US2119694A - Amplifier tube arrangement - Google Patents

Description

June 7, 1938; K, WILHELM 2,119,694

AMPL I F IER TUBE ARRANGEMENT Filed Sept. 1, 1933 INVENTIOR KABL W/L ELM ATTORNEY- Patented June 7, 1938 PATENT Fries AMPLIFIER, TUBE ARRANGEMENT Karl Wilhelm, Berlin, Germany, assignor to Telefnnken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application September 1, 1933, Serial No. 687,793

In Germany August 31, 1932 1 Claim.

This invention relates to amplifier tubes of the screen grid or pentode type and more particularly with amplifiers whose last or power stage consists of a screen grid tube or pentode tube. Usually when the signal voltage applied to amplifier tubes is increased, there occurs a growth of the plate direct current. Such a case occurs when the grid is biased slightly more than half Way between zero and cut-01f and the signal increases to a point such that the negative peaks are beyond cut-ofi, while the positive peaks are insufiicient to cause grid current to flow. The

net result will be an increase in the average screen and plate currents of the tube. Such a change in the plate current, especially in the power tube, causes a change in the load on the filter circuit or power pack, inasmuch as the power tube is usually connected direct to the power pack and constitutes the main load thereof. In other words, increase of the output of the end or power tube, as a result of the change in voltage on the output terminals of the power pack, reacts on the first stage of the amplifier since its plate voltage is also supplied from these same terminals.

An object of this invention is to provide ways and means adapted to render the output direct current of an amplifier tube essentially independent of large changes in grid voltage due to corresponding changes in the signal voltage.

A second object of the invention is to provide a screen grid or pentode tube circuit arrangement in which the screen grid voltage is automatically increased in case of a tube where the plate current decreases due to increase of the applied signal voltage, this increase in screen grid voltage being so controlled as to ofiset the decrease in the direct plate current resulting from increase of signal voltage. Such an operating condition is met with when the initial grid bias is so chosen that upon a large increase in signal voltage the grid is driven positive and rectification takes place in the grid-cathode circuit, with resulting direct current flowing through any resistance in the circuit between grid and cathode. This causes the average grid bias to become more negative thereby decreasing the average value of the plate current. Other objects of the invention will become apparent to those skilled in the art as the description thereof proceeds.

Fora better understanding of the invention reference is made to the accompanying drawing in which,

Fig. 1 shows a schematic arrangement of an amplifier embodying the invention; and,

Fig. 2 shows a slightly modified arrangement.

Inasmuch as whenan increase of signal voltage is applied to a screen grid tube, the screen grid current of necessity increases simultaneously with the plate direct current, I have taken advantage of this relationship to influence the output current of the tube by means of the variations in the screen grid current. According to this invention, the screen-grid voltage is applied to the screen grid through a suitably dimensioned resistance.

As shown in the figures the screen grid vacuum tube I has a control grid l0, cathode H, screen grid l2 and plate 13. If desired a suppressor grid It may also be used between grid l2 and the plate, as shown, but it will be understood that the invention may be used with a tube not having a suppressor grid. As shown, the grid l4 may be electrically connected to cathode H within the tube. As also shown, signal voltage is applied to control grid I0 through an audio transformer 2 whose primary may be connected to a preceding audio amplifier tube l6 of a broadcast radio receiver adapted to receive modulated radio signals. A load such as a loudspeaker l5 may be inserted in the plate circuit and a resistor 5 in series in the screen grid circuit with the voltage source 4. If in this arrangement the grid Ill is biased by any means (not shown to such an extent that the negative voltage peaks are cut off, While the positive peaks are not suflicient to cause grid current to flow, the net result will be an increase in the average value of the plate and screen currents. By a proper choice of the value of resistor 5, the screen current may be caused to decrease the screen voltage by the proper amount so that the average current drawn from the source 4 by the tube will remain substantially constant.

If it happens in any case that due to increase of applied signal voltage, the direct plate current falls, the screen grid current likewise falling, there will be a lesser drop of voltage in resistor, 5 with a consequent increase in voltage of grid l2. Such an operating condition will occur when the grid 16 is driven positive and rectification takes place in the grid-cathode circuit. The rectified current flowing through any resistance in the grid circuit, such as that of the transformer secondary winding, will cause the gridvoltage to decrease or become more negative, which causes 50 the average value of the screen and plate currents to decrease. In this system, the decreased screen current flowing through resistor 5 causes the screen voltage to become more positive; which will, in turn, cause the plate current to increase to such an extent as to maintain a substantially constant average value in spite of the net increased negative grid bias due to rectification. This increase of screen grid voltage will in turn increase the steady plate current and by choosing resistor .5 of the proper value, the increase in plate current may be made substantially to compensate for its decrease caused by signal voltage increase.

The power stage shown in Fig. 2 is arranged generally the same as in Fig. 1 except that the proper screen grid voltage is secured from the common terminal of resistors 6 and T which are connected in series across the plate battery 4 in voltage divider fashion. In this case, the resistors 6 and 1 are chosen of such values that the desired compensation of the direct plate current is secured, as above described.

Having described my invention, what I claim. and desire to secure by Letters Patent is:

A two stage audio frequency amplifier, each stage including an electronic tube having a cathode, an anode and a grid electrode, an output circuit connected between the anode and cathode of the electronic tube of the first stage, said output circuit including a transformer primary and a source of positive potential in series, an input circuit connecting the grid electrode and cathode of the electronic tube of the second stage, said input circuit including the secondary of said transformer, the electronic tube of said second stage being provided with a screen grid located between its grid electrode and anode, an output circuit for said second electronic tube, said output circuit including a sound reproducer and said source of positive potential connected in series between the anode and cathode of the said second electronic tube, increases in the amplitude of audio frequency currents in said transformer secondary winding acting to increase the average value of the current passing through said sound reproducer whereby the load on said source of positive potential is varied to such an extent that the positive potential applied to the anode of the electronic tube of the first stage is varied in accordance therewith, and means for compensating for said increase in current comprising a single resistor having one end directly connected to said screen grid and its other end directly connected to the positive terminal of said potential source whereby said resistor is not traversed by the current flowing in the output circuit of either of said stages, said resistor being adapted to maintain the load on said source substantially constant during operation of said amplifier. KARL WILHELM.

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