US2084181A - Radio signaling system - Google Patents

Description

`Fun@ 15, 1937. l.. E. BARTON 2,084,181 RADIO SIGNALING SYSTEM -v Filed Jari. 30, 1952 @vai E ""zsafaws l' v i g E n g :a E l guaung M4919 uylug 91mg .Y INVENTOR. Lvyaron,

w HIsATTORNEY.

Patented June 15, 1937 UNITED STATES- RADIO' SIGNALING. SYSTEM:

Loy E. Barton,

Radio .Corporation of' of f Delaware- America', a corporation Application January 30,1932", SerialA N0. SH-,847

l 18 Claims.

The present invention relates'to radiosighaling systems and, more particularly, to signalfamplie iers of the electric discharge orfvacuum tubetype-lA of a minimum amplitudek and at relativelylowf` plate voltages has been of increasing importance lo since the beginning oi the `radio'broa'dcastingrart. At present the demand` for high powercutput from ampliiiers, particularly. audio frequency amplifiers in radio receiving` apparatus, is,for commercial and practical reasons, accompanied by the demand for simplified and; comp'actapparatus of low cost and of relatively' low power consumption.

An amplifier capable of supplying relatively large amounts of audio frequency or other` fluctu- 20 ating electric power and *adapted for economical operation is describedv in my coepcnding' application, Serial No. 586,874, filedJanuary 15, 1932 andv assigned to Radio Corporation of America. In the said co-pending application, an' amplifier is ide-V scribed wherein a pair of balanced electricl dis-J charge devices are utilized in theoutput stage thereof to supply energy to ai load circuit having an impedance of a value-which may belower than the internal impedance of said devices and having a driver stage, the reflected impedance of which. over into the input circuit is of afvalue relatively low with respect toA the impedance of the input circuit, whereby the electric discharge devices in the output stagemay be driven tc` the 3 limit of their emission or space charge, forioaiii'.- mumpower output for a given anode-:potential and load resistance,

In an amplifier of the above character, electric.' discharge devices ortubcs having ordinarycom-- 40 mercial characteristics` as amplifiers and adaptedfor normal power outputmay bemade toI deliver' many times their rated power output without iiicreasing the applied. anode'potentials. Such anamplifier, therefore, provides' a marked improve- 43 ment in the efficiency of a signaling system in.

which it may be employed.

The advantages in the use of electricdischargc devices or tubes having ay high amplification" face tor have heretofore been limited practical-ly ,tot

59 voltage ampliers, such'. as those oi theordinary radio frequency and resistance coupledzaudiofre` quency types. The application of such tubes to output audio frequency power amplifiers, modulators and the like is also desirable because of the' increased sensitivity obtainable, whereby alowcr signaly voltagemay be applied' for the same power output.y

Itis-.therefore; an object of the present invention to` provi'd'efan` amplifier of the above high power' audio frequency output type, adapt'ed to makey available the `advantages, inherent in` the use' of electricl di'schargedevices or tubeshaving avhigh amplification factor and tovtakeadvantage; of the. high` internal impedancel of such devicesvv inv theamplienircuit, whereby" further" improve# ments are rna'deA in` the way of ec morni`calv opera; tion, a, reductionwof.' the apparatus requlr'ed,and' anvi'ncrease in the availablepower outpiit Heretofo're outputv tubes asl aperiodieor audio; frequency output amplifiers have-been operated with load vresistances4 equal'to or' not V more than two te` four'timesthe internal impedanceofthe` tube: Asa'result, l'ower'im'p'edancetubeslare most commonly usedfcroutput systems so thatavlow impedance loadi circuit may "beused' Itha'sbeen found', however; in' accordancewithl my invention', that al high mu tube',Y that-isgonehaving a` hglil a'mpli'i'cation` factor andai-highinternal or anodef'impedance, may be'- used asi an output amplifier-"to supply normal-andiincreasedy power output, provided` the grids or input elecu` trodes may beV driven into the positive; gridbiasrange bythe applied! signal. While the gridevoltiage-anodel current characteristic' of af low'impedance tube under actual load conditions maybeV affected considerably by' therseriesv load' or anode'v impedance, the latter" has practically' no effect uponitubcs having athigh` anodefresistan'c'ewithin" the spacev chargelimits; if the-load"'resistanceis1 lou/'compared with the anoderesistanccor imped' ance. An'. arrangement vof tlisch'aracterfemployf l in'g" ordinary amplie'r tubes has been described'v in my abovel named cot-pending application.v

