Radio broadcast .. (1922-30)

418 RADIO BROADCAST ADVERTISER X-L VARIO DENSER Used by RADIO BROADCAST in the Impedance Coupled Browning-Drake Hookup "I was really surprised at the fine adjustment I was able to get." GLENN H. BROWNING also specified by GERALD M. BEST Endorsed by LAURENCE COCKADAY Tested and approved by G. M. WILCOX Prof, of Physics, Armour Institute of Technology SEE THAT SCREW X-L VARIO-DENSER Results in easier tuning, more distance, volume and clarity — greater stability. Mnrlol "W A slight turn obtains correct IVIUUCI 1^ tuhe oscillation on all tuned radio frequency circuits. Neutrodyne. Roberts two tube Brown ing-Dra Ice, McMurdo Silver's Knockout, etc. capacity range \ to 20 micro-micro farads. Price $\.QQ with grid clips obtains the proper Krid capacity on Cockaday circuits, filter and intermediate frequency tuning in heterodyne and positive grid bias in all sets. Capacity range: Model G-i — .00002 to .0001 MFD Model G-s — .0001 to .0005 MFD Model G-io — .0663 to .001 MFD Price £1.50 X-L PUSH POST Push it down with insert wire, remove wire is firmly held, stantly. Pu»h Po»t Panel marked in white on ing strip. In box deiing lugs, raising screws for mounting your thumb, pressure and Releases inPrice ijc permanently black insulatincluding solbushings and etc. Price $1.50 X-L Radio Laboratories CHICAGO, ILL. No. 30 RADIO BROADCAST Laboratory Information Sheet September, 1926 Measuring the Output Voltage of a Line Supply Device REQUIREMENTS OF A SUITABLE METER (CONSIDERABLE care must be taken in measur^-' ing the output voltage of a line supply device if an accurate reading is to be obtained. The output voltage of such devices depends to a great extent upon the current being drawn from them, and if any considerable amount of current is also drawn by the voltmeter which is used in determining the output voltage, the reading will not be accurate and cannot be used. Also, if true results are to be obtained, the output voltages must be measured when the instrument is connected to the receiver and a normal load is being drawn from it. since, if these conditions do not exist at the time of the test, the voltage read with th« voltmeter will be considerably higher than would actually be applied to a receiver during operation. A reading taken without any load on the line supply device will sometimes be 100 volts higher than the reading taken with load. The voltmeter used to measure the output voltage must have a very high resistance in order to prevent large currents from (lowing through it. On Shtet No. 27 is given information regarding the construction of a home-made voltmeter which can be used. It is also possible to purchase suitable units for use in measuring the output of B eliminators. In any event, the voltage cannot be at all accurately read if one of the cheaper low -resistance type of meter is used. A numerical example might make more evident the errors which will be intrcxluced in the reading, if the incorrect type of voltmeter is used. As an example, suppose that we desire to measure the output voltage of a Raytheon B line supply device such as was described in the December, 1925, RADIO BROADCAST. If the receiver was drawing from the eliminator 20 milliamperes, the output voltage would be about 120 volts. However, if this output was measured with a low resistance meter, itself drawing about 20 milliamperes, the voltage read would be 75, an actual error of 38 per cent. However, if a high-resistance . , meter is used, such as is described on Laboratory Sheet No. 27, only about 2 milliamperes will be required by the voltmeter, and then the voltage read would be practically the same as the actual voltage, and a truer indication of the voltage being supplied to the set would be obtained. The care which is necessary in measuring the output voltages of B line supply devices is not necessary in measuring B batteries, since a drain of 20 or 30 milliamperes will make very little change in the voltage of a B battery. Therefore, it becomes possible to read the voltages of these units with an ordinary voltmeter whether it have a low resistance or not. No. 31 RADIO BROADCAST Laboratory Information Sheet September^ 1926 Distortion in Receivers SOURCES AND REMEDIES 'T'HERE are several points in a receiver where •*• distortion can occur. In the first place, if the radiofrequency amplifier or detector circuits are tuned too sharply, distortion will occur due to the fact that the side bands of the radio frequency waves which carry the voice or music will not be equally transmitted by the tuned circuit, and in this way unequal amplification is obtained. If a grid leak and condenser system of detection is used, it is not at all impossible to overload the detector tube on strong local stations. If this occurs, the various frequencies will not be properly amplified by the detector tube and serious distortion will