US1878740A - Push-pull amplifier - Google Patents

Description

P 20, 1932- i H. A. WHEELER 1,878,740

PgSH-PULL AMPLIFIER Filed July 16. 1929 2 Sheets-Sheet 1 T6 INVENTOR WHEELER Y. r BY QM, 9w,

ATTORNEYS Sept. 20;, 1932. H. A. WHEELER PUSH-PULL AMPLIFIER Filed July 16. 1929 2 Sheets-Sheet 2 rqlff'll INVENTOR 1 hiA. MIE LE)? BY kit-0J4 ATTORNEY 5 Patented seal-2o, 1932 UNITED 'STATES BATION, A CORPORATION OF DELAWARE rusn-rnnr. AMPLIFIER Application filed July 16, 1929, Serial No. 878,630, and in Canada June 26, 1930.

This invention relates to improvements in thermionic'amplifiers of the low frequency type adapted principally for amplifyinfiurrents in the audio frequency range. ore particularly, the invention is directed to improvements in thermionic amplifiers operating in so-called push-pull circuits.

An object of the invention is to provide an amplifier of the type mentioned which will furnish substantially distortionless amplification without the necessity for utilizing a biasing battery or an initial negative biasing potential on the control circuits or grids, of

- current delivered to the output circuit is caused to flow in series thru the plate circuits of a pair of tubes oppositely connected thereto, such arrangement permitting the most eificient delivery of distortionless power to the output circuit.

A further object is to rovide a meter in the amplifier circuit w ich gives a continuous indication of the signal intensity.

As a general proposition, one of the essential requisites to the distortionless amplification of the signaling currents by means of thermionic amplifiers consists in applying to the grids of the thermionic tubes a suiiiciently large initial or permanent negative biasing potential to maintain them at all times negative relative to the corresponding cathodes or filaments. Otherwise, if the incoming signal wave has a suificiently' great peak value to carry the grid positive during a portion of each cycle, space current will flow in the grid circuit during such portion thereby causing an increased voltage drop in the input circuit associated with the tube with resultant decreasein voltage impressed between the grid and filament thereof as compared to that of the correspondingnegative PATENT OFFICE morn A. WHEELER, or GREAT NECK, NEW ronx, ASSIGNOR TO HAZELTINE conro' portionof the signaling wave. As a conse-,

quence of this, the amplified signaling wave will contain a flattened portion during each interval that the grid is positive, which effect, of course, is distortion.

It is possible by means of a well designed thermionic amplifier utilizing a suificiently large permanent negative biasing potential on the grid, to obtain substantially distortionless amplification, but such aresult is attained only by sacrificing certain other ,desirable features. In the first place, the use of the negative biasing potential on the grid of a tube increases the plate circuit impedance thereof by a corresponding amount since the presence of the negative potential on the grid impedes the flow of electrons from the filament to the plate, thus requiring a higher plate circuit voltage for a iven plate current. The increased plate voltage required, together with the added grid bias voltage, involves an added expense to the amplifier construction.

In order to partially overcome the above enumerated disadvantages accompanying the use of apermanent negative biasing potential on the grids of the tubes, the so-called push-pull amplifier circuit was developed.

The push-pull amplifier, as frequently constructed, comprises in essence an input transformer having the secondary winding thereof connected between the respective grids of a pair of similar thermionic tubes, an output transformer having the primary winding thereof connected respectively between the anodes of saidtubes, and having the cathodes connected in common tothe midpoint of'the input transformer secondary winding and also thru a direct plate current supply source to the midpoint of the output transformer primary winding.

If now a'sine wave of signaling current is impressed upon the input transformer of such a push-pull amplifier, there will at each instant of time be a signal voltage impressed in a positive sense between grid and filament of one tube and an equal signal voltage impressed in the negative sense between grid and filament of the other tube,'and since the voltages thus impressed produce additive effects in the output transformer secondary,

it is thus insured that each positive half cycle of amplified signaling current will be identical in wave form with the corresponding negative half cycle. This, however, does not necessarily mean thatthe amplified wave will be distortionless; it merely means that the even harmonics of the fundamental frequency will be eliminated. The odd harmonics may still be present in sufiicient magnitude to cause appreciable distortion.