It is,ttherefore,a further object of my' invention-` to provide an` improvedfaudio frequency', ampli'- er'of `the type described'infmy co`pendi`ng'appli'-` cation, having vrelatively lowimpedance input and? output circuits' and' to providey such: anfamplifierwith ktheifurther advantages' inherent in electric discharge devices or tubes' of high` ampliiication'l factor andl of relatively higl1`.`interr1al` anodef impedance.

ln" the above named application the' output' stage' ofthe amp'liii'eremploys electric dischargedevicesor tubes in balanced or push pull relation" and'is arranged to op'erate'the sarnefatsubstan-` amplifier, a normal class B amplifier being.,

An amplifier of this sharper' anode or input grid voltage. substantially anode current cut ofi and the anode current increases in response to applied radio frequency signal voltages.

The inodiied class "B" amplifier or class AB audio frequency amplifier is characterized by the fact that the tubes are arranged in push pull or balanced relation, and each tube functions during only one half of the signal Voltage wave or cycle. The tubes are to the limit of their space charge or emission vices are high or maximum. For this reason a varying load is placed upor'i the driving stage or source of signal potentials, together with a varying current of appreciable magnitude through the input circuit and the source of biasing potential.

The principal difiiculty, therefore, encountered with an audio frequency amplifier of the above type. which will be referred to asa class B audio(` frequency amplifier, is the provision of a proper source of grid biasing potential and a current cut off at the operating bias. Accordingly, it is a further object of this fier of the above type which is adapted to operate at zero or `positive bias potentials, Whereby the above named difculty is overcome.

The invention will be better understood from the following description when taken in connection with the accompanying drawing' and its scope will be pointed out in the appended claims.

'In the drawing, Figs. l and' 3 are curve diagrams' illustrating the operation of an' amplifier embodying the invention:

Fig. 2 is a simplied circuit diagram of an audio frequency amplifier embodying the invention;

Figs. 4 and 5 arey modifications of of thecircuit of Fig. 2; and

Fig. 6 is a top viewrin cross-section of an electric discharge device or amplifier tube shown in connection with the circuit of Fig. 5.

Referring to Fig. l. the grid-bias anode-cur rent curve of a usual amplifier tube having a low amplification factor is indicated at 1. When the anode current is reduced to a certain low value such as indicated at 8,.by the proper biasing potential, a relatively wide applied grid or signal potential for a desired power output must be applied to cause the anode current 1p1 to increase to a certain predetermined value such as that indicated at 9, the curve 'i being drawn with respect to a zero bias linei0.

The curve indicated at H is the grid-bias a portion anode-current curve of a tube having a highv amplification factor and a high internal impedance. vThe curve is also drawn between a point It Will be noted that the applied signal potential or grid voltage swing' necessary to cause the same anode current at i3 as at 9 is the rst amplifier tube. l

Assuming that the tubes have the saine hea dissipating ability and the same emission for thi same anode potential.. there will be a particular grid bias potential for cach tube that will permit the same anode current. Therefore, in connection with the same load impedance for the two tubes, the power dissipated in the load-inipedance will be the same, and as above pointed out, a relatively lower signal potential change or grid swing will be required for such power in the case of the tube having the high amplification factor.

