Radio Broadcast (May 1928-Apr 1929)

130 RADIO BROADCAST ADVERTISER Melodies Caught in Flight with RADIO ^ TUBES As the Yule-logs crackle and music fills the air, enjoy the Christmas melodies to their utmost by having new Cunningham Radio Tubes in every socket of your radio. These "ambassadors of joy" make delightful Christmas gifts. E. T. CUNNINGHAM, Inc. New York Chicago San Francisco Manufactured and sold under rights, patents and inventions owned and/or controlled by Radio Corporation of America THE aim of the Radio Broadcast Laboratory Information Sheets is to present in a convenient form, concise and accurate information in the field of radio and closely allied sciences. It is not the purpose of the Sheets to include only new information, but to present practical data, whether new or old, that may be of value to the experimenter, set builder or service man. In order to make the Sheets easier to refer to, they are arranged so that they may be cut from the magazine and preserved, either in a blank book or on 4" x 6" filing cards. The cards should be arranged in numerical order. Since they began, in June, 1926, the popularity of the Information Sheets has increased so greatly that it has been decided to reprint the first one hundred and ninety of them (June, 1926-May, 1928) in a single substantially bound volume. This volume, "Radio Broadcast's Data Sheets" may now be bought on the, newsstands, or from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Inside each volume is a credit coupon which is worth $1.00 toward the subscription price of this magazine. In other words, a year's subscription to Radio Broadcast, accompanied by this $1.00 credit coupon, gives you Radio Broadcast for one year for $3.00, instead of the usual subscription price of $4.00. — The Editor. No. 241 Radio Broadcast Laboratory Information Sheet December, 1928 Supplying Power Devices from 220 volts A.C. USE OF STEP-DOWN TRANSFORMERS T ETTERS are received frequently from readers in which the following question is asked, " I live in a district in which the only a.c. supply is 220 volts. How can I adapt a 110volt B-power unit for operation on a 220-volt line? " There are two methods by which this may be accomplished. First, a resistance of such a value as to produce a drop of 110 volts and leave remaining 110 volts for the power unit may be connected in series with the 220-volt line. This method is not very satisfactory, however, for the value of resistance which must be used varies considerably with different power units and with the load on the output of the power unit. Also, unless one has available instruments for measuring the a.c. voltages there is no simple means of determining what value of resistance must be used in order to reduce the line potential to 110 volts. If one has available an a.c voltmeter this method can, of course, be used quite (readily. The variable resistance is con nected in series with one side of the line, the voltmeter is connected directly across the input terminals to the power unit, and the resistance then adjusted until the voltmeter reads 110 volts. The second method of adapting 110volt B-power units for operation on a 220-volt line is somewhat more expensive, however, it is much simpler and does not require that any voltage measurements be made. This system of reducing a line potential to 110 volts calls for placing a separate power transformer between the power unit and the line. The transformer should have a step-down ratio f 2 to 1 so that with 220 volts across its primary 1 10 volts will be developed in the secondary winding. The secondary is connected directly across the terminals of the B-power unit as indicated in the diagram. The same transformer may be used with any B-power unit so long as the input power to the B-power unit does not exceed the power rating of the step-down transformer. Such transformers are now made by several manufacturers. No. 242 Radio Broadcast Laboratory Information Sheet December, 1928 Resistance-Coupled Amplifiers PREVENTING DISTORTION AT VARIOUS times letters asking how to reduce distortion have been received from readers, who have constructed resistance-coupled amplifiers. The correspondent usually explains that the amplifier produces considerable distortion unless the volume is kept down very low. In this sheet we have endeavored to indicate what we consider the causes of the distortion. As proof that a resistance-coupled amplifier, when properly constructed and operated, is capable of giving excellent results, we might refer to the use of such an amplifier in the demonstration of television by the Bell Telephone Laboratories. In this work an amplifier of this type was used to amplify the output of the photo-electric cell and it was essential that the audio response curve be practically flat over a very wide frequency band. Distortion in an amplifier of television signals would be much more serious than similar distortion in the amplification of music, the eye being a much more critical judge of quality than the ear. What, then, is the probable cause of the distortion which many notice when using such an amplifier? The answer is, first overloading of the amplifier, and secondly, common coupling in the plate-voltage supply, be it batteries or a power unit, although, of course, such coupling generally will be more serious in the latter case. If any of the tubes in a resistance-coupled amplifier are overloaded so that the grid of one or more tubes goes positive, some grid current will flow and produce so-called "blocking." If the overloading is very slight, the blocking may not be noticeable as such, but the amplifier will distort. The important point is that the blocking does not affect only the signal which caused the blocking but will also affect the following signals until the blocking current leaks off through the high-resistance grid leak. If the signals were fed into a transformer-coupied amplifier some overloading might occur but the tubes would not block because the transformer windings are of low resistance in comparison with the resistance of the grid leaks used in a resistance coupled amplifier. The resistance-coupled amplifier is, therefore, much more critical with regard to overloading, than a transformer-coupled amplifier, and in the operation of the former type of amplifier the signal input must be kept down to a level at which no overloading occurs. Laboratory Sheet No. 243 discusses a second cause of distortion in resistance-coupled amplifiers, i.e., common coupling in the plate-supply circuits.