Radio Broadcast (May 1928-Apr 1929)

380 RADIO BROADCAST ADVERTISER For the A. C. Set Builder The name — "WESTON " on any meter you select is the highest guarantee of long life and dependable service with the lowest cost of instrument upkeep. The following three models are recommended for those having professional or technical interest in radio set construction — builders, transmitting and repairmen and all others who demand the best obtainable operating performance. Model 301 D. C. Milliammeter 3\"dhm. Also Model 506 2" diam. Use of Milliammeter in the Plate Circuit For checking plate current and plate and grid battery conditions. Low B and C battery voltages determined by direction of fluctuation of the pointer when strong signals are received. Placed in the Bbattery lead this instrument checks the set as a whole, or it checks any one radio or audio stage when placed in the plate circuit of that stage — Price, $8.00. Model 489 D. C. Portable voltmeter— 1000 ohms per volt resistance guaranteed. For checking output of Battery eliminators. Also made in lower resistance models for general D. C. testing service — Price $13.50 to $28.00. Triple Range A. C. Voltmeter 150/8/4 volts. A compact, light-weight, portable instrument with red and black mottled bakelite case for testing A. C, supply and tube voltages of socket power A. C. receivers. Also made as doublerange voltmeters up to 600 volts, and as single-range ammeters and milliammeters —Price, $13.50 to $18.50. 150/8/4 volts Weston Electrical Instrument Corporation 604 Frelinghuysen Ave., Newark, N. J WESTON RADIO INSTRUMENTS rf3 No. 227 Radio Broadcast Laboratory Information Sheet The Audio Transformer October, 1928 THE EFFECT OF ITS INDUCTANCE THE diagram on this sheet indicates at A a -1 single stage of audio-frequency amplification: B is the equivalent circuit, in which Eg is the signal voltage in the plate circuit, La is the leakage reactance of the transformer, L is the inductance, and C is the distributed capacity of the secondary and the tube input capacity, transferred to the primary. Rp is the plate resistance of the tube. Let us study this circuit to see what happens at various frequencies. The treatment given below is not exact but is approximately correct. At low frequencies the reactance of C in comparison with L is very large and the reactance of L is very large in comparison with that of La. Therefore at low frequencies the voltage in the plate circuit divides between L and Rp. The voltages across these two parts of the circuit are 90 degrees out of phase and the percentage of the total voltage that appears across L depends upon the ratio of the re actance of L to the resistance of Rp, and varies as indicated in the second column in the table, column 1 being the ratio of the reactance of L to the resistance, Rp. Now suppose that we desire to work the transformer out of a 201A-type tube with an Rp of about 11,000 ohms and that at 60 cycles we want to utilize at least 70 per cent, of the total voltage. Then, from the table we will have to make Xl, reactance of the coil L at 60 cycles, equal to the resistance of the tube. Therefore: Xl = 11,000 2 xfL = 11,000 6.28 x 60 x L = 11,000 L = 30 henries We might look at the problem in another way. Suppose we desire a transformer with a voltage drop at 60 cycles of not more than 1 TU. When a circuit is 1 TU down in voltage, the actual voltage loss is about 11 per cent., TABLE Xl 4.0 2.0 1.0 0.5 0.3 Percentage of total voltage across L 97 89 71 44.6 28.7 le a v i n g 89 per cent. This corresponds to a ratio of Xl over Rp of 2. Therefore, from the table the reactance