In addition to the distortional effects noted with the push-pull amplifier as commonly constructed, any grid current in flowing thru the input transformer secondary winding willcause a consirerable voltage drop therein which will reduce the net signaling voltage applied between the grid and filament of the tubes and in this way limit the amplification obtainable.

. The amplifier of the present disclosure is adapted to overcome the objections pointed out above with reference to the push-pull amplifier as commonly constructed. To accomplish this the input transformer having the tapped secondar winding as commonly used in such ampli ers is replaced or supplemented by a pair of equal high impedance elements such as a pair of equal resistances, serially connected between the grids of the tubes. The midpoint between such elements is connected directly to the cathodes in common but is not connected to the midpoint of the transformer secondary winding. For proper operation theimpedances must be large in magnitude as compared to the gridto-cathode resistances of the tubes so that when a signaling voltage is impressed on either grid in, a positive sense the cathode-togrid resistance of the tube will effectively short circuit the corresponding impedance element.

If with the arrangement of-elements as described, a signaling voltage wave is impressed between the grids of the tubes, near- 1y all of the voltage will at any instant exist across the tube havingthe more negative grid. The reason for this-is that the more negative grid draws no grid current and hence presents substantially an open circuit connected across its impedance element. The tube whose grid'terminal is positive at any instant offers a relatively low resistance to the flow of grid current, and thus substantially short-circuit the impedance connected thereacross. Inasmuch as the grid current at any instant flowing in this short-circuiting path must also flow through the impedance connected to the more negative grid, it is so limited thereby that the voltage distribution between tubes will be such that nearly the full voltage at each instant will be impressed across whichever tube has the more negative grid at that in tant. The positive grid becomes in effect an instantaneous connection directly between the transformer and the filaments.

The advantages of such a mode of operation are apparent. In the first place, neither grid can ever become highly positive and draw excessive grid current. Furthermore, the grid current can not in this case cause a voltage drop in the input circuit, thereby eliminating this distorting and power-limiting effect observed in common push-pull amplifiers.

The major portion of the signal voltage is impressed at each instant on the negative grid. Therefore, one tube amplifies the half permit the maximum power output from the present circuit. Due to the fact that only one tube at a time is active, the output transformer in such case acts as a threewinding transformer having a single primary connected to the active plate and a pair of windings connected respectively to the inactive'tube and to the output circuit. The dynamic plate circuit resistances of both tubes are thus connected effectively in parallel with the output circuit so that the inactive plate acts as a dissipative shunt across the output circuit.

It is well known that in the operation of thermionic tubes the maximum power output consistent with good operation is obtained when the load impedance is substantially greater than the plate resistance. In the above case, the eflfective load impedance of inactive plate and output circuit in parallel is always less than the active plate resistance. I

In contrast with the above arrangement wherein the output circuit and plate circuit resistances are effectively in parallel, it is possible to so arrange the elements that the amplified signal current flows in series thru the active and inactive plate resistnces and thru the output circuit. The above ideal condition can then be realized and nearly the entire output power from the active delivered to the output circuit.

active tube at any instant.

The series arrangement for the output circuit is obtained by replacing the commonly used output transformer having the tapped primary winding, with a pair ofequal unlate be much smaller signal current now flows thru the incoupled high impedances such as choke coils or resistances, which are serially connected between the plates of-the tubes. The direct plate current supply is then connected.be tween the filaments and a point intermediate the impedances. Only the steady direct plate current supply flows thru the high impedances. The major part of the signal cur rent-due to the choking action thereof is forced to flow in series thru the active and inactive tubes and thru the output circuit.

The series type of output circuit is inferior to the usual output transformer when applied to the common push-pull amplifier usingahighly negative grid bias. The reason is that with such an amplifier both tubes are about equally active at each instant in delivering current to the output circuit, and the lessactive tube is the one with the higher instantaneous plate resistance.

The expressions divided input section and divided output section as used in the specification and claims denote the push-pullconnection of the amplifier tubes in which the cathodes of the push-pull connected tubes are joined together and connected fespectively to an intermediate point of the input circuit and output circuit, while the grids are connected to respective opposite'outer terminals of the input circuit and the anodes are connected to opposite'outer terminals of the output circuit.

