Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST across the grid leak-condenser combination in the equivalent circuit by the action of the —0.002 volts is the amount of 1000-cycle voltage drop in grid potential existing in the actual detector, and is the amount of 1000cycle voltage which is applied to the input of the audio amplifying system. (This does not mean the voltage applied to the primary of the first audio transformer, for example, but is the audio-frequency voltage impressed on the grid of the detector, which the author considers as the beginning of the audio system. If the mu of the detector tube is 8, the maximum voltage across the primary under these conditions would be 8 x 0.002 or 0.016 volts — Editor.) The negative sign of the rectified grid voltage is caused by the fact that the voltage constant, Vg, of the grid is negative, and this merely means that the voltage acts in a direction opposite to that indicated by the arrow in Fig. 4. It is apparent from a study of Fig. 4 that the fraction of the rectified grid voltage which is usefully used to produce change of grid potential is determined by the ratio of impedance to the rectified grid voltage, which the grid leak-condenser combination offers to the grid-resistance, Re. The higher this ratio, the more sensitive will be the detector, but in no case will the change of grid potential ever exceed the rectified grid voltage. In order that the detector may reproduce the high notes as well as the low notes it is necessary that the impedance of the grid leak-condenser combination at the highest note desired be sufficiently great relative to the grid resistance, Rg, as to cause most of the rectified grid voltage of this high frequency to be used up as voltage drop across the grid leak and condenser. Then the high notes will be reproduced with full sensitivity and, as the low notes are already as loud as possible, the detector will give good quality output covering the entire audio-frequency range. The quality of the detector output will be worse for operating points which give a high grid resistance, Rg, than for conditions which give a low grid resistance. Thus, high-resistance grid leaks give poorer quality than lowresistance ones. With a given size grid condenser, however, the quality is not unproved appreciably after the grid resistance gets less than a critical value to be discussed later. The maximum allowable grid resistance, Rg, is determined by the highest audio frequency to be reproduced at full sensitivity, and by the size of grid condenser. The grid-leak resistance has little effect on the quality at the high notes except as a means of controlling the operating grid potential, and hence of controlling the grid resistance, Rg, because the rectified grid currents of high audio frequency very largely go through the grid condenser shunting the grid leak. Detection Data THE most satisfactory way to represent detector characteristics is to plot grid voltage constant, Ve, as a function of grid 226 Tube' Ep=42Volt3 1 I \ G -V \ Volta £C Constan 0 0.2 0.4 0.6 0.8 1.0 12 1.4 GRID RESISTANCE IN MEGOHMS Fig. 5 — Rectifying characteristics of a 226type tube used as a gridleak grid-condenser detector resistance, Rg, at the operating point. Since the sensitiveness of the detector is proportional to the rectified voltage and this in turn is determined by Vg, while the possible quality is dependent upon Rg, such a curve can be considered as showing the relation between sensitivity and quality. A typical relation between the grid voltage constant, Vg, and grid resistance, Rg, is shown in Fig. 5. This figure also shows the operating grid potential required to give different values of grid resistance. In examining the Vg-Rg characteristic it is to be remembered that, since the rectified grid voltage is inversely proportional to Vg, the sensitiveness is greatest when the grid voltage constant, Vg-, is smallest. In Fig. 5 it is accordingly seen that as the operating grid potential gets more negative, and the grid resistance, Rg, increases, the sensitivity rapidly increases until Rg equals about 150,000 to 200,000 ohms. For all grid resistances higher than approximately 150,000 ohms the sensitivity as indicated by the Ve curve is substantially the same, and is the maximum sensitivity which is obtainable from this particular lube. While Fig. 5 gives the Vg-Rg characteristic of a particular tube at particidar values of \\\ — 1^ 0 100.000 200,000 300,000 Fig. 6 — Grid current as a function of grid resistance and Ve plate and filament voltages, an investigation in which over 1000 measurements of Vg were made showed that every tube tested had a Vg-Rg characteristic similar in shape to Fig. 5. In every case there was the same rapid decrease in Vg at increasing values of grid resistance and this was followed by the low flat part of the curve at all grid resistances above a critical value. Not only does every tube have the same type of Vg-Rg characteristic, but every tube of the same type was found to have substantially the same characteristic for all plate and filament voltages (provided there was sufficient electron emission from the filament). Furthermore using the tube, or even rejuvenating it (in the case of thoriated filaments) had no effect on the Vg-Rg relation as long as the filament was reasonably active. The only point on which tubes of the same type differ is in the grid voltage required to give a particular value of grid resistance. At high plate and low filament potentials the operating grid voltage must be slightly more positive to obtain a given grid resistance than at low plate and high filament potentials. Even at the same filament and plate conditions different tubes of the same type will sometimes require operating grid voltages differing in extreme cases by as much as 0.5 volts to give the same Rg. Characteristic Curves The Vg-Rg characteristics for standard types of tubes are given in Figs. 8 and 9. These curves are all for a plate voltage of 42 • march, 1929 . . . page 305 • V UX 222 Tub e R ited E ( 'reen-gri :p=123, & Detect( Esg=40 r ~z — ' Spa \r ce-cnarg( Ep=80, '-grid Detector Escg=40 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 GRID RESISTANCE IN MEGOHMS Fig. 7 — Detection characteristics of four-element tubes and rated filament conditions, but would be substantially unchanged if measured at other plate and filament voltages. The important differences between the various tube types are (a) the value of voltage constant on the flat part of the curve, and (b) the value of grid resistance at which the low flat part of the curve begins. These characteristics are tabulated in Table I. The values in the third column determine the sensitivity of the detector as a rectifier. The tube with the smallest Vg is the best rectifier, and will produce the greatest change of grid potential with a given signal. The second particidar in which different types of tubes differ, i.e., the point at which the flat part of the curve begins, has an important influence in determining the effectiveness with which the high notes may be reproduced. The lower the value of grid resistance at which the low flat part of the Ve— Rf, curve begins, the better the detector. Practical Detection THE principles and data of the preceding paragraphs will now be applied to the problems involved in selecting detector tubes and adjusting their circuits. In selecting detector tubes it is necessary to remember that the sensitiveness depends upon, first, the maximum change of grid potential obtainable, which is inversely proportional to the value of Vg over the low flat part as tabulated in Table I, and second, the amplification (produced in the tube) of this change of grid potential. While both the 201a and the 199 have substantially the same Vg, the 201a tube is a better amplifier because of its higher mu and lower RP and is, therefore, superior. The 227 tube is a more sensitive detector than the 226 tube because, although they are equally good amplifiers, the 227 tube has a smaller Vg, and so gives a greater change of grid voltage to amplify. On this basis the 227 heater-type tube is the most sensitive detector, closely followed by the 226 and the 112a types. Other tubes, such as the 201a, 199, 171a, 120, and 12 varieties are distinctly less sensitive, either because of high grid voltage constant or because of low audiofrequency amplification per stage. The 200a gas tube and the 240 high-mu tube are no better rectifiers than the 201a tube, but as both have a high mu they are more sensitive than other detector tubes in resistancecoupled circuits. Securing Sensitivity IN ORDER to realize the full sensitivity of the detector tube the operating grid potential must be such as to give a grid resistance that is on the low flat part of the VgRg characteristic. No detector tube should be operated at a grid resistance lower than the value given in the fourth column of Table I. If this rule is violated great loss in sensitivity will result. Since Rg is not the same as the grid-leak resistance the next step is the selection of a value for the latter that will give the best operating grid potential. In general, the most