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Fiq.6. Method by which a mast- mounted boost er is connected to its power supply and TV receiver using one antenna transmission line. condenser C,. The greater the feedback through C,, the lower the gain of the amplifier and the better the noise factor. The "Q" of the grid circuit is reduced by this arrangement, and the frequency response extended slightly. Fig. 5B is the schematic of a commercial version of the same circuit using a bifilar coil in place of the two coil sections of Fig. 5A. The only tricky adjustment is that of the variable neutralizing condenser C,. Without equipment for adjustment, this condenser can be set using a receiver. Normal procedure is to tune the plate and grid slugs to the proper channel with C, set at minimum capacity. The booster will oscillate as the second slug is brought to resonance. C, is adjusted (higher capacity) until the oscillation stops. This is the point of maximum gain, but not that of best noise factor. Adding more capacity to C, will result in lowered gain and internal noise. In making bifilar coils only one slug is used and both sections of the coil are wound on a common form. The center or r.f. ground point of both coils is located at one end (the other taps are one turn down), and the windings "criss- crossed" at opposite sides of the coil. This method has been employed in the interests of economy rather than engineering perfection. Since at any instant opposite sides of the coils are 180 degrees out - of- phase, a small voltage cancellation occurs at the crossover points of the individual windings. The arrangement assumes that tube and distributed capacity is equal across the four separate coils involved. Production tubes and coils cannot be held to these exacting tolerances, and small circuit unbalances occur which vary the amount of effective feedback controlling the circuit gain and bandwidth. Tunable boosters definitely have their place in multichannel areas, but do not represent the ultimate in engineering design. Probably the best approach to signal preamplification is to design the booster for a specific frequency and to incorporate proper circuit parameters for low noise and high selectivity. Single -channel boosters have recently become popular in fringe areas due to their improved performance vs the tunable types. A booster operating at a single frequency has the advantage of controlled feedback, which directly determines the amount of gain, and the all- important noise factor. Since the plate and grid coils can be designed for optimum "Q," selectivity can be predetermined. When the booster is attached to the TV receiver, the plate and grid slugs may be adjusted to balance out the reactive components of the transmission line, resulting in better selectivity. The cir- Fig. 7. (A) Typical cascade type booster amplifier circuit using one of the new dual -triode tubes developed for this application. (B) High -band coupling circuit. V2 661(7 VI. OL2 0 CI 2 C3 LT R,- 100,000 ohm, r/2 w. res. Re ---500 ohm, t/2 w. res. Co C,-5 µµld. cond. C,, Co C. -300 µµfl. ceramic tond. L, -Ch. 2.6 3 t. #20 hook -up wire. Ch. 7.13 2 t. #20 hook -up wire. 72 2 -4 10 t. #20 en. Ch. 3.6 8 t. #20 en. Ch. 7 -13 3 r. #20 en. 17 r. #20 en. L L,-Ch. 2 -6 ce 1/2 6B1(7 VI. Op J - L.O/ a R2 etA11111N . (A) LB NOV PLATE OF VIA Ch. 7 -13 8 t. #20 en. 12 I. #20 en. Ch. 5.6 9 I. #20 en. Ch. 7 -13 4 I. #20 en. 3 t. #20 en. Ch. 7.13 2 t. #20 en. L.-Ch. 2.4 L,,-Ch. 2.6 CATHODE Cl- 20YV1a. L1- 4 TURNSa203/16"qA. (B) L.-16 t. #28 c.c., 3/16" form. V, -68K7 tube Note: L,-L. wound on sí" s 2" form. Slugs are 1 /s' + V." threaded ¡or form. cuit of Fig. 5B is supplied with values for single -channel design. In the quest for extended television reception, mast -mounted equipment has been used extensively in fringe areas. Circuitry is the same as that of the receiver booster with the exception of the power supply. In the case of mast -mounted equipment, a maximum of twenty -four volts is permitted on transmission lines under the national electrical code. This means that the power supply at the receiver must be capable of overcoming the resistive loss of the transmission line and still deliver twenty -four volts to the mast -mounted booster. Variable supplies are necessary to insure proper operation of such external equipment. Fig. lA is a popular mast - mounted booster, and Fig. 1B a variable power supply. Fig 6 is a commercial method for employing a common transmission line for both the power and r.f. signal without loss to either. Mast -mounted equipment has two distinct advantages over single -channel boosters installed at the receiver. Noise picked up by the transmission line is overridden by preamplification. The r.f. losses of the transmission line are minimized by the higher signal - to -noise ratio at the booster output. <strong>Com</strong>mon 300 -ohm polyethylene twin - lead has an average loss of 1.2 db per 100 feet on the low channels, and 3.2 db per 100 feet on the high. Incorporation of cascode r.f. amplifiers by set manufacturers has created quite an interest in the industry for improved receiver front ends. These sets show much better pickup in areas of weak signal level. This may be directly accounted for by the quieter operation of the cascode r.f. stage. However, if a comparison is made between the cascode circuit and a properly designed push -pull triode amplifier, the difference would be barely noticeable. Push -pull amplifiers were never extensively used by set manufacturers mainly because of mechanical circuit complexity. The cascode circuit, on the other hand, adapts itself nicely to pentode circuitry. Fig. 7A is the schematic of a commercial version of a cascode amplifier. The controlling element in this circuit is the feedback condenser C,. Positive feedback is accomplished from the plate -to-grid circuit. The grid circuit "Q" and the first stage gain is controlled directly by this feedback path. As with the circuit in Fig. 5, noise and gain go in opposite directions, and the value of C, is a corn - promise between the two. The inter - stage coupling transformer is important in the design of low- channel cascode boosters, as the parallel coils L, and L, (in shunt) represent a broadband tuned load. Throughout Channels 7 to 13 the input impedance of the grounded -grid stage is of such a low value that the coupling arrangement of Fig. 7B is recommended. RADIO & TEL<strong>EVIS</strong>ION NEWS
