplifiers June, 1928 pUSH-PULL type amplifiers in many cases exhibit a tendency to howl at some audio frequency due to feedback through the interelectrode capacity of the tubes. When this occurs it is obviously impossible to obtain satisfactory operation from the amplifier. The howling in push-pull amplifiers can generally be readily prevented by connecting a choke coil or resistance at the point marked X in the circuit diagram. When constructing an amplifier of this type it is wise to include such a choke or resistance in the circuit; no by-pass condenser should be placed across the unit. The inclusion of choke or resistance in this circuit will not affect the quality for this circuit does not have to carry any audio-frequency currents. In some instances it will be found necessary to prevent howling to include also a choke coil in the lead from the center tap of the output transformer and the B-plus terminal ot the plate supply. If a resistance is used in the grid circuit it should have a value of about 50,000 ohms. Since it does not have to carry any current, any ordinary grid leak type of resistance unit may be used. The chokes used may be any type with an inductance of about 10 henries or more. The primary of an old audiofrequency transformer might be used in the grid circuit but is not satisfactory for inclusion in the plate circuit between the center tap of the output transformer and the plate supply for when connected at this point, the choke must carry the plate current of the two tubes, which may be enough to burn out the windings of an ordinary audio transformer. Use at this point some device designed to carry 50 or 60 milliamperes. The circuit given on this sheet also shows the use of a resistance Ri to supply C bias to the two tubes. Its value, depending upon the type of tubes used, is given below Type of Tube Ri 112A 750 ohms 171-A 1000 ohms 210 1100 ohms Input transformer