While the so-callizd grid swing for a given anode current change is less for a tube having a high amplification factor, the power output relatively less than fo under the same conditionsl is the same in both' cases. It is obvious that the instantaneous anode potential is the same in each care for thesaine initial anode potential and for the same load resistance or impedance because* Instantaneous anode/ voltage -:EtltRp in which En: Supply voltage IhIAnode current RpzLoad resistance or impedance Because of the above relation to power. it is 'obvious that for maximum power output It must be relatively large, and in order to have I, relatively large Rp must be small compared to 'rp in high impedance tubes, rp cina the internal output or anode impedance of said tubes.

From the foregoing, it will be seen that it is a load impedance of changes may be enumerated as follows:

1. It has been found that in an amplifier of this character the grid biasing potential may be low or positive, although at vpresent zero bias is preferred for reasons which will hereinafter appear.

2. The input or grid circuit may be of a. relatively low impedance whereby the electric discharge devices may be operated as a class B" audio frequency amplifier.

3. With zero bias the supply potential for the amplifier may be reduced by the amount of the bias potential.

4. The potential stresses in the apparatus may be less by the amount of the bias potential.

5. Tubes of high internal impedance and high present found to be uniform'in their operating characteristics, thereby simplifying and rendering more uniform operation of an amplifier embodying them.

6. A considerable variation with load impedance is permissible while maintaining a substantially constant current output.

7. The maximum output power is essentially constant over a relatively wide range of load impedance change.

Referring now to Fig. 2, an output audio fretransformer 25.

cludes a pair of electric discharge devices I5 arranged in balanced relation to each other between a. push pull or balanced input circuit I6 and a push pull or balanced output circuit I1. The output circuit is connected to a load indicated by the terminals I8 by a suitable output transformer or coupling device I9, while the input circuit is provided with an input coupling device interposed between said circuit and the output circuit 2I of 0 an intermediate amplifier 22 of any suitable type.

In the present example, the amplier 22 includes a pair of balanced electric discharge devices 23 which are supplied with signals from an input amplifier 24 through a suitable input or coupling Signal potentials are applied to the amplifier 24 at the input terminals 26.

The output amplifier stage I4 is, therefore, preceded by two stages of amplification, whereby a.

strong input signal potential may be applied 0 thereto.

The input and output devices 20 and I9 respectively, are designed in accordance with the principles set forth in my co-pending application hereinbefore mentioned. They provide, in the input circuit I6, a lower input operating im- 5 pedance than the lowest input operating impedtrodes or grids are driven to a predetermined.

maximum positive potential, whereby they are 3 drawing a maximum grid current.

In the present example, a common grid return lead 21 from the secondary of the device 20 is carried directly back to the filament circuit 28 and to'a common cathode return lead and negative anode supply lead 29 for the amplifier. The input circuit for each of the devices I5 is connected electrically directly with the cathode circuit and the devices I5, therefore, are operated normally substantially at zero grid bias.4

The output coupling -device I9 is also of such a ratio that the load circuit impedance or resistance may be less than the internal anode or output impedance of the devices I5, and the load impedance is maintained at a minimum whereby the output may not be limited by the output circuit resistance. n

It will be noted that the devices I5 are of the multi-grid type, each having a plurality of grids or control electrode structures in addition to the usual cathode 30 and anode 3|. With reference to the grids, the grid 32 is a cathode or suppressor grid of the type employed in the usual electric discharge device now known commercially as a Pentode. The grid. 33 is a usual control grid adjacent to the cathode and the grid indicated at 34 is a usual screen grid interposed between the control grid and the anode. It will also be noted that the latter grids are connected together and are then connected with the input circuit I 6. whereby they act jointly as a control grid in two parts, each part being therefore in a different plane between the cathode and the anode.