You, ecutiAtea a &LtAufoP4Lo ctZ& build a better car radio repair business Yes you can Radio and here is why: nearly of Delco door ore Delco Radio- the help pass your you with one -half the cors that dio alone can supply arts Plus equipped; co Rd; Delco P Radio original equipment replacement can supply Deco parts; Delco up replacement Delco Radio and comprehensive i ° you with a uamp issues of "Testing P Service Manual monthly factory information on the very latest the bulletin giving tpa Y Delco cor radios-even testing and repairing "Favorite Station Signal-Seeking new radio repair toward T) a better cor T get this need only on the Delco Rad10 tusiness you need only get together with your CO nearest United Motors Distributor. Electronics DECO RADIO Don't delay act today! i RADIO KOKOMO,INDIANA DIVISION OF GENERAL MOTORS CORPORATION, MI lEO MOIOES A (.ENEAAL MOIOES I.ODUCI til2 DISTRIBUTED BY ELECTRONICS WHOLESALERS EVERYWHERE Apr 1953 73
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T EVIS i N APRIL 1953 IN THIS ISSUE
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YOU PRACTICE COMMUNICATIONS with Ki
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I NOW...from D.T.I.s MILLION DOLLAR
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"I Read You Five." Welcome Words! H
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A LOOK INSIDE PROVES WHY PAY A SERV
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TELEVISION RADAR ELECTRONICS RESEAR
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BETA RADIATION GAUGING METHODS By G
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I voltage regulation', '.'. as well
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DECAY CONSTANT IN VIBRATING SYSTEMS
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I I (5000), and Curie point* occurr
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COAXIAL CAVITY TUNING ELEMENT Fig.
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Make your U H F circuits as simple
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(// Capacitors shown 21 /7 times ac
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RCA N E E DS E NGI N E E RS who won
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O N FREQUENCY CONTROL FOR MILITARY
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i Gpoßt i. NOW YOU CAN 4 pvRAVE ti
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Ticnife "FILTER DESIGN DATA FOR COM
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Converter Calibration (Continued fr
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Page 53 and 54: Sweeping the Country! Collins Audio
Page 55 and 56: ULTRA FAN series-Complete VHF -UHF
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Page 67 and 68: A ROBOT ELECTRONIC TURTLE Closeup v
Page 69 and 70: 7. Solder to the ends of the Fa " s
Page 71 and 72: THE semi -automatic or "bug" key is
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Page 75 and 76: Fig. 1. The compact, versatile mobi
Page 77 and 78: access to a surplus TU -10-B tuning
Page 79 and 80: CHANNELS Pertinent frequencies and
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Page 85 and 86: THE U T SPEAKER ENCLOSURE Design de
Page 87 and 88: a drop of 6 db -per- octave above 5
Page 89 and 90: Fig. 1. (Left) Front panel view of
Page 91 and 92: Fig. 3. Bottom view of the 30-watt
Page 93 and 94: A LOWR[SISTA NCE OHMMETER _ By J. P
Page 95 and 96: By HAROLD HARRIS Vice -President, E
Page 97 and 98: erticle, the results are not too fa
Page 99 and 100: Sete/t SHORT -WAVE Compiled by KENN
Page 101: dual triodes with the necessary int
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Page 133 and 134: 1 BA" None None Top Top THE BEST IN
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Page 137 and 138: 59 .n 89 60 .89 54.95 . .69 . 1.59
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Page 149 and 150: nllent 95 D.e.198 e B d. Adjust the
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impedance" and is an import ant con
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- 1 Junius Heard with news 0730 on
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allo is French session 0630 on 15.1
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Fringe Antennas (Continued from pag
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a theater TV in the frequency spect
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Receiver Change s (Continued from p
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:nrh BF "BASIC ELECTRONIC TEST INST
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S0.62 f tical and easily- understoo
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B2 IT'S TERRIFIC! Fig. 3. Circuit t
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VARIABLE PULSE LIGHT SOURCE HUNGER
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1 -Improving Ir71 vision Technician
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i 2 Rate 50c per word. Minimum 10 w
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Assembling the components of Raythe
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