22 RADIO BROADCAST MAY, <strong>1927</strong>type of Rice- Kellogg cone loud speakeris usedas the filter choke for the B supply.Third, the necessity of a ground connectionis eliminated by the use of a 2-mfd. condenserbetween one side of the I lo-volt line and thecenter-tap of the power tube filament winding.In inserting the lo-volt plug in a socket or otherioutlet, the plug should first be tried one wayand then the other as best results are generallysecured when the grounded side of the line is theside to which the condenser is connected.RADIO BROADCAST PhotographTHE HANSCOM "SUPERUNIT"This pick-up is constructed froma Baldwin phone unitRADIO BROADCAST PhotographTHE GRIMES PICK-UPAn excellent exampleThe rear of the baffleboard loud speaker is seen in the center of this picture.A metal funnel is placed over the UV-876 tube qual-to deflect the heat ity of the radio set is poor.phonographare at least four different types of1 pick-ups. The electro-magnetic and theof theelectro-magnetic type of crystal types function by generating voltages,pick-upwhile the condenser and the carbon forms operateby variations of impedance.as will be seen from Fig. 3, and is connectedOf these four, the only one of commercialbetween the pick-up and the amplifier. Theimportance, and, it would seem, the most practical,is the electromagnetic form. To this classfilter is so located before, rather than after theamplifier, in order to prevent unnecessary overloadingtrope," andbelongs those used in the " Electrola," " Pana-amplifier."Vitaphone." This latter is inThe volume is controlled by a wire-wound reality a potentiometerspecial electric phonograph designedwhich permits the audio-frequencyfor synchronization with a motion picture pro-voltage supplied to the amplifier to be made jector.anydesired fraction of the One of the pick-up output.large corporations interested in theThe amplifiers employed in the present commercialdevelopment of radio isexperimenting with amodels of the electric phonograph oper-FIG. pick-up of the crystal type but as yet this 3uniqueate entirely from a. c. The circuit, as will bedevice is still inThe experimental stage.seen from Fig. 2, comprises a stage of electrical connections of the pick-up, scratchvoltagePick-upsamplification, using a filter, volume control, and adapter. The scratchtype have been in existencefor a number rectified a. c., and approximately a 4600years but only recentlyux-igg tube with filament filter as shown is tunedlighted from stage of has onepowercapable ofcycleshigh-quality reproduction"amplification using a ux-2io tube with filamentbeen perfected. This is the Bristophone,"and to adapt the complete phonograph to power The loud speaker, as unitalready mentioned, telephone receiver.islines of various voltages located in different sectionsof the country.the amplifier due to the use of its field emphasize certain notes and, unless used inof the cone type and is electrically "tied into" Generally, however, such devices tend to over-windingSecond, the field winding of the baffleboard as a filter inductance.connection with a "trick" amplifier, will sound"flat." An exception is the unit from the Western540 AW cone loud speaker. Thislighted from raw a. c. This power tube shown inissuppliedpotential of close to 400 volts, andThisa photograph on page 23.A special input jack makes the pick-up, whichamplifierloudspeaker unit available for operation di-upon the change of its electrical resistance withdepends for its operationthe requisite negative grid bias. Transformercoupling is employed between the two stages of rectly from the detectoramplification as well as between output of any radio vibrations of the needle,set,in a somewhat differentclassthe amplifier or better yet, a small two- orthan the others as it isthree-tube set, suchdesigned to operatea speaker.as the two-tube R. B. "Lab." receiver, de-speaker without the use of an ampli-output and the loudThe power supply, which is the same as that scribed in the fier.January RADIO BROADCAST byemployed in the RCA-IO4 loud speaker, embodiesunique features.used in the construction of anJohn B. Brennan, may be built just for the Some pick-ups are made of the samepurpose.ordinary radioparts asFirst, a line ballast tube isemployed to compensatevoltage so as to pregraphcase isTerminals are also provided on the type headphono-telephone. That such should be thesupplying B power to the radio receiver.pick-up is exactly the reverse of the loud speakerquite reasonable, for the function of thevent changes in