Referring now to the drawings: Fig. 1 shows in circuit .diagram form a push-pull amplifier having the input section thereof arranged in accordance with the present invention and utilizing generalized impedance elements in the grid circuit. The output circuit is terminated in a standard manner by means of an output transformer having a tapped primary winding for connection to the filaments.

Fig. 2 shows an amplifier of the above type wherein both the input and output sections are arranged inv accordance with the present invention utilizing generalized impedances therefor.

Fig. 3 shows a modification of, Fig. 2 wherein generalized impedance elements and an input transformer are associated with the input circuit while the output circuit is terminated in choke coils and condensers suitably arranged.

In Fig. 4 each of the generalized impedance elements of Fig. 2 is replaced by a separate transformer. The operation, however, is the same as that of Fig. 2.

Fig. 5 shaws two amplifier stages connected in cascade. The arrangement of each stage is in accordance with Fig. 1 with the exception that the second stage shows an auto-transformer and condensers in the output section. i

Fig. 6 shows a two-stage amplifier in accordance with the invention wherein a-spe cial type of loud speaker unit is connected directly in the output section of the second stage.

Like elements are designated in the same manner ineach of the. drawings.

Referring to Fig. 1, the amplifier comprises similarvacuum tubes V and V each having a grid G, filament or cathode F and plate P electrodes. The filaments F are heated in any suitable manner, not shown, and are conductively connected as shown by lead 4. Between the grids of tubes V and V are serially connected the similar and electrically distinct impedance elements Z and Z These impedance elements are shown in generalized form indicating that any suitable type of high impedance may be used such as resistances or choke coils. Preferably, resistances should be used for this purpose having a value of .1 to 1.0 megohm dependent upon the characteristics of the ticular tubes used.

The filaments F in common are conductively connected to the point 3 between elements Z and Z by means of lead 1. A milparliammeter M is interposed in lead 1 for purposes of continuously indicating the power level of the signaling current. (In the usual grid-bias amplifier, the grid current meter does not indicate the signal intensity except when overloading.) The primary P of an output transformer T is connected between.

theplate electrodes P of tubes V and V The midpoint of winding P is connected in the usual manner thru a plate current supply source B to the filaments in common by means of lead 2. The secondary S of transformer T terminates in output terminals 0 to which an output impedance indicated by rectangle L may be' connected as indicated by the dotted-lines.

The input section comprises terminals 1 connected respectively to the grids of tubesv Y and V The source of incoming signalmg current, indicated by A, may be connected directly to terminals I or may be applied thereto thru an input transformer T in the manner shown.

As the general operation of the amplifier of Fig. 1 has been explained above, only a few specific points need be noted here. At an instant when the applied signaling current is such that the lower terminal I is negative relative to the upper terminal I, the grid of tube V will be negative and that of tube, V positive relative to the filaments. The impressed signaling current will flow, theresmall portion flowing thru impedance'Z to the upper terminal I and the major portion thru lead 1 and milliammeter M, and thence fore, from the lower terminal I thru imped ance Z to point 3 where it will divide, a-

thru thespace path from filament to grid of tube V to the upper terminal I. Since the grid circult of tube V; is of low impedance relative to element Z, the grid circuit practically short-circuits the element, so that substantially all of the voltage drop due to the impressed signal is across impedance Z and hence across the grid circuit of the tube V which being biased negatively thereby is practically an open circuit.

Thus during the interval that the lower terminal I is negative relative to the upper terminal, tube V is active in delivering'amplified signaling current to transformer T Tube V on the other hand, is relatively inactive but the small signaling current flowing in the plate circuit thereof produces an effect. in transformer winding S which aids v the effect therein of the current flowing in the plate circuit of tube V During the next half cycle, i. e., while the upper terminal I is negative relative to the lower termnal, the above conditions will be reversed, the grid of tube V being negative and hence active while that of tube V is positive and hence relatively inactive.

tinct transformer windings.

