Radio Broadcast (May 1928-Apr 1929)

.RADIO BROADCAST ADVERTISER. Safeguard Your A. C. Installation SATISFACTORY and economical operation of A. C. receivers is contingent upon maintaining close regulation of operating voltages, by means of suitable A. C. measuring instruments. This is necessary because of the wide fluctuation in the potential of secondary lines furnishing current to house lighting circuits. Set manufacturers, dealers and electric light and power companies everywhere are cooperating to the end that voltage regulation, both on supply lines and in connection with voltage control equipment of the receivers themselves, may be effected for the better operating service of all set owners. For this reason, as well as for other testing requirements outlined in the following, all purchasers of A. C. receivers are urged to provide themselves with an instrument such as is shown in the illustration — known as the Weston Model 528 A. C. Voltmeter, range 150/8/4 volts. When you find that there is an excessive in-put voltage, it follows that there is too high a voltage on the filament which shortens the operating life of the rectifying tubes. The Model 528 Voltmeter therefore checks the line supply voltage at all times and indicates when adjustments should be made to manually operated line voltage regulators between the power supply and the power transformer. This voltmeter also indicates when the line voltage is over -rated, thus enabling the operator to make an adjustment in the set for the higher line voltage so that normal life can be obtained from his tubes. The Model 528 is also made as Ammeters which are especially useful in checking the total load of the A. C. Set — in conformity with set manufacturers' instructions. The determination of A. C. filament flow in A. C. tube filament circuits is easily obtained by means of this instrument. Write for your copy of Circular J fully describing the Weston Radio Line. Weston Electrical Instrument Corporation 604 Frelinghuysen Ave., 6>» Newark, N. J. WESTON RADIO INSTRUMENTS No. 261 Radio Broadcast Laboratory Information Sheet February, 1929 Where A. C. Hum Originates 'T'HE amount of a.c. hum audible in a loud speaker connected to a radio receiver depends upon various factors. With a given installation, however, the hum depends to the greatest degree upon the amount of a.c. ripple introduced into the plate circuit of the detector tube. This hum voltage may come from the B-power unit or from a.c. tubes, and in a.c. sets some hum is, of course, obtained from bot h of these sources. It is important to realize the importance of any hum in the detector circuit. Consider an ordinary transformer-coupled amplifier using, say, a 3 to 1 ratio transformer between the detector and first a.f. tube, and assume that the first a.f. amplifier tube has a mu of 8. Between the plate circuit of the detector tube and primary winding of second a.f. transformer the gain is, therefore, 24. It follows from this calculation that, if a given amount of hum is obtained from a loud speaker when there is a certain hum voltage in the plate circuit of the first a.f. tube, that the same amount of hum will be obtained with only one-twentyfourth as much hum voltage in the plate circuit of the detector tube. For these reasons it generally is found that ampli fiers which are noisy under normal operation are generally quiet if the output of the detector tube is short circuited — a definite indication that the major part of the hum arises in the detector circuit. Let us consider a concrete example. Suppose that we have a two-stage a.f. amplifier with 3 to 1 transformers, the first, audio tube having a mu of 8 and the power amplifier having a mu of 3 and that the load resistance in the output is equivalent to 4000 ohms. Assume tbat a hum potential of 0.1 volts is existant in the plate circuits of the detector tube and also the first audio amplifier. The hum voltage from the plate circuit of the first a.f. amplifier circuit will produce a hum potential of a 0.6 volts across the 4000-ohm load resistance. The hum voltage in the detector circuit will produce 14.4 volts in the load circuit. Even assuming that these two voltages are 180 degrees out of phase so that they oppose each other the voltage in the load circuit would be 13.8 volts. It follows from these figures that practically all the hum in the output will come from the detector circuit. The importance of proper design in the detector circuit to eliminate any small hum voltages cannot be overemphasized. No. 262 Radio Broadcast Laboratory Information Sheet February, 1929 Advantages of Dual Push Pull TJIGH-GAIN a.c. -operated power amplifiers, designed particularly for use in public-address systems, frequently make use of two 250-type tubes in push pull in the output. If these tubes are operated at their rated voltage in order to obtain the maximum amount of undistorted power a total value of peak signal voltage across the secondary of the push-pull transformer feeding these tubes must be about 160 volts. Assuming that this transformer has a ratio of 3 to 1, the voltage across its primary must be 160 divided by 3, or approximately 53 volts. If the tube feeding this transformer has a mu of 8 then the voltage on its grid must be about 7 volts, and, in order to prevent the possibility of overloading, the grid bias should, therefore, be twice this value plus about 10 per cent, or 15 volts. We might consider using a 226-type tube to feed the push-pull stage, but the maximum rated voltage of this tube is 180 volts with a corresponding grid bias of 13.5 volts which, from the figures given above, is not sufficient. It is for this reason that we find many of the power amplifiers disigned for public-address work consisting of two push-pull stages, the power output stage being fed by a preceding push-pull stage using 227 or 226-type tubes. Through the use of the push-pull arrangement we are able to handle voltages somewhat greater than twice that which can be handled by a single tube. These tubes in pushpull can then handle without difficulty the voltages required to load up two 250-type tubes in push-pull. It follows obviously, from these figures, that any power amplifier using 250-type tubes in push-pull must be preceded by a push-pull stage if maximum output is desired, since a single 226 or 227-type tube will be badly overloaded when called upon to supply the necessary voltages. The above discussion, of course, does not consider the possibility of using a small power tube in the circuit preceding the push-pull stage. If we assume that we can obtain from the detector circuit about 0.3 volt and that 160 volts are required on the grids of the power tubes, it follows that the gain in the amplifier must be about 530 (160 divided by 0.3). The gain of an ordinary amplifier, is about 100 and, consequently, when using 250 s in push-pull it is essential that a threestage audio amplifier be used. No. 263 Radio Broadcast Laboratory Information Sheet February, 1929 Wavelength-Kilocycle Chart 8,000 i a 25,000 --i2| 53 23,000 21.000 ; z 20,000-15 g 2 19,000 18,000-' 15,000-20 14,000- 9000 23 :r24 8000 12,000 --25 ■19" S 3 S S 3 :-,7£ 3 -21 ■22 ;r28 ^r29 ,000 -HO 31 32 -33 26 g P, 7000 9000 J 299.800 FREQUENCY IN KltOCYCtES =WWUENGTh 3000 u s o 2 2000 ■70 ■75 80 ■85 ■90 '100 as ■125 5 ■150 s --200 : -250 300 -400 -500 • february, 1929 page 276 •