of L at 60 cycles must be twice the resistance of the tube or 22,000 ohms. 2 xfL = 22,000 L = 59 henries lo. 228 Radio Broadcast Laboratory Information Sheet October, 1928 The Dynamic Loud Speakers THE FIELD MAGNET 'TTHE dynamic-type loud speaker depends for -t its operation on the production of a very strong magnetic field in the air-gap in which the moving coil is placed. This air-gap is indicated in the sketch on this sheet. The useful magnetic flux is that indicated by the light solid lines flowing directly across the gap, and the leakage flux — that part of the magnetic field which serves no useful purpose — is indicated by the dot-dash lines. The flux which any given amount of magnetic material, such as iron or steel, can handle efficiently is definitely limited by saturation. When the iron is saturated its r e s i stance — reluctance is the technical term — to the flow of magnetic lines through it increases and then the leakage flux increases. The flux will tend to take that path which has the lowest Moving ^ Coil" -Air Gap reluctance. To prevent leakage the pole pieces are frequently shaped in some peculiar manner, such as indicated at B, in order that the actual airgap will be a very much lower reluctance path foi the flux than any other path. The leakage flux in sketch A does not have to travel a path much longer than the actual air-gap, i.e., the two paths have about the same reluctance. In the pole shape indicated at B the flux path outside the air-gap is much longer than the path through the air-gap. The latter arrangement therefore tends to reduce the leakage flux. Assuming that the iron does not saturate, the flux in the air-gap will increase very rapidly as the size of the gap is decreased, and in practice the gap is always made as small as possible, leaving just sufficient room for the coil to move without any danger of its striking the pole pieces. Radio Broadcast Laboratory Information Sheet October, 1928 The Telephone Transmission Unit No. of Power Ratio No . of Power Ratio No. of Power Ratio No. of Power Ratio TU Gain Loss TU Gain Loss TU Gain Loss TU Gain Loss 0. 1 1 023 .977 2 7 1 862 .537 5 3 3 39 .295 7 9 6.17 .162 0.2 1 047 .955 2 8 1 906 .525 5 4 3 47 .288 8 0 6.31 . 158 0.3 1 072 .933 2 9 1 950 .513 5 5 3 55 .282 8 1 6.45 .155 0.4 1 096 .912 3 0 1 995 .501 5 6 3 63 .275 8 2 6.61 .151 0.5 1 122 .891 3 1 2 04 .490 5 7 3 72 .269 8 3 6.76 .148 0.6 1 148 .871 3 2 2 09 .479 5 8 3 80 .263 8 4 6.92 . 144 0.7 1 175 .851 3 3 2 14 .468 5 9 3 89 .257 8 5 7.08 .141 0.8 1 202 .832 3 4 2 19 .457 6 0 3 98 .251 8 6 7.24 . 138 0.9 1 230 .813 3 5 2 24 .447 6 1 4 07 .245 8 7 7.41 . 135 1.0 1 259 .794 3 6 2 29 .437 6 2 4 17 .240 8 8 7.59 .132 1.1 1 288 .776 3 7 2 34 .427 6 3 4 27 .234 8 9 7.76 .129 1.2 1 318 . 759 3 8 2 40 .417 6 4 4 37 .229 9 0 7.94 .126 1.3 1 349 .741 3 9 2 45 .407 6 5 4 47 .224 9 1 8.13 . 123 1.4 1 380 .724 4 0 2 51 .398 6 6 4 57 .219 9 2 8.32 . 120 1.5 1 413 .708 4 1 2 57 .389 6 7 4 68 .214 9 3 8.51 . 118 1.6 1 415 .692 4 2 2 63 .380 6 8 4 79 .209 9 4 8.71 .115 1.7 1 479 .676 4 3 2 69 .372 6 9 ' 4 90 .204 9 5 8.91 .112 1.8 1 514 .661 4 4 2 75 .363 7 0 5 01 .200 9 6 9.12 . 110 1.9 1 549 .645 4 5 2 82 .355 7 1 5 13 .195 9 7 9.33 .107 2.0 1 585 .631 4 6 2 88 .347 7 2 5 25 .191 9 8 9.55 .105 2. 1 1 622 .617 ' 4 7 2 95 .339 7 3 5 37 .186 9 9 9.77 .102 2.2 1 660 698 .603 4 8 3 02 .331 7 4 5 50 .182 10 0 10.00 . 100 2.3 1 .589 4 9 3 09 .324 7 5 5 62 .178 20 0 100 .01 2.4 1 738 .575 5 0 3 16 .316 7 6 5 75 .174 30 0 1,000 .001 2.5 1 778 .562 5 1 3 24 .309 7 7 5 89 .170 40 0 10,000 .0001 2.6 1 820 .550 5 2 3 31 .302 7 8 (i 03 .166 50 '1 100,000 .00001