Where an input transformer T is used, all of the current flowing in the secondary thereof must flow in series thru the high impedance Z or Z as the case. may be which thus limits the current magnitude toa small value and hence prevents appreciable magnetic saturation of the transformer or appreciable impedance drop therein.

With the arrangement of Fig. 1, the plate circuits of tubes V and V are effectively in parallel with the output impedance L, withthe resultant disadvantages pointed out above. In Fig. 2, on the other hand, these same elements are in series, due to the presence of the similar high impedance elements Z and Z serially connected between the plate electrodes respectively of tubes V and V These impedances should be of the order.

tube tends to decrease the current in the corresponding impedance by a like amount, but a small current change therein sets up a choking action which forces the major portion of the signaling. current to flow in series thru the plate circuits of the active and inactive tubes and thence thru the load impedance L with the resultant advantages pointed out above.

Assuming the active tube capable of delivering a certainmaximum current value without appreciable distortion, the maximum energy will be delivered to the impedance L when its magnitude is large com,- pared to the dynamic plate circuit resist ance of the tube. Since, however, the impedances Z and Z must be large compared to L, this would mean a corresponding increase of the same with resultant added resistance in the battery supply path which would in turn call for an increased battery B and would at the same time introduce certain other undesirable features well known in tube operation. It has been found theoretically and in practice that the optimum arrangement is obtained when the impedance L is about four times the dynamic plate circuit resistance of a single tube. On the same basis, with the arrangement of Fig. 1,.

the impedance L should also be about four times the dynamic resistance of a single tube, if the total primary and secondary turns are equal in number. v

Fig. 3 shows an arrangement identical in operation with Fig. 2, but one wherein an input transformer T ispermanently associated with the input circuit and wherein the generalized impedance elements Z and Z, are specified to be similar choke coils L and L The capacities C are inserted in the output leads 0 to keep the direct plate voltage off the output circuit.

Fig. 4 shows an arrangement similar electrically with Fig. 2, but one wherein the generalized impedance elements Z to Z inclusive, are specified to be separate transformers T to T inclusive. The rimaries of transformers T and T are serlally connected between the input terminals I, and the secondaries thereof serially connected between the grids. The midpoint between the secondaries is connected to the filaments by means of lead 1. A corresponding arrangement is shown for the output transformers T and T It should be pointed out that each of transformers T to T inclusive, operates independently since each is on a, separate magnetic core. In this respect the operation differs essentially and advantageously from that .of transformer T of Fig. 1, in which all windings are magnetically coupled.

Fig. 5 shows a two-stage amplifier with tubes V and V comprising the first stage and tubes V and V comprising the second stage. The output of the first stage and the input of the second stage aresimilar in arrangement to Fi 1, while the output of the second stage uti izes an auto-transformer or tapped choke coil T and is thus similar electrically to the output of the first stage.

Fig. 6 shows a two stage amplifier utilizing a transformer T and equal resistance elements R and R for-input of the first stage, and resistances R and R for the output thereof. The successive stages are intercon- I atria-:40

nected by transformer-T with the input to a the second stage utilizing equal generalized a permanent magnet 8 having affixed to poles thereof two opposed U-shaped pole pieces 9 and 10.

.The pole pieces are so dimensioned as to leave an an gap between theopposite legs 7 thereof, in which spacethe armature 11 is suspended in such manner as to be free to vibrate only along the line of the diaphragm connecting rod 12, so that the cone 7- will be vibrated in accordance with the oscilla tions of the armature.

The coil 5 is placed about the upper leg of the pole piece 10, and the corresponding coil 6 about the lower leg of pole piece 9. Coil 5 is connected directly in the plate circuit of tube V and coil 6 in the plate circuit of tube V in such manner that the flux set up by the direct space current in each case8 aids the flux of the permanent magnet i The armature 11 has a length about equal to the distance between centers ofthe legs of the pole pieces 9 and 10, and'being posi tioned symmetrically in the air gap is thus pulled equally in opposite directions by the steady fluxes in'the two gaps, respective ly, between the pole pieces; I

It now a signal wave be impressed upon the input terminals of the amplifier, the action of the amplifier is such that the current in signal coil 5 decreases when the grid of tube V is negative, and at the same time the current in signal coil 6- remains constant at the normal direct current value or increases very little because the grid of tube V is positive. Likewise, when the grid of tube V is positive, the current in signal coil 5 is practically at the normal direct cur-' rent value and the current in signal coil 6 is decreased. From the foregoing, it is .seen that the signal coils are alternately operative, and furthermoreflhe coil which is inoperative exerts no influence upon the coil which is operative. This gives the result that the inactive tube of low plate resistance is not eiiective a shunt across the output of the active tube, as would be true if a single output transformer were utilized.

