Radio broadcast .. (1922-30)

246 RADIO BROADCAST ADVERTISER Model A. C. 20 Height, including bane.ZIJiIn. Far ahead of to- day's accepted standards—the new Amplion Cone withM. A. 1 type unit In quality of reproduction it sets a new high mark for others to strive for remarkable reproducer has an unusually wide musical range due to the striking developments embodied in the new Cone Unit. One important feature of this unit is the felt-lined stylus protection bar which serves a double purpose. First, it pro- tects the stylus itself against any pos- sible injury. The felt lining, or stylus anchor, neutralizes the harmonics of the stylus itself. This model employs a new principle in Cone construction—a 14" cone being mounted on an 18" sound board which extends toward the center in back of the cone to form a resonating chamber. The unit, cone and sound board are assembled on a rigid bronze bracket with a handsome bronze base. The sound board is finished in dark walnut, which, with the gold-finished cone in the center, gives this instrument both graceful beauty and dignity. Write to us for price list and literature illustrating and de- scribing the new Amplion line. THE AMPLION CORPORATION OF AMERICA 280 Madison Ave., New York City The Amplion Corporation of Canada LtrL, Toronto No. 118 RADIO BROADCAST Laboratory Information Sheet August, 1927 Audio Amplifiers FREQUENCY AND LOAD CHARACTERISTICS A NY audio amplifying system has two character- • ri istics, equally important, which determine how well it will function. They are generally known as the frequency characteristic and the load charac- teristic. The frequency characteristic indicates the relative amplification of the amplifying system of various frequencies between the limits over which the amplifier is to be operated. The frequency characteristic is generally shown in the form of a curve and, of course, a flat curve indicates equal amplification at all frequencies. Slight rises and depressions in the curve in the order of 10 per cent, can be neglected because they are too slight to be noticeable to the ear. The load characteristic of an amplifier, while not in such common use, is just as important as the frequency characteristic. The load characteristic will show how the total amplification of the system varies with different input voltages at a constant frequency generally of about 1000 cycles. If the amplifier is a good one the amplification will remain constant over the entire range of input voltages at which the amplifier would normally be worked. If a two-stage amplifier is operated with a 201-A type tube in the output with 90 volts on the plate, it will overload very quickly because a 201-A cannot deliver much power. Consequently, the load characteristic curve of such an amplifier would begin to fall off comparatively quickly, but if a 171 tube with the proper voltages were to be used in place of the 201-A, then the load characteristic would indicate that it was possible to obtain much more power from the amplifier without overloading it. Both of these characteristics depend upon the type of tubes used and the voltages with which they are supplied, and upon the design of the coupling devices connecting the output of one tube to the input of the next. Frequency and load character- istics can be taken on any part of the complete amplifier but such curves may have very little in common with the characteristics of the complete system. Consequently, although curves on in- dividual units are useful in designing an amplifier, curves on the completed system should always be made to make certain that some factor, such as common coupling in the batteries, is not seriously altering the overall characteristic. No. 119 RADIO BROADCAST Laboratory Information Sheet Radio-Frequency Choke Coils August, 1927 THEIR PLACE IN CIRCUIT TK A very high-gain radio-frequency amplifier is to - 1 be constructed, it is essential that radio-frequency choke coils be used in the amplifier to prevent it from oscillating. Neutralization will prevent the production of oscillations due to feed-back through the tube but will not prevent the production of oscillations due to coupling in the battery leads or in a B socket-power device. To prevent instability due to these effects it is necessary that choke coils, L, and bypass condensers, C, be used in the plate circuits of the radio-frequency tubes, as indicated in the diagram on this Sheet. These choke coils offei a very high impedance to the flow of radio-frequen- cy currents and all these currents therefore flow through the bypass condenser connected between the choke coil and the negative filament, instead of through the plate battery. What size choke and what size condenser should be used? To keep down the cost they should both be as small as possible whereas their effectiveness be- comes greater as their size is made larger. The plate impedance of a 201-A type tube is about 12,000 ohms and it is essential that the con- denser which is incorporated to bypass the r.f. cur- rents does not introduce in the plate circuit any great amount of impedance. A 0.003-mfd. condenser will increase the total circuit impedance from 12,000 to about 12,120 ohms, a negligible amount. This value is correct at 500 kilocycles, the lowest fre- quency used in broadcasting, and at higher broad- cast frequencies the effect of the condenser will be even less. The choke coil's impedance must be large in comparison with that of the condenser so as to cause practically all the current to flow through the con- denser and not through the choke. If the choke coil's impedance is made 1000 times greater than that of the condenser only one-tenth of one per cent, of the total radio-frequency current will flow through the choke coil and therefore good filtering action will be obtained. If the choke coil's impedance at 500 kilocycles is to be 1000 times greater than the impedance of the condenser then it must be 12,000 ohms. The inductance of a choke coil with an impedance of 12,000 ohms at 500 kilocycles is 38 millihenrys. Most radio-frequency choke coils have an inductance of much more than this. No. 120 RADIO BROADCAST Laboratory Information Sheet AugUSt, 1927 A-Battery Chargers TRICKLE VERSUS HIGH-RATE CHARGERS '"THERE are many different types of A-battery -*• chargers now available; some of them are satisfactory for use as trickle chargers and others only efficient when used to charge the battery at comparatively high rates of charge. The charger employing an electrolytic type of rectifier, for example, is very well adapted for use in trickle charging. It is very efficient, requires little attention, and has long life. Another very satisfactory type of rectifier for a trickle charger is the so-called dry crystal, which was developed rather recently. A third type of rectifier that can be used for trickle charging is the Tungar but it is not especially efficient as a trickle charger, because of the comparatively large amount of power required to heat its filament. There are three types of rectifiers that are satis- factory for use in high-rate charging. They are the Tungar, the Vibrator type, and the new cartridge recently developed by Raytheon. All of these chargers are capable of delivering fairly large amounts of rectified current for charging a battery and are fairly efficient when delivering these cur- rents. There is little to be said regarding the com- parative efficiency of the two methods of charging. Trickle charging has the advantage that it requires somewhat less attention than does high-rate charg- ing but it has the disadvantage that it is somewhat difficult to determine just what the best rate of trickle charge should be in order to prevent the battery from being overcharged or undercharged; also slow rates of charge used in trickle chargers are hard on a battery. With a trickle charger, a low- capacity storage battery can be used because it is not called upon to supply any great amount of current for a long period of time. With high-rate charging, on the other hand, it is usual to charge the battery every one or two weeks and also a fairly large storage battery is necessary in order that it will have sufficient capacity to supply the receiver between charges. It seems to be generally agreed among battery manufacturers, however, that the high charging rate is somewhat better for the battery in that it makes possible longer life. For best results the charging rate should gradually taper off as the battery becomes charged.