This circuit is shown only by way of example and indicates a circuit which has been found to be satisfactory in connection with tubes of the type known commercially as the Radiotrons RCA-241. These tubes are of the usual Peniode type having the grid construction as illustratcd, and it has been found that by connecting the control grid and the screen grid together as a single input grid. each tube may be operated, as indicated, without grid bias. The use of the two grids whereby a spaced grid structure is provided results in a reduction of the anode current substantially to zero or cut off even at zero bias, thereby simplifying the circuit and obviating the necessity for a grid bias potential supply means. It will be appreciated that this may effect a relatively great saving in the cost of the apparatus not only because of the elimination of the means for the bias potential supply, but also because the entire supply potential for anode and the grid bias may be reduced by the amount ofthe grid bias formerly required.

A condition for zero'bias operation may be obtained by employing a very fine meshed grid to obtain the desired anode current cut off, but such construction is found to limit the anode current, and the grid current becomes relatively high when the grid is driven far into the positive range. Therefore, it has been found that a plurality of grids of coarser mesh are preferable, such as those provided by the'tube employed in the circuit of Fig. 2. y

With a low impedance input circuit and output circuit as described, a pair of RCA- 241 Pentode output tubes at normal anode potentials and operated in a circuit shown in Fig. 2, a power output of approximately l0 Watts was permissible without appreciable distortion, while the same tubes connected in a conventional class A arnplifier with the same anode potential and with normal negative bias potentials and screen grid potentials could produce only substantially 4 to 41,/2 watts without appreciable distortion. Accordingly. the improved operation through the use of tubes of high internal impedance and high amplification factor in a class audio frequency amplifier circuit, without bias, may be appreciated.

In the present example, the coupling device or transformer 20 employed is a step-down transformer having a turn ratio of 8 to 1 from the total primary to each side of the secondary in circuit I6, or an impedance ratio of 64 to l. The load impedance Rp was about 8000 ohms or approximately 2000 ohms per anode during the time each tube was in operation. Further advantages of this arrangement will be described hereinafter.

Attention is now directed to Fig. 3 in which plate current and grid current curves are plotted with respect to grid bias for two different conditions of operation of the output stage of Fig. 2.

Referring first to the zero bias operation, the zero vbias axis is indicated at 35 for the grid bias anode current curves 36 and 31 of the balanced amplifier devices or tubes I5. The curve 37 is shifted along the hlorizontal or grid bias axis until it coincides wit curve 36 in a common operating line indicated by the dotted line 3'8. This arrangement provides an indication of the operation of the two tubes. since each one is alternately in operation while the other is idle. The operation will be clear from an assumed condition wherein a signal potential wave 39 is applied to input circuit I6 and being arranged about the zero bias axis 35. Starting at a point 40 for a zero signal potential, it will be seen that as the signal wave increases in a positive direction the anode current Ibi of one of the devices I5 increascs in curve 36 to a maximum point 4I then decreases to the normal or zero bias value 42 as the signal wave decreases to zero, and upon a reversal of thev signal wave the other tube goes into operation to provide the anode current It: in curve 31 in the same manner. It will thus be seen that each tube oper- 5 ates alternately, the other being idle.

It will be noted that after the zero bias axis is passed in a positivedirection, the grid current Ici indicated at 43 increases from zero toa certain maximum point 44. The grid current curve l0 for the other tube 'is not shown in order to simplify the diagram, but increases in the same manner upon reversal of the input wave. Therefore, it will be seen that with the zero bias operation a current is caused to flow in the input circuit substantially continuously, whereby the load on the driver stage is maintained substantially constant.

Curves 45, 46, and 41, corresponding respectively to curves 36, 31 and 43, are drawn with respect to a positive bias axis selected at 48 to illustrate the operation of the devices I5 when given a normal positive bias. It will be seen that the grid current indicated by curve 41 is initiallyA relatively high as indicated by the point 49, but does not increase as rapidly as Iin the preceding example. It is, therefore, possible to provide a substantially constant load at a positive bias, although the load upon the driver stage is higher in the case of the positive bias than in the case of the bias at zero. For this reason the zero bias'operation is at present'preferred. It should be noted that the curves shown in Fig. 3 are taken with the tubes I5 under load.

