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Show i I i , f " n ' , , l" J I I e ' ,, , i r 1 c5 6 o o (Oi n 1 -sf-- At B- B-r -A M " ThreTube Receiver, Square Wound Toroid Colls, Two Stages of Radio Frequency and Detector. Can Be En larged to Five-Tube Set. By CARLTON E. BUTLER (MwintiT American Institute "f EltM-t rlral KiiKlftnorrt and lfintttute of Kudio Enl-nMnrn Enl-nMnrn t .Styles In rudlo receivers lnive cliuugcd wllli Hie changing conditions of tin: broitilt'MHtliiK Held. A three-tubo three-tubo regenerative net with two Hitmen 6f radio frequency amplification was ruuuble of giving Hatlsfuctory reception recep-tion a few yearn ago, but totltiy, witli the worst congest Ion of the etlter lunen ever experienced In the history of radio, nnd the Increasing use of high-power broadcasting stations, selectivity se-lectivity requirements demand the use of radio frequency amplifying circuits. A three-tube, circuit today should uo Home form of tuned radio-frequency amplification that successfully ropes with conditions and fur nishes an Inexpensive set that later on niay be enlarged Into u five-tube receiver re-ceiver by the addition of two stages of audio-frequency amplification. Leave Room on Panel. In the diagram shown here, however, how-ever, Just, the three-tube circuit Is Illustrated Il-lustrated ns It makes a complete set In Itself. Hoom should be left on the panel and In the cabinet for the other stages at a later date. For this reason rea-son u 2-1 or 2G Inch panel should be lined, with a cabinet and baseboard largo enough to allow for this future expansion. The three-tube set will tiring In Just as much distance and possess all of the desirable qualities of tho high-priced live-tube sets on the market, excepting volume, which is obtained ob-tained when audio nmplillcation Is added. Little apparatus is needed to construct con-struct this set, and with reasonable care an excellent receiver can be had for very little outlay of time and money. To construct the set you will need the following list of apparatus: 1 square wound toroid coll, type Tl. 2 square wound toroid coils, type T2, Indicated as T2 and Tli. 8 straight-line frequency variable condensers, .OOOlio ml'd. capacity. Indicated In-dicated as CI, ."-', and C3. 1 .00025 mfd. fixed grid condenser nnd 2 megohm grid leak 23-ohm rheostats. rheo-stats. 1 phone Jack. 3 tube sockets. Binding posts, wire, screws, etc. The 25-ohm rheostats are to be used with 'CV 201-A or C 301-A type of tubes. If the I'M type are to be used, then 40-ohm rheostats should be substituted. sub-stituted. For the WD U or WD 12 type of tube, G-ohm rheostats throughout will be correct. Both radio-frequency lubes are operated from one rheostat If desired, but it is usually best to have a separate control for the detector detec-tor tube. Use Double Circuit Jack. The detector tube Is using the same voltage of "I!" battery as the radio-frequency radio-frequency tubes. This will only be possible as long as all three tubes are of the same type nnd not more than 45 volts are to be used. If a higher voltage of "15" battery is desired, then the detector Jack should be supplied from the 45-volt tap. If a soft tube is to be used, reduce the voltage to 22'a volts, or less. A single-circuit Jack is shown in the detector stage. 1 If at a later date It is practically certain that amplification amplifi-cation is to be added, It is best to use a double-circuit Jack In tills position to avoid the need for changing the present circuit. Two antenna terminals are provided on the first toroid coil. These nre to adapt the set to a long or short antenna. an-tenna. It might be well to bring these two terminals out to binding posts on the rear of the cabinet so that the antenna enn be qnickly changed over from one to the other as conditions require more selective tuning, or greater great-er volume is desired. The position of the apparatus in laying lay-ing out the receiver baseboard is not critical. The coils may be placed side by side without intercoupling or causing caus-ing the receiver to become unstable, due to their confined magnetic fields. |