US1959540A - Method and means for increasing the power output of a thermionic tube - Google Patents

Description

May 22, 1934. F. B. MacLAREN 1,959,540

METHOD AND MEANS FOR INCREASING THE POWER OUTPUT OF A THERMIONIC TUBE Filed Sept. 2l, 1929 ATTORNEY Patented May 22, 1934 i UNITED STATES PATENT OFFICE METHOD AND MEANS FOR INCREASING THE POWER OUTPUT 0F A THERMIONIC TUBE Fred. B. 'MacLaren, Malba, N. Y., assignor, by mesne assignments, to Revelation \Patents Holding Company, a corporation of Delaware *Application september 21, 1929, serial No. 394,172

15 claims. (ci. 179-171) number of turns of ne wire and forms a high resistance which is interposed in the grid cathode or input circuit. Whenever any current flows in the grid cathode circuit, therefore, there is a potential drop across the secondary of this transformer. When the grid' of the thermionic tube is negative no current fiows through the grid cathode circuit but when the grid of the thermionic tube is positive it will receife some of the electrons emitted from the cathode and a current will therefore ow. When no current fiows in the grid cathode circuit variations of potential on the grid will be in exact proportion .to the variation impressed on the secondary of the transformer but when current flows in the grid cathode circuit the variations of potential on the grid will not be exactly the same as the variations impressed on the circuit due to the resistance of the secondary of the transformer which causes the potential drop between the grid and cathode. In other words since the grid is maintained normally at zero potential the incoming signal wave will cause it to swing first negative and then positive. The grid will respond accurately to the negative side of the wave and produce true amplification in the plate circuit of the tube but the positive side of the wave will cause grid current to flow between the grid and the cathode and the potential drop in the secondary of the transformer will prevent the grid potential varying in accordance with the incoming signal. This results in a distortion of the output current, half the wave coming through accurately andthe other half being distorted so that the signals produced are muddy and indistinct even though the volume is sufiicient for perfect hearing under proper conditions.

.To overcome this defect in the amplifier vit has been expedient to place a negative bias on the grid of the thermionic tube and this bias is made suiciently l large toA maintain the grid always negative and prevent the positive waves of the incoming signal from swinging the grid beyond the zero mark. The use of the grid bias entirely eliminates the distortion just described but it has one very serious defect. It limits the power output of the tube in such a Way that it not only prevents the tube from producing as great an output as it is capable of doing but it also causes distortion when a modulation of extremely large limits is impressed on the input circuit as when a The secondary of the inputvery loud or high note is produced at the microphone or other translating device forming the input or source of signals.V This will be readily seen hereinafter -as the invention is described more in' detail. s

One of the objects of the present invention is to provide a means for increasing the range of Ypower output of a thermionic tube.

Another object of the invention is to providea method and a means for compensating for the ordinary distortion produced when the grid of an amplifying tube ispositive so that the grid may swing either negative or positive and still produce true amplification at the output of the tube. v

Another object of the invention is to provide an input circuit for an audio frequency thermionic tube which will not have a potential drop therein to cause distortion.

Other objects of the invention and objects relating to methods of connecting the various parts of the amplifier together will be apparent as the description of the invention proceeds.

One embodiment of the invention has been illustrated in the accompanying drawing in which:

Fig. 1 is a circuit diagram illustrating the invention; andv sov Fig. 2 is a graphic representation of the plate I current characteristics of a thermionic amplier tube.

In Fig. 1 is shown an input transformer 10 having a primary winding 11 connected to a source of input cmrent and a secondary winding 12, and an output transformer 13 having a primary winding 14 and a secondary winding 15, these transformers being the ordinary transformers used in an ordinary amplification systern. The primary 14 of the output transformer is connected at one end directly to a plate 16 of a thermionic amplifying tube 17 while the other end of the primary 14 is connected to the positive terminal of a B battery 18 whose negative terminal is connected to a lead 19 which is connected in turn to the cathode 20 of the tube 17, these connections following the usual connectlons for an amplifier tube. A

Now in order to permit the grid 21 of the vacuum tube 17 to swing positively Without causing distortion in the output circuit I provide what I choose to call a compensating tube 22 in the input circuit. This compensating tube 22 may be exactly lI ke the amplifying tube 17 and having a grid 23 which I connect to one endA of the secondary 12 of the input transformer 10 while the other end of this secondary is connected to the lead 19 and therefore to the cathode 20 of the amplifying tube 17 and also to the negative side of the B battery 18. y

The cathode 24 of this compensating tube I connect directly to the grid 21 of the amplifying tube 17 by means of the lead 241, while the plate 25 is connected to a point on the B battery having a positive potential. 26 may be connected across the terminals of the' battery 18 and 'a second by-pass condenser 27 may be connected if desired between the negative terminal of the battery and the connection with the plate 25 of the tube 22. These condensers provide the feed-back for the plates of the two tubes in a manner well known to those skilled in the art.