The principal advantage of connecting the loud speaker directly in the output circuit is that all frequencies operate on the speaker unit to equal advantage, whereas if the speaker were connected to the output terminals of 3, the choke coils would offer-a relatively small impedance to the lower signaling frequencies and hence would shunt a considerable portion thereof away from the speaker unit. Another advantage of Fig. 6lies in the elimination of the high impedance elements in the output circuit, their places being taken by coils 5 and 6 of the speaker unit. This, of course, represents an economical advantage. The greatest distortionless power output is secured when coils 5 and 6 are practically without magnetic coupling and when the eflective impedance of each coil is approximately three times the output impedance'of the tube connected thereto.

I claim:

1. An electric wave repeatin apparatud comprising divided input and vided output sections with a pair of electron discharge devices each including a control electrode connected in opposition therebetween, and high impedance means associated with said input section adapted to cause a signal wave applied between said control electrodes to be repeated at each instant in substantial entirety by the said device which receives a negnected in opposition therebetween and high resistance means'directly associated with the 1 input sections of said devices adapted to cause a signal wave applied between said control electrodes to be impressed in substantial entiretv at each instant upon the said device which receives a negative potential therefrom, whereby one device repeats the positive portions and the other device the negative portions of said signal wave.

3. An electric wave repeating apparatus comprising divided input and divided output sections with a pair of electron discharge devices each including a control electrode connected in opposit on therebetween, and electricallv uncoupled high impedance means individual respectivelv to'said divided input sections adapted to causea signal wave ap plied between said control electrodes to be impressed in substantial entiretv at each instant upon the said device which receives a negative v po ential therefrom whereby the latter said device repeats the signal.

4. An electric wave repeating apparatus comprising divided input and divided output sections with a pair of electron discharge de- 4 repeats the signal, and additional high im- I tential therefrom, whereby said latter device 55 an amplifier using matched choke coils as in pedance means associated with said output section adapted to cause the repeated wave to flow in serles thru the space paths of both said devices.

5. An electric wave repeating apparatus comprising divided input and divided output sections with a pair of electron discharge devices connected in opposition therebetween, high impedance means associated with said input section adapted to cause a signal wave applied to said input section to be impressed in substantial entirety at each instant upon the said device which receives a'negative potential therefrom, whereby said latter device repeats the signal, and additional high impedance means interposed in the space current supply paths of said devices adapted to cause the repeated wave to flow serially thru the space paths of both said devices.

6. An electric wave repeating apparatus comprising divided input and divided out- .put sections with a pair of electron discharge devices connected in opposition therebetween,

high resistance means associated with said input section adapted'to cause a signal wave applied to said input section to be impressed in substantial entirety at each instant upon the said device which receives a negative potential therefrom, whereby said latter device repeats the signal, and high impedance means individual to the respective divided portions of said output section adapted to cause the repeated wave to flow serially thru the space paths of both said devices and thru said output section.

7. An electric wave repeating apparatus comprising divided input and divided output sections with a pair of electron discharge devices connected in opposition therebetween, high impedance means individual to the said divisions to said input section adapted to cause a signal wave applied to said input section to be impressed in substantial entirety at each instant upon the said device which receives a negative potential therefrom, whereby said latter device repeats the signal, and additional high impedance means individual to the respective divided portions of said output sect-ion and adapted to cause the repeated wave to flow serially thru space paths of both said devices and thru said output section.

