Radio Broadcast (May 1928-Apr 1929)

.RADIO BROADCAST ADVERTISER. for Television Reception This lamp is made in numerous types and styles, which provide suitable light sources and lightsensitive relays for all systems. List Price, $7'50 L€OH V Foto-CeU / for Television Sending This is an extra-sensitive broadcasting tube, supplied in either hard vacuum or gas-filled types, and in two sizes of each. Information and prices on application LONG LIFE RECTIFYING TUBE for "B" Power Eliminators Over a hundred different makes of k,B" Eliminators require this tube, and take no other. There are millions of them in daily, satisfaction-giving use. List Price, $4,-50 Write for ftirtlier information on any of this equipment RAYTHEON MFG. CO. CAMBRIDGE, MASS. No. 258 Radio Broadcast Laboratory Information Sheet February, 1929 An Analysis of Filter Circuits r)N LABORATORY Sheet No. 259 are given ^ a circuit diagram and set of curves showing the output voltage from a typical full-wave rectifier using two 281-type tubes. These curves will prove helpful in determining what voltage is necessary across the power transformer to deliver a given voltage to the filter system. The curves show the output of the rectifier with transformer voltages ranging from 550 volts per plate up to 700 volts per plate. Two sets of curves are given, one set being obtained with the standard filter system indicated in the circuit diagram and the other with a special circuit recommended by the E. T. Cunningham, Inc. The solid curves show the voltages with a standard filter system and the dotted curves show the voltage with a special filter system. In determining the latter curves the first filter condenser, Ci, was omitted. When using the standard type of filter system the load on the tube is quite heavy and the peak value of current, which the rectifiers are called upon to supply under full-load conditions, reaches values as high as 310 milliamperes, although the average current drawn from the filter system is only 125 milliamperes: the filament must be capable of supplying the maximum value of current, i.e. 310 milliamperes. With the first condenser, Ci, removed from the filter system the voltage output for a given transformer voltage decreases considerably, as indicated by the curve, but with this condenser removed the tube operates under much more satisfactory conditions. The peak value of current used in such a circuit is only 140 milliamperes when the load current is 125 milliamperes. In other words the peak current has been reduced from 310 milliamperes to only 140. This reduction increases the life of the filament, and a tube having a total emission of 150 milliamperes will still give satisfactory operation in the special filter circuit although itwould not function satisfactorily in an ordinary filter circuit where the plate current reaches values up to 310 milliamperes. It is recommended that this special filter system be used wherever possible. No. 259 Radio Broadcast Laboratory Information Sheet February, 1929 Filter Circuit Characteristics 900 Output T. 40 80 120 160 MILLIAMPERES OUTPUT No. 260 Radio Broadcast Laboratory Information Sheet. February, 1929 Voltage Gain in Resistance-Coupled Amplifiers T ABORATORY Information Sheets Nos. 242, ' -J 243, 249, and 250, discussed resistance-coupled amplifiers: the latter two sheets gave the circuit diagram and a list of parts for the construction of a good amplifier of this type. In this sheet further data is given regarding resistance-coupled amplifiers in comparison with other types. The overall voltage gain in a resistance-coupled amplifier is generally much greater than that of a transformer-coupled amplifier. For example, a standard two-stage transformer-coupled affair has a voltage gain of about 100 from the input to the grid of the power tube. The usual three-stage resistancecoupled amplifier using high-mu tubes has a gain of about 400 from the input to the power tube's grid. This additional gain is not always an advantage. If such an amplifier is used in a receiver operated entirely from batteries this high gain will simply have the effect of increasing the loudness of the signals, but if such an amplifier is used in a receiver operated from a B-power unit it is probable that the hum output will be much greater than it would be if a two-stage transformer-coupled amplifier were substituted for it. This is due to the fact that, as pointed out in Laboratory Sheet No. 261, the hum voltage developed across the loud speaker is a direct function of the overall gain of the amplifier and the amount of hum introduced into the detector circuit. Since the amplifiers have a ratio of about 4:1 in gain, the hum voltage developed when using the resistance-coupled amplifier will be about four times as great, assuming that all other conditions remain the same. For these reasons it frequently is advisable to construct the resistance-coupled amplifier with somewhat lower gain. For example, if instead of using two 240 tubes we use one 201a and one 240 then the overall gain will be about 150 which is a very satisfactory value. For some reason the resistance-coupled amplifier has not found wide use in manufactured or homeconstructed receivers although when properly designed it is certainly capable of giving results as good as any other type of amplifier. • february, 1929 page 274 •