'Ihe theory of operation of the circuit just described is believed by me to be somewhat as follows: Signals impressed upon the input transformer 10 cause a variation of potential on the grid 23 of the compensator tube 22. This variation of the grid of the compensator tube causes a variation in the electron stream between the cathode 24 of this tube and the plate 25 which therefore, as the cathode 24 is connected directly to the grid 21 of the amplifying tube 17, causes a variation of potential on the grid 21 of this tube which of course controls the electron stream between the cathode 20 and plate 16 and therefore the Aoutput of the circuit. Any variations impressedon the grid 23 of the compensator tube 22 therefore will be amplified by the tube 17 and will cause increased output current to flow in the outputcircuit.

Now if .the grid 21 of the amplifier tube 17 is made negative by the incoming signal no electrons will ow between the cathode 20 andgrid 21 and no current will flow in the lead 24,. Variations of the incoming voltage therefore which maintain the grid 2l negative will be truly amplified by the amplifying tube 17 asis well known in the art. However when the grid 21 becomes positive due to the variations impressed upon the grid,23 by the input circuit, electrons will 'ow between the cathode 20 and grid 21 thereby causing a current to flow inthe lead 24';

, However electrons are flowing between the cathode 24 and the plate 25 of the compensating tube 22 due to the difference in potential therebetween and as the current flowing therebetween is in the same direction as the current flowing in the wire 24' this current flows through the plate 25 down to the battery 18, to the lead 19,

` and back to the cathode 20 forming a complete path for the grid current. Inasmuch as the resistance between the cathode and plate of the compensating tube is affected by the variations of the grid of this tube, any drop in potential across this resistance will also vary with the grid and will therefore cause no distortion when impressed on the grid of the amplifying tube and hence the amplification of the tube 17 is just as accurate for this side of the wave as it is for the negative side. As a matter of fact I have found that the grid of the amplifying tube is rarely negative due presumably to the potential difference between the cathode and plate of the com pensator tube and the consequent tendency for current -to flow therebetween.

While this may not be an exact explanation of the theory of operation of the invention, the results which are produced indicate to me that it operates in this manner. These results may be more readily seen by reference to Fig. 2 of the drawing. This figure shows a plate current characteristic curve for a vacuum tube with grid voltage plotted on the horizontal line and plate current in milliamperes plotted on the vertical line. Before my invention it has been the custom to selecta point A on the characteristic curve where A by-pass condenser.

the grid voltage is negative to prevent the positive side of the incomingsignal wave bringing the grid beyond the zero grid voltage point and is between B and C as indicated on the curve.

Any swing lof the grid beyond the point C as would be caused by a large variation coming over the input circuit will ofcourse cause a distortion in the output circuit.

The characteristic curve however continues in a straight line beyond the zero grid voltage point so that the capabilities of -the vacuum tube are considerably greater as to its power output than has been heretofore used as indicated by the limits B and C. "With my invention the midpoint of the grid potential swing, instead of beinglat A. may be at the zero point or on the positive side as at A' where it appears to come due to the operation of the compensating tube and the swing i of the grid is now not limited to the negative side of the curve but it is free'to swing on both sides making undistorted amplification possible when the grid swings between the points B on the negative and C' on the positive side of the curve.

From an inspection of this curve it is immediately evident that I have greatly increased the power output of a thermionic tube. It will also be evident that any potential drop occurring in the grid circuit of the amplifying tube when the grid is positive varies with the input variation so that there will be no distortion in the output circuit of the tube.

While I have indicated the positive potential on the plate of the compensating tube as being less than the potential on the plate of the amplifying tube I have found that this potential may be increased even above the amplifying tube potenial with an accompanying increase of power output from the amplifying tube.

In the drawing I have not indicated a means of energizing the cathode of .the two tubes used and I prefer to use independently heated cathode tubes commonly called heater type tubes which may be energized by alternating current. These tubes however, should have no connection between the cathode and filament in which cas'e they may both be operated from the same transformer winding but Where a connection is made inside of the tube between the cathode and filament there would have to be a separate transformer winding for each tube. Direct current tubes of course, may be used in which case a separate battery would necessarily have to be used for the compensating tube 22.

I vhave illustrated and described merely one stage of amplification but it is evident that other stages may be added to the construction as shown and described and in fact it is my purpose to put several such stages in series to provide sufficient output at the loud speaker or other translating device.

Modifications of the invention may be resorted to without departing from the spirit thereof and I do not therefore desire to limit myself to what has been shown and described except as such limitations occur in the appended claims:

What I claim is:

l. In an audio frequency amplifying system a thermionic vacuum tube, an input circuit for said tube, a transformer secondary coil included in said input circuit, and means to by-pass any current owing in said circuit around said transformer coil, the last mentioned means including another thermionic vacuum tube having its cathode conductively connected to the grid of the first mentioned tube, and circuit arrangements for imparting a positive bias to the grid of the first tube and a negative bias to the grid of the second tube.