8. Thermionic electric wave repeating apparatus for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements,

comprising a pair of electron discharge devices connected in opposition to an output section, each said device including anode, cathode and a control electrode, a pair of uncoupled high impedance elements serially connected directly between said control electrodes, a connectionfrom said cathodes to a-point between said impedances, and .an

input section connected to said control elec- .tro'des, said high impedance elements being adapted to cause a signal wave applied to by said latter device alone repeats the signal.

9. Thermionic electric wave repeating apparatus for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements, comprising in combination, a pair of electron discharge devices connected in opposition to an output section, each said device including anode, cathode and a control electrode, high resistance means connected directly between said control electrodes having an intermediate point thereof connected to said cathodes,- and an input section having its opposite'terminals connected respectively to said control electrodes,-said high resistance means-being adapted to cause a signal wave applied to said input section to be impressed insubstantial entirety upon the device which receives a'negative potential therefrom, whereby said latter device repeats the signal.

10. Thermionic electncwaverepeating apparatus for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the resistance of said devices during operation I serially connected between said grids, a connection from said cathodes to the point between said impedances, and an input section connected to said grids, said impedance elements being adapted to cause a signalwave applied to said input section to be impressed in substantial entirety upon the device which recives a negative potential therefrom,

whereby said latter device repeats the signal.

11. Thermionic electric wave repeating apparatus for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements,

applied to said input section to repeated comprising a pair of vacuum tubes each hav- .7 ceives a negative potential therefrom, whereby said latter tube repeats the signal.

12. Thermionic electric wave repeating ap 5 paratus for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements, comprising in combination a pair of vacuum tubes each including anode, cathode and grid electrodes, a pair of impedances serially connected directly between said grids withthe 4 I point between said impedances connected to said cathodes, said impedances being adapt ed to prevent the flow of excessive current in the grid circuits of said tubes when large signal voltages are applied between said grids, and also being adapted to cause said voltages to be im ressed in substantial entirety upon the tu e which receives a negative potential therefrom,

13. Thermionic electric wave repeating apparatus for providing high degrees of'substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic, elements, comprising in combination a pair of vacuum tubes each including anode, cathode and grid electrodes, resistance means connected direct- 1y between said grids, said means being connected at an intermediate point thereof to said cathodes and adaptedsubstantially to suppress the flow of space current in the grid circuits ofsaid tubes for voltages impressed between said grids, and also being adapted to cause said voltages to be applied in sub stantial entirety upon the tube which receives a negative potential therefrom, whereby said 49 letter tube repeats the signaL.

14. Thermionic electric wave repeating apparatus for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements, comprising a pair of electron discharge de v1c es, each including anode, cathode and grld, connected in opposition to an output section, an input section comprising a pair of uncoupled high impedance elements serially connected between said grids with the point between said elements connected to said cathodes, wherebyin operation a signal wave applied between said grids is repeated at each 5 instant in substantial entirety by the said device which receives a negative potential therefrom.

15. Thermionic electric wave repeating apparatus for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements, comprising a pair of electron discharge de vices, each including anode, cathode and grid,

connected in opposition to an output section,

whereby said lat-1 ter tube repeats the signal.

and an input section comprisinga pair of uncoupled high impedance elements serially connected between said grids with the point, between "said elements connected to said cathodes, vwhereby, in operation, a signal wave applied between saidgrids isat each instant impressed on the said device which receives a negative potential-therefrom for causing said latterdevice to repeat sald Slgnal.

16. Thermionic electric wave repeating apparatus'for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements, comprising a pair of vacuum tubes each havin grid,;cathode and anode, a pair of electrically distinct high impedance elements serially connected between said anodes with the point between said impedance elements connected to said cathodes, means for supplymg space current to 'said tubes thru said high impedance elements respectively, a second pair of electrically distinct'high impedances serially connected between said gr ds with the point between said second pair of impedances connected to said cathodes, an

input section connected to said grids, said section connected only to said anodes whereby the repeated signal follows in series throu h saidoutput section and the anodecathode 'space paths of said tubes, so that. the low impedance space path of the tube having the momentarily positive grid is in series with the output section.