The zero bias circuit has the advantage that a source of bias potential supply is unnecessary and the substantially constant flow of grid current in the input circuit represents an essentially constant load resistance on the preceding driver stage, or source of signal potential. Fur- 40 thermore, by arranging the devices themselves with a pluralityof grids in different planes and of coarse mesh, connected together as a common control grid, or a grid structure of that cha-racter, or a type of tube known as a Pentode, all of the advantages of a class "B audio frequency amplifier and all of the advantages pointed out hereinbefore in connection with the use of tubes having a high amplification factor and a high impedance may be obtained.

Furthermore, an arrangement of control grids vin separate planes as a means of reducing the 'anode current to a desired minimum value for class ,B operation without bias, permits a coarser mesh of the grid structure which in turn 5'5` consumes lower power when driven into the necessarily extreme positiverange. The grids in different planes may be connected together at any suitable point either inside or outside the tube structure' when connected together, to operate as a single control grid.

With a grid construction ,as above described and when connected in an amplifier circuit as shown in Fig. 2, the anode current cut-off of the amplifier devices is relatively sharp, as will be noted from the anode current curves of Fig. 3. The sharp cut-off tends to reduce distortion. In other words, the curves 36 and 31 together with curves 4 5 and 46 conform closely to a straight line such as the line 33 in Fig. 3. The tubes thus operate on a substantially straight line characteristic curve.

It will be noted, however, that about the point 42, and between that point and the zeroy plate current axis and an equal distance along curve 36 in the opposite direction, andalso over alike portion and strictly class "B.

of the curve 31 about a corresponding point formed by the intersection of the curve 31 with the zero bias axis 35, theY tubes are jointly o'perating both at the same time inpush-pull relation and, therefore, substantially as a class A amplifier. As long as the operation is symmetrical about the point 42 and the corresponding point on the curve 31, within the limits mentioned, the operation is strictly class "A forlow signal levels. The operation becomes class B, whereby the tubes operate alternately, the one acting beyond the point of operation represented by the plate current curve of the other reaching the zero. plate current axis. For the higher signal levels each tube ceases to act and the other tube acts alone alternately.

The amplifier is, therefore, over a limited range of low signal strength, strictly a class "A" ampliiler, with the vtubes operating simultaneously in push-pull relation, while beyond that range, for higher signal levels, the tubes operate alternately The amplifier is, therefore, at the Sametime a zero bias class "A and a zero bias class B amplifier when operating in accordance with the curves 36 and 31. The tubes may operate in a similar manner about another bias axis as shown by the curves 45 and 46.

A further advantage in the use of electric discharge devices or amplifier tubes having inherently a high internalimpedance and a high amplication factor as output amplifier devices, lies in the fact that, as pointed out in connection with Fig. 1, a lower signal potential is required' to drive the grids positive for the same power output and, therefore, a higher step-down ratio may be provided in the input coupling device such as the device 20 in Fig. 2. The impedance of the secondary or input winding itself may, therefore, be lower, resulting in a lower impedance drop in response to grid Ycurrent flow and in response to signal currents in the input circuit, all tending to make for amplicatlon with less distortion.

It has been found that in all cases it is not necessary to employ a cathode grid, such as the grid 32, in high amplification tubes in accordance with the invention, and in Fig. 4 the circuit of Fig. 2 is shown provided with a pair of amplifier devices 50 in which, in addition tol the usual cathode and anode, a single pair of grids 5| spaced in different planes between the anode and .the cathode, are connected with the input ci;cuit` i6 as a common control grid in each tube. The circuit is otherwise the same as that shown in Fig. 2 and is arranged without bias, whereby the spacing and number of the grids serve to reduce the anode current to a desired cut-off or minimum value at normal anode potentials.

