Radio Broadcast - 1927, May - 61 Pages, 4.9 MB ... - VacuumTubeEra

22 RADIO BROADCAST MAY, 1927type 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