17. Thermionic electric Wave repeating apparatus for providing high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements. comprising a pair of vacuum tubes each having cathode, anode and grid, high resistance means connected between said grids tapped at an intermediate point to said cathodes, a pair of choke coils serially connected between said anodes, a space current supply source connected from said cathodes to a point between said choke coils, an input section conwhereby said latter tube alone repeats the signal and an output section connected nly to said anodes whereby the repeated signal flows in series through said output section and through the anode-cathode space paths "of said tubes, so that the low impedance space path of the tube having the momentarily positive grid is in series with said output section.

18. An electric wave repeating apparatus comprising a pair of vacuum tubes each having cathode, anode and grid, a pair of im pedance elements of large magnitude relative to the lowest gridfilament resistance of said tubes in operation serially connected between said grids, a connection from the point between said impedances to said cathodes, a second pair of high impedance elements serially connected between said anodes, a connection from the point between said second pair of impedances to said cathodes thru a space supply source, and input and output sections connected to said grids and to said anodes respectively, said impedance elements connected between said grids being adapted to cause a signal wave applied to said input section to be impressed in substantial entirety upon the tube which receives a negative potential therefrom, whereby said latter tube alone repeats the signal, and said repeated signal flows in series through said output sections and through the anode-cathode space paths of said tubes so that the low impedance space path of the tube having the momentarily positive grid is in series with said output section.

19. An electric wave repeating apparatus in accordance with claim 7 wherein the effective impedance of said output section is substantially four times the output impedance -of either said electron discharge device. 35

20. An electric wave repeating apparatus comprising a pair of electron discharge devices connected in opposition between input and output sections, each said device including anode, cathode and control eletrodes, an

input transformer having a secondary windmg connected between saidcontrol electrodes respectively and having a point thereof connected thru high impedance means to said cathodes, whereby said secondary winding is normally maintained at substantially the potential of said cathodes, saidhigh impedance means comprising a pair of noninductively related impedance elements, one of which is connected between one of said control electrodes and said cathodes and the other of which is connected between the other of said control electrodes and said cathodes, whereby a signal wavepresent in said secondary-winding is impressed at each instant in substantial entirety upon the device whose control electrode receives a negative potential therefrom, so that said latter device repeats the signal.

21. An electric wave repeating apparatus comprising a pair of electron discharge devices connected in'opposition between input and output sections, each .said device including anode, cathode and control electrodes, an

' input transformer having a secondary winding connected between said control electrodes respectively, and having a point thereof connected thru high resistance means to said cathodes, whereby in the absence of signaling waves said secondary winding is maintained substantially at the potential of said cathodes, said high resistance means comprising a resistance connected between the cathodes whereby in the absence of signal waves said input section is maintained substantially at the potential of said cathodes, said high impedance comprising non-inductively related impedance elements connected between the control electrode and the cathode of each discharge device, whereby a signal wave present m sa1d input section is 1mpressed in substantial entirety upon the device whose control electrode receives a negative potential therefrom, whereby said latter device is effective to repeat the signal.

23. An electric wave repeating apparatus comprising a pair of electron discharge devices connected in opposition to an output section, each said device including anode, cathode and control electrodes, an input section connected between said control electrodes and having the opposite terminals thereof con-' nected thru high impedance means and a current indicating device in. series to said cathodeswherebyintheabsence of signaling waves said input section is maintained substantially at the potental of said cathodes, and whereby the direct grid current indication upon said device is proportional to the signal wave intensity at all times, said high impedance means comprising a pair of non-inductively related impedance elements one of which is connected between the control electrode and cathode of each electron discharge device,

whereby a signal wave applied at-said input sectionds repeated at each instant in substantial entirety by the device which receives a negative potential therefrom..

24. In an electric wave repeating apparatus the combination of a pair of vacuum tubes each including cathode and grid electrodes, an input transformer having a'secondary winding connected'between said grids respectively, and a high impedance connecting a point on said secondary winding with said cathodes, whereby-in the absence of sig- I tube whose grid receives a negative potential therefrom.

