Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST Note: favorable operating point is the one that gives the lowest permissible grid resistance, as indicated in the fourth column of Table I, except that it is best not to operate with Rg less than 100,000 to 75,000 ohms in ordinary cases because of losses in the grid circuit. The value of grid-leak resistance controls the operating point, as has been explained. The greater the leak resistance, the more will be the voltage drop in the leak, the more negative will this make the actual operating grid potential, and the larger will be Rg. The value of grid-leak resistance giving a desired operating grid resistance can be determined exactly by certain obvious measurements, which, however, require apparatus frequently not available, or they can be determined approximately with the aid of Fig. 6, which shows the grid current that will flow when the operating point is on the low jiai part of the Vg~Rg characteristic, and when the tube's Vg is on the flat part, and the desired Rg is known. To select the grid leak in this approximate way one first determines the grid current that will flow at the desired operating grid resistance, using Fig. 6, and then computes the resistance this current would have to flow through to produce a voltage drop equal to the voltage drop in the filament. The resistance thus obtained when used for the grid leak will give the desired grid resistance usually without more than 20 per cent, error for all plate voltages within the usual operating range, and for all tubes of that type. Thus in the case of a 201a tube to be operated at l\z = 150,000 ohms, the grid current as determined from Fig. 6 (Vg = — 0.47) is approximately 1.57 microamperes and the grid-leak resistance for rated filament potential of 5.0 volts would be 5/1.57 = 3.2 megohms. The grid return lead would then be brought back to the positive leg of the filament. This approximate method can be satisfactorily applied to all tubes except the 200a and the 227. With 227 tubes satisfactory results will be obtained when the grid leak is such as to give a drop of 0.9 volts when the grid current at the desired operating grid resistance is flowing through the leak. Grid-Condenser Delerm inat ion AFTER selecting the tube, the proper operating grid resistance, and the grid leak that will give the operating RK desired, there remains the determination of the grid condenser. The grid condenser capacity is determined by the highest audio frequency that is to be satisfactorily reproduced, and by the operating grid resistance. The rule is that the reactance of the effective grid condenser capacity (which is the actual grid condenser capacity plus the input grid-filament tube capacity to audio frequencies) at the highest note to be reproduced at least 70 per cent, as well as the low notes must be equal to the grid resistance. Therefore,| if f is this highest frequency and Cefr is the capacity, then 1 Ceff = The actual grid condenser size is Ceff minus the tube input capacity — about 70 mmfd. for tubes with mu = 9 and for the other tubes it will be roughly proportional to mu. In the case of the 201a tube considered, if the highest note is to be 5000 cycles, then C 0.000212 a it X 5000 X 150,000 mfd. As the tube input capacity is about 70 mmfd. a grid condenser capacity of 0.000142 mmfd. will be required. With this capacity notes of 10,000 cycles will be reproduced half' as well as the low notes. In Table I there is tabulated the value of grid-leak resistance which will put the operating grid resistance at approximately the value corresponding to the lowest permissible figure as given in the fourth column of the Table I DETECTION CHARACTERISTICS OF THREE-ELEMENT TUBES grid Type Mu Vg Rg at start Leak resistance Ceff for 70% (volts) of flat part to give Rg in reproduction of (approximate) column four 5000 cycles (ohms) (megohms) (mfd.) 201a 9 —0.47 150,000 3.20 0.000212 200a 20 —0.47 50,000 1.06 0.000636 240 30 —0.47 150.000 3.20 0.000212 199 6 —0.50 125,000 1.50 0.000255 120 3 —0.45 125,000 1.67 0.000255 171a 3 —0.28 200.000 7.2 0.000160 112 a 8 —0.26 150,000 5.8 0.000212 226 8 —0.29 150,000 1.6 0.000212 227 8 —0.23 50.000 3.9 0.000636 12 6 —0.27 50,000 4.0 0.000636 given in fourth Ceff minus tube Values of Vg are averages for a number of tubes. Values of Ceff are values for grid resistance as column. The actual grid condenser capacity is input capacity. Values of Ceff twice the value given in table reproduce 5000 cycles 45 per cent, as well as the low notes instead of 70 per cent. All tubes are RCA or Cunningham. fRg Table. This table also gives the value of