It is alsoy feasible to include a compound gridstructure and a push-pull or balanced electric discharge device arrangement in asingie envelope in connection with a circuit embodying the invention and in Fig. 5, to which attention is now directed, a tube or electric discharge device of this character is shown in connection with the input circuit I6 of Fig. 2. In this case a pair of control grids 52, spaced in different planes between a common cathode 53 and two separate anodes 54, are enclosed in a common envelope 55. A tube of this character is particularly well adapted to the circuit of the present invention and it is preferably provided as indicated in Fig. 6, by winding the two grids 52 as a continuous wire 55, about two vertical pins l, and bringing the end out as a terminal at 58. The remaining pair of grids are made in the same manner by winding another wire 59 about two spaced posts t@ and bringing out the end as a terminal wire This arrangement, as-will be seen from the diagram, places a double mesh grid structure or two grids in spaced relation to each other, between the cathode 53 and the anodes 54. It should bc understood that the wires 55 and 59 are wound about the posts or spaced pins 51 and liti with a relatively large lead to provide a coarse mesh, and because of the helical winding, the wires of one grid will slope in one direction while on the opposite side it will slope in another, tending to provide a control effect upon the electronic stream.

From the foregoing description it will be seen that in accordance with the invention, means are provided in an audio frequency amplier whereby the electric discharge amplier devices therein may bev operated without bias or at substantially zero bias. This in itself is a desirable feature for the reason that it not only eliminates the necessity for an additional source of biasing potential which in the usual type of amplifier reduces the available anode potential supply by the amount of the bias potential, but it also simpliies and improves the circuit and effects a reduction in the apparatus required and therefore the cost of construction.

The Zero bias operation of a pair of electric discharge amplier devices in a class B audio frequency amplifier, in the manner herein described, has thefurther advantage that the" input or grid circuit load upon the driver stage or other source of audio frequency signals is more uniform. This is for the reason that grid current is caused to flow in each half of the balance circuit at all times and at least throughout each positive half wave of the signal potential cycle for each half of the input circuit.

In an apparatus embodying the invention, the structural elements of the electric discharge devices may advantageously be included in a common envelope thereby further simplifying the apparatus required for the transmission of large amounts of audio frequency or other pulsating or iiuctuating electric power.

lit is obvious that an audio frequency amplifier which requires no biasing potential and which is capable of delivering a large power output relative to the normal power output capabilities of the electric discharge devices employed therein, may have a relatively wide field of useful application, and that it is particularly advantageous in connection with battery operated amplifying apparatus, for example as audio frequency amplifiers for portable radio receivers and the like.

While the invention has been illustrated and described for purposes of convenience in its ,present preferred application to audio frequency amplifiers generally, it is obvious that it. is not i ni-ted thereto, but may be applied to any amplifylng apparatus requiring a relatively high power output for nuctuating electric power, and wherein it is desired to simplify the circuit and to eliminate the necessity for supplying a biasing' po thereto. I

claim as my invention:

i. in an audio frequency power amplifier, an .tric discharge device including a pair of Aloden a cathode structure located therebetween, .ns interposed between the cathode structure i di and each of said anodes providing a pair oi' spaced grids, a single envelope enclosing all of said elements, and a balanced low impedance in put circuit connected with each of said grid means and electrically directly with the cathode structure, the impedance of said input circuit being lower than the impedance between the cathode and either of said grid means when driven into a positive range of potentials, and the spacing of the grids in each of said grid means being such that the normal anode current is substantially zero with said direct connections to the cathode.

2. A class B audio frequency amplifier including a pair of balanced electric discharge devices each having an anode, a control grid structure, a cathode and circuit connections therefor, including an input transformer for supplying power to said grid structures without appreciable potential drop, and an electrical bias supply connection for said grid structures electrically directly from the transformer to the cathode, whereby said devices are operated at a normal substantially zero bias potential, and said grid structures being spaced in each device between the cathode and the anode and interconnected to reduce the anode current substantially to zero with said normal substantially zero bias potential.