25. In an electric wave repeating apparatus for providing a high degree of substantially distortionless amplification in the-absence of a negative biasing potential in the controlcircuits of said apparatus the combination of a pair of vacuum tubes each including cathode and grid electrodes,-an input transformer having a secondary Winding connected between said grids respectively, and high resistance means connecting the opposite terminals of said secondary winding with said cathodes, whereby in the absence of signaling waves said secondary winding is maintained substantially at the potential of said cathodes and during reception of signaling waves of relatively large am litude excessive current is prevented from owing in the grid circuits of said tubes, said high resistance means being adapted to cause a signal wave applied to said input transformer to be repeated at each instant in substantial entirety by the tube whose grid receives anegative potential therefrom.

26. An electric wave repeating apparatus for providing a high degree of substantially distortionless amplificatlon in the absence of a negative biasing potential in the control circuits of said apparatus comprising a pair of vacuum tubes each having grid, cathode and anode, an output transformer having a. primary winding connected between said anodes and having a oint thereon connected to said cathodes thru igh impedance and space current supply means in series, and an input transformer having a secondary winding connected between said grids respectively, and having a point thereon connected to said cathodes thru high impedance means, where-.

by in the absence of signaling waves said secondary winding is maintalned substan tially at the potential of said cathodes, said high impedance means being adapted to cause a signal wave applied to said input trans-,

former in substantial entirety by the tube whose grid momentarily receives a negative potential therefrom.

27.-An electric wave repeating apparatus comprising a pair of vacuum tubes each having grid, cathode and anode, an output transformer having a primary winding connected between said anodes respectively, and hav-' ing a point thereon connected to said cathodes thru high resistance and a space current supply means in series, and an input transformer having a secondary winding connected between said grids respectively, and having a point thereon connected to said cathodes thru high resistance means, whereby in the absence of signaling waves said sec ondary winding is maintained substantially atthe potential of said cathodes, said high resistance means being adapted to cause a signal wave applied to said input transformer to be repeated in said output transformer at each instant in substantial entirety by the tube whose grid receives a negative potential therefrom.

28. An electric wave repeating apparatus comprising a pair of vacuum tubes each having grid, cathode and anode, an output section connected between said anodes and having a point thereon connected to said cathodes thru high impedance and a space current supply means in series, and an input section connected between said grids respectively, and having a point thereon connected to said cathodes thru high impedance means, whereby in the absence of signaling Waves said input section is maintained substantially at the potential of said cathodes and whereby during signal reception the amplified signaling current is caused to flow in series thru the anode circuits of both said tubes and said output section, said high impedance means being adapted to cause a signal wave applied to said input section to be repeated at each instant in substantial entirety by the tube whose grid receives a negative potential therefrom.

29. An electric wave repeating apparatus in accordance with claim 28 wherein said output section has an efiective impedance of substantially four times the anode circuit impedance of either said tube.

"30. An electric wave repeating apparatus in accordance with claim 2 wherein said divided output section comprises a pair of serially connected coils having associated apparatus in accordance with claim 2 w ereln said divided output section comprises a pair of serially connected. coils having associated therewith a movable armature so arranged that opposite current changes in said coils produce additive actuating forces upon said armature, said coils having substantially zero magnetic coupling, and each coil having an effective impedance which is several times the output impedance of the electron discharge device to which it is connected.

32. A distortionless thermionic amplifier comprising a air of vacuum tubes each havmg anode, cat ode and grid, an input transformer having a secondary winding con-' nected between said grids respectively, a pair of high resistances serially connected between points of said secondary winding and between said grids the point between said resistances being connected to said cathodes whereb a signal wave present in said input tran ormer is impressed in substantial entirety upon the tube whose grid is 10 rendered positive by said signal wave whereby said latter tube is efiective to repeat the signal, and a sensitive direct current meter in series in the connection between said resistances and said cathodes indicating the 15 signal intensity, a space current supply source having its negative terminal connected to said cathodes and a loudspeaker having a pair of substantially uncoupled coils connected between the anode of said tubes rem spectively and the positive terminal of said space current supply source and an armature magnetically related to said coils and so arranged that opposite current changes in said coils produce additive actuating forces upon said armature, whereby the repeated signal moves said armature in accordance with signals in said input transformer regardless of which tube is repeating the signal m In testimonfiwhereof I aflix m signature.

AROLD A' W EELER.

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