Ceff that reproduces 5000 cycles 70 per cent, as well as the low notes when the grid resistance is the minimum value giving full sensitivity. Values of Ceff twice as big as those given in the table will reproduce 5000 cycles one half as well as the low notes. In general it is best to use the largest size grid condenser that is consistent with the quality of reproduction desired. Large grid condensers use up less of the radio-frequency signal voltage, as explained in connection with Fig. 2. There is not much to be gained by going to condensers over 0.00025 nifds., however, while capacities much less than 0.0001 mfd. are also to be avoided if possible. The final decision to be made regarding the detector is the choice of plate voltage. Since the rectifying action in the grid (i.e., the form of the Eg-I;, curve — Editor) is unaffected by the plate potential, it is desirable to make the plate voltage as high as possible consistent with the safe or allowable plate current. Such a plate potential will give a low plate resistance and hence good amplification. Unusually high plate potentials cannot be used with moderate mu tubes, however, because of the high plate current with the grid operating point near zero potential. Measurements of Vg of four-element tubes operated as space-charge-grid and screen-grid (222 type) tubes show that the rectifying action in the control grid of four-element tubes is of exactly the same character as in three-element tubes. Fig. 9 shows the VgRg characteristic of four-element tubes com pared with the 201a tube. The curves for four-element tubes are independent of filament, plate, and auxiliary grid potentials for both space-charge-grid and screen-grid tubes. The only difference between the two connections is that the low flat part of the Ve-Rg curve extends down to very much lower values of grid resistance in the case of the space-charge-grid tube. The screen-grid and space-charge-grid tubes can, accordingly, be used very satisfactorily as grid-leak detectors. In particular, the space-charge grid tube combines fair rectification with unusual amplifying properties. When arrangements are worked out to satisfactorily utilize the tremendous amplifying properties of the screen-grid tube for audio-frequency amplification grid-leak detection with the screen-grid tube can be used with great success. Comparison of Detector Tubes THE indications are that the merit of a detector tube as a rectifier depends primarily upon the characteristics of the filament, or the electron emitting cathode, and only secondarily, if at all, upon other features such as the mu, electrode voltages, number of elements, power capacity, and the like. The oxide-coated filament is definitely superior to the thoriatedtungsten filament, which, in turn, is better than straight tungsten. At the same time there is some difference between different types of tubes with the same kind of filament material. In selecting a detector tube the choice depends upon several conditions. If the audio system is resistance coupled the high-mu 240 and 200a tubes are best. When the detector is transformer coupled and is operated by storage battery, the 112a is best, being definitely superior both in amplification and rectification to the 201a which consumes the same filament power. Of the dry-cell filament tubes the obsolescent cx-12 is much more sensitive than the 199. Of the a.c. tubes, the 227 type is the best detector, being better than any of the d.c. or other a.c. tubes. It is interesting to note that the gas-filled 200a "super-sensitive" detector is no more sensitive than woidd be a 201a tube built with the same high mu. The gas apparently contributes substantially nothing to the 200a tube but an objectionable hiss! In conclusion it is worth pointing out that the value of the grid-voltage constant, Vg, over the low flat part of the curve, and the value of grid resistance, Rg, at which the flat part begins, are tube constants which should be published by tube manufacturers. Both of these quantities are as truly characteristic of a given make and type of tube as is the mu and plate resistance. Both detection constants are substantially independent of age, filament, and plate voltages within the operating range of values. ■0.8 -0.6 0 4 •0.2 4 \ Ep = 42 -Rated E ' ^ se: 200-A -201-A~£ 227-. 199 c'112-A 0.2 -0.8 -0 6 -0.4 -02 Ep=42. Rated Ef A I —X 120 T" 1-171-A 1.2 1.4 0.4 0.6 0.8 1.0 GRID RESISTANCE IN MEGOHMS Fig. 8 — Detecting characterization of typical tubes • march, 1929 . 0 0.2 1.2 1.4 0.4 0.6 0.8 1.0 GRID RESISTANCE IN MEGOHMS Fig. 9 — Detection characteristics of three-element tubes page 306 A