2. In an audio frequency amplifying system a thermionic amplifying tube, a grid in said tube, an input circuit connected to said grid, and means connected to said input circuit to prevent a distortion of theamplification of said tubewhen said grid swings to a positive potential, the last mentioned means including a compensating tube having its anode directly connected to a source of positive potential and its cathode directly connected to the grid of the said amplifying tube, and circuit connections including said source of potential tending to bias the grid of said amplifying tube positively and the grid of said.

an output transformer, a primary winding on said' output transformer, a connection between one end of said primary winding and the plate of said second thermionic tube, a connectionbetween the other end of said primary winding and a source of positive potential, and a connection between the plate of said first thermionic tube and a source of positive potential.

4. In an amplifying system the combination of a first electron emitting cathode, a control grid associated with said cathode, a second electron emitting cathode, a controlgrid associated with said second cathode, a steady potential source tending to bias the' second grid positive with respect to its cathode, and circuit connections for causing said potential source to maintain the first grid negative with respect to its cathode.

5. A system according to claim 4, in which each cathode is provided with an associated anode, and said potential source serves as the operating potential for said anodes.

6. An amplifying system comprising a first electron emitting cathode and a control grid therefor; a second electron emitting cathode and a control grid therefor; an anode for the first cathode: an anode for the second cathode; a connection between the first cathode and the second grid; a series circuit including in sequence the positive terminal of a source of potential, the first mentioned anode, the first mentioned cathode, the second mentioned grid, the second mentioned cathode, the negative pole of said source; and a connection from the said negative pole to the first grid whereby said first grid is negatively biased with respect to the first cathode.

'1. An amplifying system comprising a first cathode, a first grid, a second cathode, a second grid, means for connecting the respective grid cathode spaces in series, said means including an input impedance, and circuit connections and a source of steady potential for simultaneously imparting some positive bias to the second grid and some negative bias to the first grid, substantially independently of the signal variations.

8. An amplifying system comprising a rst electron emitting cathode and a control grid as'- sociated therewith, a second electron emitting .cathode and a control grid associated therewith,

the first mentioned grid being somewhat negatively biased with respect to its cathode substan- 10. A system according to claim 8 in which the first cathode and second grid are conductively connected together by a connection of substantially z'ero impedance.

11..In a system of the character described a thermionic amplifying triode, an input circuit for said triode comprising a compensating triode, a connection from the cathode of said amplifying triode to the negative terminal of the anode supply, a connection of substantially zero impedance from the grid of said amplifying triode to the cathode of said compensating triode, a connection of substantially zero impedance from the anode of said compensating triode to a positive terminal of the anode supply, whereby said anode supply tends to bias the grid of the compensating triode negatively, and the grid of the amplifying triode positively.

12. A system of the character described comprising a first triode, a second triode, the `anodecathode space of the second. triode being connected across the grid-cathode of the first triode to form part of the input impedance of the said first triode, and circuit connections tending to bias the grid of the first triode positively and the grid of the second triode negatively.

13. A system according to claim 12, in which the means for biasing one grid positively and the other grid negatively includes the source of anode potential for both triodes.

' 14. A system of the character described comprising an amplifying triode, a compensating triode, a source of anode potential for both triodes, a connection of substantially zero impedance from the anode of the compensating triode to a positive terminal of said potential source, a connection of substantially zero impedance from the cathode of the compensating triode to the grid of the amplifying triode, means for impressing the signals to be amplified on the grid of the compensating triode, and circuit connections for causing the signal Variations on the grid of said compensating triode to be in substantially the same phase as the signal variations on the grid of the amplifying triode.

15. In an audio frequency amplifying system, a thermionic amplifying tube, a grid in said tube, an input circuit connected to said grid, a second amplifier tube connected in said input circuit, a grid anda cathode in said second tube, said grid and cathode being connected in series with said first mentioned grid, a source of positive potential connected to the plate of said second amplifier tube, and means whereby said source of potential tends to bias the grid of the first tube positively andthe grid of the second tube negatively, the last mentioned means including a conductive connection from the grid of the first tube to the cathode of the second tube, and a connection from the grid of the second tube to the cathode of the first tube.

FRED. B. MACLAREN.

Disclaimel Y 1,959,540-lred B. M aeLawm, Malba, N. Y. METHOD AND MEANS Fon INCREAS- ING THE Powxpn OUTPUT oF A THERMIONIC TUBE. Patent datedMay 22, 1934. Disclaimer filed Nov. 2, 1950, by the assignee, Remo Electro/nie, Ine.

Hereb` enters this disclaimer to claim 14 of said patnt.

[ #cz'al Gazette December 5, 1.950.]

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