filament current of the 199 tube,unit can be converted into a very fine, but needlesslyexpensive, pick-up. One of the tricks tosuch a conversion is the counter balancing of thetone-arm so that the unit, which isquite heavy,does not press too hard on the record.As a rule, however, for best results, a unit designedfrom the "ground-up" as a high gradepick-up should be used. Several units of thistype are now being manufactured.USING A PICK-UP WITH ANY SETREMOVING the base from an old 2OI-ABYtube (this may readily be accomplishedby heating over a gas flame to melt the cementin the base and the solder on the prongs), andconnecting the leads from a pick-up to the plateand filament prongs, as shown in Fig. 3, thehome-made pick-up shown in iFig. may beused with the amplifier in any radio set. Merelyremove the detector tube and in its place insertthe "adapter." Fig. 3 also shows the inclusionof a scratch filter and volume control.The audio amplifier channels of many radiosets in use to-day are so poor as to make sucha system of using the pick-up most undesirable.RADIO BROADCAST Photograph There is little goodWITHIN THE "PANATROPE"purchasing or constructinga pick-up for obtaining the same quality from a
MAY, <strong>1927</strong> THE ELECTRICAL PHONOGRAPH 23AHOME-CONSTRUCTED PHONOGRAPH-RADIO CO<strong>MB</strong>INATIONBuilt by the author. A more detailed description of this particularinstrument is scheduled to appear in RADIO BROADCAST for next monthA desirable thing to do is to constructa new high-quality lamp socketoperated audio channel of the type describedby the writer in recent issues ofRADIO BROADCAST so as to secure wellnigh perfect phonograph and radioquality.Where the amplifier in the radio setis to be used with a pick-up, the fol-RADIOlowing essentials of good quality shouldbe kept in mind:1. Use of high-grade coupling mediums. Theuse of the new large-size audio transformers,resistance coupling, or impedance coupling, is tobe recommended.2. Use of a power tube in last audio stagewith proper B and C voltages to prevent overloading.~).Use of an output device. Inductancecapacityunits are, in the author's opinion, preferableto the transformer units.4. Use of a high-grade loud speaker.SCRATCH FILTERSan electrical filter circuit of theWHILEtype indicated in Fig. ^ will remove allobjectionable scratch from the music issuingfrom the loud speaker, it will not prevent onehearing the un-amplified scratch noise directlyfrom the record. For this reason the lid of theturntable compartment should be kept closedwhile records are playing.A high-resistance potentiometer, such as theCentralab or the type L Electrad Royalty unit,makes a very excellent volume control, whenwired as indicated in Fig. ->,.As a rule, it willgenerally be found rather convenient to mountthis volume control on a small square or discof bakelite in the turntable compartment of theRADIO BROADCASTphonograph. This same panelPhotographmay also wellcontain the amplifier control switch.To get the full benefit from a home-constructedelectric phonograph,it is essential to use thenew electrically cut records and a good loudspeaker of the cone type. Of course the pick-upand amplifier must be good, but generally, afterdevoting much attention to them, the finalresults are spoiled by failure to use a good loudspeaker.In the June RADIO BROADCAST, details of aconsole phonograph cabinet in which a two-tubeR. B. "Lab" receiver, baffleboard loud speaker,and lamp-socket powered amplifier with recordBROADCAST Photographpick-up, will be described. Data on the constructionof a special amplifier, entirely lamp-socketTHE "BRISTOPHONEoperated and designed primarily for use with aA pick-up of the carbon type which operatespick-up device, will also be given.a loud speaker without using an amplifierR
- Page 1 and 2: RADIO BROADCASTVOLUME XIMAY, 1927,
- Page 3 and 4: INDEX.ContinuedPAGEThreshold of Hea
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- Page 11 and 12: MAY, 1927WITH MACMILLAN TO THE ARCT
- Page 13: THE MARCHInterpretation of Current
- Page 16 and 17: 18volved the law in precarious lega
- Page 18 and 19: The ElectricalA Non-Technical Expla
- Page 22 and 23: A Balanced Short-Wave ReceiverA Des
- Page 24 and 25: What About the A BatteryThe Importa
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- Page 34 and 35: AS THE BROADCASTER SEES ITDrawings
- Page 36 and 37: 38 RADIO BROADCAST MAY, 1927Interes
- Page 38 and 39: Some Facts About Coil DesignThe Osc
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