occur. For real quality on local stations, a C battery detector is advisable since it can handle comparatively larger amounts of signal strength without overloading. Distortion can occur in the audio frequency amplifier. To prevent this, good transformers should be employed, if this form of coupling is used, and a C-battery bias should always be placed on the grids of all the audio amplifiers in order to prevent the tubes from overloading. If overloading does occur, the peaks of the voice waves will be cut off, and serious distortion results. Also, if the wiring of the audio amplifier is rather poorly done, it is not at all impossible that the audio amplifier will begin to oscillate, sometimes at inaudible frequencies and sometimes at audible frequencies. If the oscillation is audible it can be fairly rasily checked up and corrected, but if it is inaudible, il is sometimes quite a while before we realize just what the trouble is. The only practical method that can be used to dettct these inaudible oscillations, is to place a milhammeter in the platecircuit of the tube of the suspected circuit. After putting this meter in the plate circuit, the input to the tube is short-circuited (if a transformercoupled amplifier is used, a lead would be connected between the G post and the K post on the transformer) and no change should take place in the reading of the plate mijliammeter. If a change in the reading does occur, it is a fairly good indication that the circuit is oscillating. Of course, during this test, no signals whatsoever should be received. The final point at which distortion might occur is in the reproducing device. In order to obtain best reproduction from a cone speaker, it is necessary to use a semi-power tube in the output stage, with sufficient voltage to prevent overloading. It is also essential that the impedance of the loud speaker be fairly closely matched with the plate impedance of theoutput tube. If any discrepancy between the two impedances does exist, the tube should be preferably of a lower impedance than the speaker. No. 32 RADIO BROADCAST Laboratory Information Sheet September, 1926 Matching Tube and Loud Speaker Impedances THE USE OF AN OUTPUT TRANSFORMER TV/fANY recent articles dealing with quality amiVA plincation have stressed the point tliat an endeavor should be made to approximately match the impedance of the output tube with the impedance of the loud speaker. This fact is important from two standpoints; lirst, from the standpoint of quality and, secondly, from the standpoint of efficiency. Regarding the first point, if a low-impedance cone speaker is used with a high-impedance tube, such as the 201 -A, the low frequencies will IK? lost and undue prominence will be given to the high frequencies. In order to eliminate this drawback, and at the same time mane it possible to obtain a considerably greater amount of undistorted power, the new type 112 and 171 lubes have been developed; both of ihesr have quite a low plate impedance. The characterist ics of these two tubes were pi i n i c< 1 on Laboratory Sheets Nos. 7 and 12 respec lively. Hy the use of" such lulx-s. the frequency distortion (produced when a high-impedance output tube is used) is practically eliminated. When we use a low-impedance tube and thereby better the quality output of our receiver, we at the same time increase the efficiency with which the power developed by ihe tube is delivered to the loud speaker. Maximum power will be delivered to the oulput when its impedance is equal to the tube impedance, so that, for best results, the loud speaker impedance al a medium frequency, say UXX) cycles, should match fairly well Ihe output impedance of the tube. A simple method whereby tubes and loud speakers of different impedances may be used together, is by the inclusion in the circuit of a suitable output transformer, several of which are now on the market. When this plan is resorted to, it is necessary for the impedance of the transformer primary to approximately match that of the tube. The secondary should have an impedance similar to that of the loud speaker. In this way, it Iwromes possible to use a low-impedance speaker with a high-impedance tube, although it is not particularly advisable since the highimped a nee tubes are not capable of handling any great amount of power and will very likely overload, if they are used to supply a loud speaker. If a semi-power tube is used in the output, it is not generally advisable to connect the loud speaker directly into the plate circuit of the tube since, if this is done, the d. c. plate current will pass through the loud speaker windings and will harm the magnets used in the loud-sj>eaker unit. In order to eliminate the d. c. from the loud sjx-aker windings, either an output transformer or a combination of a choke and condenser should be used. Tested and approved by RADIO BROADCAST