3. A class B audio frequency ampliiier including a pair of balanced electric discharge devices each having a cathode, an anode, and a con-- trol grid structure connected with the cathode and so spaced between the cathode and anode thereof that said devices operate at substantially zero normal anode current and bias potential, and signal supply means in said connection having an impedance sufficiently low with respect to the minimum input impedance of said devices to drive said devices to the limit of their space charge or emission for maximum power output without appreciable signal distortion, said last named means having an impedance lower at all times than the impedance between the grid structure and the cathode.

4. A class B" audio frequency amplifier including in combination, a pair of balanced electric discharge devices having control grid structures so disposed and spaced in the electronic stream thereof that said devices may be operated at substantially zero normal anode current and bias potential, and balanced amplifier circuit connections therefor, having impedance values lower at all times than the impedance between the tube elements connected therewith, whereby said devices may be driven in response to signals to the limit of their space charge or emission without appreciable signal distortion.

5. In a class B audio frequency amplier, a zero bias electric discharge amplifier device having a cathode, an anode and a control grid structure located therebetween and including at least two spaced grid elements and a circuit connection between said grid elements and the cathode whereby said elements derive a normal substantially zero bias potential from said cathode, an input circuit for said device connected with said grid elements and having an operating impedance value lower at all times than the minimum operating impedance between said elements and the cathode.

6. )In a class "B audio frequency amplifier, a.v zero bias electric discharge amplifier device having a cathode, an anode and a control grid structure located therebetween and including at least two spaced grid elements, and a circuit connection between said grid elements and the cathode whereby said elements derive a normal substantially zero bias potential from said cathode, said connection including means for supplying signals to said grid structures having an impedance lower at all times than the impedance between said grid structure and the cathode.

7. In a class B audio frequency ampliiler, the combination with a pair of electric discharge devices each having an anode, a cathode and an input electrode structure located therebetween, of means providing a' balanced input circuit for said devices connected tween the input electrode structure of each of said devices and its cathode, said input circuit providing an impedance in series with each of said input electrode structures lower at all times than the impedance between said control electrode structures and the cathodes.

8. In an audio frequency amplifier, means providing a balanced input circuit, means providing a balanced output circuit, and a pair of electric discharge devices connected therebetween in balanced relation to each other, each of said devices including an anode, a cathode, a suppressor grid, a control grid, and a space charge grid, and circuit connections therewith whereby said space charge and control grids are connected thru said input circuit to the cathodes, and said last named grids are normally provided with a zero bias potential, and said input circuit having an impedance lower than the lowest operating impedance between said connected grids and the cathode by an amount sufficient to permit said grids to receive power without appreciable signal distortion.

9. An audio frequency amplier including in combination, a pair of electric discharge devices each having a cathode, an anode, and means providing a plurality of spaced input electrodes between said cathode and anode, and means providing input and output circuits for said devices whereby they are connected in balanced relation to each other, said input electrodes being connected thru said input circuit electrically direct'- ly with the cathodes, whereby said devices are arranged to operate normally at substantially zero bias, said input circuit having an impedance lower at all times than the impedance between the input electrodes and the cathode, whereby power at positive potentials may be supplied to the input electrodes without appreciable signal distortion, and said 'input electrodes in each of said devices being so spaced that thenormal anode current of said devices may be of a relatively low predetermined value at substantially zero bias.

10. An audio frequency ampliner including a pair of electric discharge devices, each of said devices having an anode, a cathode and a grid structure therebetween providing at least two spaced control grid elements, a balanced input circuit each half of which is connected with said grid elements of one of said devices and electrically directly with the cathodes, whereby said devices are connected with said input circuit in balanced relation to each other and provided with substantially a zero bias potential, saidbalanced input circuit having an impedance in each half thereof in series with the grid element of each of said electric discharge devices lower at all times than the impedance between said grid structure and the cathode to permit power at a positive potential to be applied to the grid strucelectrically directly be-` ture of each device without appreciable sim distortion.

