Radio broadcast .. (1922-30)

Record Details:

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Engineering Data on Its Characteristics DEVELOPMENT OF THE PENTODE TUBE imiiiimmiiiiiiimmiiiiiimiiMimiiiiiiiiiiiiii iimiiiiiHiiitiiiiiiiiiimiiiiiiimmmniiniiimt BY FRIEDRICH OSKAR ROTHY Chief Engineer, Philips Radiorohren, G. ro. fc. /I"., Vienna IT is necessary that the output tube of a modern radio receiver furnish con- siderable power to the load into which it works—the loud speaker. It is also desirable that considerable amplification take place in this tube. However, usual tubes deliver the power at the expense of amplification, because they are almost invariably of the low-mu, low-resistance type. By the construction of three-grid tubes in Europe (Philips B-443) with very great spacing, it has been found possible to raise the mu to 100, and to maintain the power output at a high level. In other words, the use of three grids makes possible the construction of a power tube with high output and high amplification. It is well known that due to the electron emission from the filament a space charge of a cloud of negative electrons is formed. These negative charges make it more difficult for the other electrons to leave the filament, and in addition they apparently reduce the plate voltage by their own potential. Although this so-called space charge can be overcome by increasing the plate voltage, practically there is a limit, in the case of receiving tubes, beyond which this can not be done. The Second Grid It is a relatively recent discovery that the space charge can be reduced by in- troducing a second grid into the tube. This principle is employed in the construc- tion of the normal double-grid (space grid) tube. In this case it is possible to reduce the plate voltage greatly and still have normal plate current. (See Fig. 1.) The auxiliary grid of this double-grid tube is placed between the cathode and the exciting grid, and the positive voltage applied to it is sufficiently high to reduce the space-charge effect. A second difficulty encountered in the construction of radio tubes is the so-called plate-reflex effect. This limits the maxi- mum undistorted energy that a tube can give. Considering the effects of space charge, with a constant filament tempera- ture, the emission is a function of the filament voltage. Assuming a resistance AUXILIARY GRIDv load in the plate circuit, which at the same time does not have any direct-current resistance, such as would be the case in an anti-resonant circuit, it is obvious that the plate voltage is the same as the battery voltage only so long as there is no alternat- ing voltage on the grid. Therefore, we must understand that the plate voltage is the real voltage produced between the plate and filament, which is less than the battery voltage, as shown in Fig. 2. If the grid goes more positive, the plate current increases and there is a drop in the external resist- ance, Ra, due to this increase. This op- poses the plate voltage, so that the voltage on the plate is no longer equal to the battery voltage. Similar effects are present when the grid is made more negative. AUXILIARY GRID Fig. 1 Fig 2 CATHODE- GRID The reflex action of the plate which is described above naturally causes a de- crease in the swing of the plate current. This is the reason for the often repeated rule governing the use of tubes in practical circuits which states that the tube does not work on the same characteristic curve when it is loaded as when it is worked into a short-circuit. In other words, the dynamic characteristic is not the same as the static. Reducing Plate-Reflex Effect Inasmuch as the dynamic characteristic always shows a smaller slope than the static, it is necessary to know this charac- teristic of the tube when worked under average conditions. There is a second re- quirement, therefore, in the construction of a radio tube, i.e., to reduce the plate- reflex effect, so that the dynamic charac- teristic may be as near to the static as possible. With such a tube considerable output can be obtained with a value of alternating grid voltage that produces only a small amount of power from a conventional tube. If now, in accordance with Fig. 3, a second grid is inserted between the normal exciting grid and the plate, and a positive voltage is applied to it, the plate-reflex effect is compensated to a greater or lesser extent. This second grid will have a constant positive voltage with respect to the filament irrespective of the external resistance, Ra. Also, if a voltage drop is experienced in the plate circuit, due to the effect of the second grid, the effective voltage will not be reduced. Under these conditions it is obvious that from time to time the auxiliary grid will be at a higher positive potential than the plate. This fact can cause a phe- nomenon which may be described as fol- lows. It is well known that secondary elec- trons can be produced by bombardment of the plate. The bombardment usually, as in the case of transmitting tubes, merely produces heat at the plate. How- ever, the dissipation of the energy pro- duces the second phenomenon of secondary electrons. In the usual tubes, they merely fall back on the plate, since the field is directed only towards the anode. In the Fig. 3 0-10 020 AMPERES dp) Fig. 4 0-30 50 100 PLATE VOLTAGE Fig. 5 150 116 DECEMBER 1929