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116 RADIO BROADCAST ADVERTISER 1 // < ymagine *an air-column • • concealed in a cabinet onlylSlong" ET that is exactly what has been accomplished in the Amplion Patrician. A 48" air-column am- plifier is assembled with the world-fam- ous Amplion unit in a mahogany cabinet 18"xl2"x9". That is the reason why this splendid in- strument has such a wide range of reson- ant, natural tone — mellow. -fc -12 -18 AMPLION PATRICIAN AA18 $50 Hand-carved mahog- any grille on all four sides, giving a non-dir- ectional reproduction. OTHER AMPLIONS : Amplion Cone AC12 ..... $30 Amplion Grand AC15 135 Other models as low as $12 Write for illustrated litera- ture giving full details of the Amplion line. The Amplion Corporation of America Dept. L, 280 Madison Avenue New York City The Amplion Corporation of Canada, Ltd. Toronto, Ont. -36 AMPLION The Radio Broadcast SHEETS TNQU1RIES sent to the Questions ami Anders department of RADIO BROADCAST were at one time ^- answered either by letter or in " The Grid." The latter department has been discontinued, and all questions addressed to our technical service department are now answered by mail. In place of " The Grid," appears this series of Laboratory Information Sheets. These sheets contain much the same typt of information a$ formerly appeared in " The Grid," but we believe that the change in the method of pre- sentation and the wider scope of the information in the sheets, will make this section of RADIO BROADCAST of much greater interest to our readers. The Laboratory Information Sheets cover a wide range of information of value to the experimenter, and they are so arranged that they may be cut from the magazine and preserved for constant reference. Wt suggest that the series of Sheets appearing in each issue he cut out with a ra^or blade and pasted on 4" by 6" filing cards, or in a notebook. The cards should be arranged in numerical order. Several times during the year an index to all sheets previously printed will appear in this department. The first index appeared in November. Those who wish to avail themselves of the sen-ice formerly supplied by " The Grid," are requested to send their questions to the Technical Information Service of the Laboratory, using the coupon which appears on page 127 of this issue. Some of the former issues of RADIO BROADCAST, in which appeared the first sets of Laboratory Sheets, may still be obtained from the Subscription Department of Doubleday, PageSf Company at Garden City, New York. No. 97 RADIO BROADCAST Laboratory Information Sheet June, 1927 Methods of Generating High-Frequency Energy T3EFORE the invention of the three-electrode J tube, and its subsequent use as a source of large amounts of high-frequency energy, the arc was a common type of continuous-wave generator. In the drawing on this Sheet is given the circuit diagram of a simple arc. The ordinary arc light used for street lighting might be used, hut much more efficient operation is obtained from an especially designed arc. The elementary theory of the arc is given below. The drawing indicates the simplest arrangement of the apparatus. "G" is a direct current generator, "r" is a resistance to control tht current, Li and L: are two choke coils to keep the r. f. energy out of the generator and to keep the current practically constant, "K" is the arc, and "C," "L," and "R" are respectively, the capacity, inductance, and resis- tance of the oscillating circuit. The arc, which consists of two electrodes, is differ- ent from ordinary electrical conductors in one important respect, which is that its resistance is not a constant quantity but a variable one, depending on the current flowing through it. At high current values the resistance is low and at low current the resistance is high. Consequently, an increase in current will produce a decrease in resistance. Now, when the switch is closed, certain currents flow and the condenser begins to charge, and, therefore, part of the current is diverted from the arc. Since the current through the arc is decreased by this action, the voltage across the arc must rise. and it continues to rise as long as the condenser con- tinues to charge. As soon as the condenser becomes fully charged, the arc voltage stops rising and the condenser begins to discharge itself through the arc. When the discharge is complete, the cycle of charge and discharge repeats itself with a frequency deter- mined by the constants of the inductance L and the capacity C. By carefully choosing these values, large amounts of high-frequency energy can be obtained. No. 98 RADIO BROADCAST Laboratory Information Sheet Audio Amplifying Systems June, 1927 RESISTANCE-COUPLED AMPLIFIERS A VERY satisfactory method of audio amplili- *» cation is that employing resistance coupling. The usual resistance-coupled amplifier requires three stages of amplification in order to obtain sufficient overall gain to satisfactorily operate a loud speaker. The introduction, however, of a new tube with a very high amplification constant, makes it possible in some cases to obtain sufficient amplification using only two stages. This new tube is known as the type 240 and data on it will appear on Laboratory Sheet No. 106 (July, 1927). Several factors must be given attention if satis- factory results are to be obtained from a resistance- coupled amplifier. The mere fact that it is resistance coupled will not insure good quality. A poorly de- signed resistance-coupled amplifier is capable of creating as much distortion as can be obtained from a poorly designed amplifier of any other type. Some data regarding the constants of a resistance- coupled amplifier were given on Laboratory Sheet No. 74 (March, 1927). The constants given were for an ordinary tube for use in the resistance-coupled amplifier with an amplification constant of about 20. For the new type high-mu tube, however, with an amplification factor of about 30, it is necessary to use somewhat different values of resistance. See Laboratory Sheet No. 106. The coupling condenser is a very important factor, and it is essential that this condenser have a very high insulation resistance, otherwise some of the B voltage will leak through the condenser to the grid circuit, and the amplifier will no longer function satisfactorily. In building up a resistance- coupled amplifier the best condensers should be used. It is essential that high-quality plate and j.rid resistances be used to prevent noise in the amplifier. Also, the plate resistor should be capable of carrying the plate current of the tube without overheat- ing. Another important point is the amount of plate voltage used. It should be realized that most of the plate voltage supplied to the amplifier is lost in the resistance in series with the plate circuit of the tube. For this reason, it is necessary that fairly high voltages be available in order that there will be sufficient voltage left at the plate of the tube to obtain satisfactory operation. At least 135 volts should be used, and it should preferably be 180. The C-battery voltages should he kept as low as possible. It will generally be found that in an ordinary resistance-coupled amplifier a C-battery voltage of about 3 volts will be necessary on the grid of the tube preceding the last tube, if the latter is of the 171 type. The C voltage on the first tube of the ampifier need not be more than one volt.