11. An audio frequency amplifier including i combination, -a pair of electric discharge devici each having a cathode, an anode and means prc viding a plurality of spaced input electrodes be tween said cathode and anode, means providin input and output circuits for said devices, where by they are connected in balanced relation t each other between said input and output cir cuits, said input electrodes being connected thri said'input circuit electrically directly with thi cathodes whereby said devices are arranged t4 operate at a zero bias potential and said inpui electrodes in each of said devices being so spacec that the normal anode current of said devices ii substantially zero, means connected with said 1nput circuit fo'r applying to said input electrodes audio frequency signal potentials of a relatively high maximum positive value. coupling means in said input circuit providing a tially lower than the input impedance of each of said devices in response to a certain predetermined maximum positive signal potential, a source of load in said output circuit, and a coupling means between said source of load and said anodes having a coupling ratio such that the effective series'load impedance in circuit with the anode of each of said devices may be lower than the internal anode impedance thereof.

12. A class B audio frequency amplifier including in combination, a pair oi'l electric discharge devices having control grid structures so disposed and spaced in the `electronic stream thereof that said devices may be operated with normal anode potential and relatively low initial anode current at substantially zero normal bias potential, and balanced audio frequency input and output circuits therefor, having impedance values lower at all times than the impedance between the tube elements connected therewith, whereby said devices may be driven in response to signals to the limit of their space charge or l emission without appreciable signal distortion. 13. A zero bias audio frequency ampliiler including a pair of electric discharge ampliiler deof said devices is adapted of and the other device is adapted to repeat the other half of a signal input wave, said input and output circuits having impedance values lower at all times than impedance between the electrode elements of said devices connected therewith, and means for causing said device to operate adjacent to anode current cutoi as a pushpull class A ampliiler over a limited range oi low signal strength, and alternately,v as a class B amplifier signal strength.

over a higher range o! ,15. in an audio frequency amplifying system, the combination of means providing a signal source, a pair of electric discharge amplier devices coupled thereto in balanced relation, means for causing each of said devices to operate under conditions Which cause Vrappreciable grid current to flow in the grid circuits thereof in response to signals, and means for operating said devices to cause the load reacting on said source to be substantially constant, said last named means providing a low impedance grid circuit for each of said devices.

16. In an audio frequency amplifying system, the combination of means providing a signal source, a pair of electric discharge amplifier devices coupled thereto in balanced relation, means for causing each of said devices to operate under conditions which cause appreciable grid current to ow in the grid circuits thereof in response to signals, means for operating said devices to cause the load reacting on said source to be substantially constant, said last named means comprising input and output circuits for said devices having at all times a lower impedance than the lowest input and output operating impedances of said devices, and control electrode structures providing like grid voltage and anode current characteristics therefor in balanced relation for conjoint pushpull operation over a limited signal range and in-l dependent alternate operation over a wider range.

i7. In an electric discharge amplier, the combination with balanced input and output circuits,

of two electric discharge amplifier devices oppositely connected in said circuits and having input electrode structures for so controlling the space current thereof that one of said devices is adapted to repeat one half of and the other device is adapted to repeat the other half of a signal input wave, said input and output circuits having imy pedance values lower at all times than impedance between the electrode elements of said devices connected therewith.

18. In combination. a source of audio frequency energy at a predetermined potential level, a power ampliiler stage including a pair of electron discharge tubes having their inputvcontrol electrodes connected in balanced relation to said source, means providing a load for' said tubes, said tubes having their output electrodes connected in balanced relation to said load, the control electrodes of said tubes being at the same direct current potential as the cathodes thereof throughout the operation of the stage, and means for coupling said source to the input electrodes of said tubes in such a manner that the potential level of the audio frequency energy is decreased to a predetermined lower potential level, each of said tubes having a plate current-grid voltage characteristic such that the stage operates in part as a class B amplii'ler and in part as a class A amplier and has a cut-off point at a point substantially negative with respect to a common bias axis of operation.

LOY E. BARTON. t

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