Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST. volts. Then the power output can be calculated from the usual formula: p _ (Imax-Imin) (Emax-Emin) 8 current being expressed in amperes and powers in watts, and the percentage distortion from the formula: D _ 100 [\ (Imax + Imin) — Ino] (Imax — Imin) Ino = no-signal plate current If the curves of Fig. 4 are carefully examined it will be seen that for high values of load impedance, above about 6000 ohms, the peak value of the input voltage to the control grid must be reduced below the figure equal to the negative C bias in order to avoid distortion due to the curvature of the tube characteristics at low plate voltages. With a load of 50, 000 ohms, the input must not exceed 3 volts peak value to avoid distortion. This is not a peculiarity confined to the pentode, for it occurs with triodes also, but it usually passes unnoticed for with a triode grid current usually occurs first. Curve b of Fig. 1 shows the power output with all loads, and with the input constant at 3 volts for all loads; this curve shows also that the greatest gain, not undistorted output, is obtained when the load impedance is approximately equal to the internal plate resistance of the tube. Throughout this article the word "gain" is used to mean the relation between the output power and the voltage input to the control grid, therefore, maximum gain is obtained when the output per input grid volt is greatest, which occurs when the load resistance and plate resistance are equal. It will be seen that for the higher values of load impedance the output for a given input voltage is much greater than with any three-electrode power tube. \ 1000 ohms -■> 6000 ohms -6 Q 50,000 ohms V T> -lb -IS -">] ANODE VOLTS Fig. 4 at different frequencies the gain is different, with the result that both high and low notes are reproduced less strongly than those in the middle of the scale. Usually, however, this loss is not serious with a triode, and if desired adequate compensation can be obtained, either in the Table I Constants of Mullard P. M. 22. 2 0 .3 amp. Plate Volts 75 Neg. C. Bias -3 Power Output of Pentode The maximum undistorted power output of which the pentode is capable when the grid peak voltage is the maximum which can be applied for a given load is shown by curve a of Fig. 1, in which the input voltage is constant at 10| volts peak value for loads up to 6000 ohms only. For loads of higher values the input steadily decreases to 3 volts at a load of 50,000 ohms. It will be seen that the maximum output when the load resistance has a value of about 6000 ohms, or roughly one fifth of the tube resistance, is totally different from the results with triodes; and considering the plate voltage used the figure for the output with this load is quite high, being 640 milliwatts. The grid peak voltage required to produce this output is only lOf volts. Compare this with a ux-171a tube, with an output of 700 milliwatts with 180 volts on the plate, at an input peak voltage of about 40. As far as the gain per stage is concerned, then, it is obvious that no other power tube can compare with the pentode. But, unfortunately, there is the relative gain at different loads to be considered. No loud speaker which has yet been invented offers the same load impedance to the tube at all frequencies, consequently Max. Fit. Volls Filament Current Max. Plate Volts Auxiliary Grid Volts Plate Resistance Amplification Factor Mutual Conductance The anode and auxiliary grid should always have the same potential applied to them. 150 150 62,500 82 100 125 150 -7 J -10J In Fig. 5a is shown the circuit of the output stage when a pentode is used, and in Fig. 5b the equivalent electrical circuit assuming that the transformer is free from losses and that the leakage inductance is negligible. In this circuit L represents the inductance of the loud speaker and the effective resistance, both multiplied by the square of the transformer ratio. Ra is the tube's plate resistance, and G is an alternator supplying a voltage equal to mEg where m is the amplification factor of the tube and Eff is the r. m. s. voltage applied to the control grid. At any frequency, then, the current flowing through the circuit is given by : I = mEg at anode and auxiliary grid 100 volts, control grid zero. Fig.A Fig. 5 amplifier or in special output circuits. There are two ways in which the relative gain at various frequencies can be found. One way is similar to that employed to find the output of a tube with various load resistances. But, as the loud speaker load is usually inductive, a straight line cannot be drawn on the tube curves to represent it accurately; it is necessary to draw an ellipse, and the method then becomes more complicated. Another way, and by far the easier, is to calculate the current through the loud speaker at various frequencies. It is, of course, necessary to know what the relative currents at various frequencies should be, otherwise any results would be useless, for there would be no standard of comparison. It is unfortunate that data as to the constants of so many of the loud speakers on the market are unknown, except to their makers, and sometimes even to them! The writer has chosen for illustration one of the best English reed-drive type loud speakers (similar to an ordinary magnetic cone — Editor), the Amplion "Lion," and its effective resistance and reactance are given in Fig. 2, curves a and b, respectively. It is necessary to give a curve showing the reactance at various frequencies, as the inductance is not constant, but decreases with an increase of frequency; the effective resistance increases with frequency owing to losses in the iron. V [(R + Ra)2 + 0)2L2] where <o= 2%i + In Fig. 3 curve c, this current is shown for a pentode of 27,000 ohms plate resistance with the Amplion loud speaker, and in curve d for the same tube and speaker when the output transformer has a step-down ratio of 4-1 instead of 1-1. It must be emphasized that these curves do not show actual values of current, but merely relative values, and that for easy comparison they are all made to coincide at one frequency. Now these two curves alone are useless, for the shape of the correct curve is unknown ; that is, the shape of the curve which, in practice, would give the best results. Curves a and b of Fig. 3 give the relative current with triodes of 1000 and 2000 ohms resistance respectively, and it can be taken that any curve falling between these two limits will be satisfactory; for the loud speaker manufacturers recommend for best results an output stage having an impedance of between 1000 and 2000 ohms. It is worthy of note that the current must be very much greater at low than at high frequencies, as with all reed-drive type loud speakers; and that at any frequency the sound output is proportional to the square of the current. Moving Coil Speakers and Pentode The curve (c) for a pentode with a 1-1 output transformer shows that the current variation over the entire frequency band is but small, as the variation of the speaker impedance at different frequencies ia almost completely swamped by the high tube resistance. This curve cannot be • OCTOBER 1929 • • 361