US1909153A - Thermionic amplifier - Google Patents

Description

.May 16, 1933. G. sMlLl-:Y 1,909,153

THERMIONIC AMPLIFIER Filed July 27, 1951 2 Sheets-Sheet 1 NvENToR ATTORNEY May 16, 1933. G. SMILEY 1,909,153

THERMIONIC AMPLIFIER Filed July 27, 1931 2 Sheets-Sheet 2 INVENTOR Wy/fm ATTORNEY Patented May 16, 1933 I AUNi'ran .STATES GiLBEnT SMILEY, or BRIGHTOEMASSACUSETTS, AssIGNoB, BY MESNE assicmmivrs, .fro s. ii. Covert oolrrm, :Neger QUINcY, MASSACHUSETTS.' A CORPORATION or MASSACHUSETTS 4Animation niediiy'a'z,

, The present 'invention relates to an' ap paratus utilizing thermionic devices or vacuum. tubes for amplifying rfeeble electric currents, electric waves, or signals;

' An object of my invention is to provide a thclfinionic electric Wave amplifier'which is lsimple in construction and' loW'in costiof manufacture, and which isL very eiiicient in affording amplification of the signals `without the introduction of noticeable distortion or other disturbances resulting from the action of the tubes per se inpassing signals,

or asa resultof the action imparted to `them by the chai-acterof the source of current supply.

Anothei'object of myinvention is yto provide an apparatus of this characterwhicli may ybe operated from a local source of* al'- ternating current such as house lighting or public service circuit mains,` Without requiring the use of additional apparatus suchas rectifying devices, and the like, which have been heretofore deemed necessary'for main'- taining steady potentials at the electrodes of the vacuum tubes. v

A more specific ob'ect of the .present invention is to providle a novel 'circuit ar- -i'angeinent for vacuum tubes, coupled into a common output,y utilizing both the ampliiying and rectifying Aproperties of such tubes and possessing certain advantages all of which Will be hereinafter described.

Other objects of the invention will become apparent from the following descrip'- tion taken `in connection with'the accompanying drawings invvhich Fig. l 1s al connecting the tubes in airs.

I vhaveillustratedin Y ig. 1 a simplified embodiment of .my invention which comprises our therinionic devices which for the purpose of illustration are shown. as three element vacuum tubes but may be any 'rimmnoivic mamma 1931'. serial No. 553,305.

conventional or commercial form of tube. The four tubesshown in the several figures are indicatedv bythe numerals 1,2, 3 and 4. These tubes are as nearly as practical identical V,to one another and, as will be under'- lstood, provide an amplication of an incoming signal delivered at the input lterminals a and V'received eithery from a stage or stages of preamplifcatiton, or directly from a. 4signal voltage source such as a pick up, demodulator, microphone transmitter, photo-electric cell or the like.` -The respectivek grids 5 and'f ofthetubes 1 and 2fare connected vtoygether by a conductor 10, which is joined to one end 11 of a secondary vWinding or input portion 12 of an input coupling means 14. The respectivegrids l7 and 8 of the tubes 3-and-' 4areconnected together by aconductor 15 tofthe other terminal 16 of said secondaryr winding 12. The input coupling Vmay also comprise the primary winding 17. The 'secondary Winding 12 has an intermediate oricenter tap which is preferably'connected by Lak conductor 19 to the pointof grid Vbias potential for the tubes and to an optional ground 20. While a transformer has been illustrated it Will be understood that itisk merely representative of a means for supplying signal voltage in phase oppositionito the grids'of the separate pairs of tubes. A

PATENT' .oFFicET The respective filaments 21, 22, 23 and 24 Y ductors 25 and 26 from a secondaryl Winding or current-supply portion 27 of a trans-V former or power-'supply device'29, the pri'- mary Winding30 of which isconnectled to the terminals of a sourcev ofalternating current. `TheY'mid-point of saidsecondary AWinding 27 is connected bya conductor 32 to a grid-bias resistance 34, and the latter is connected by a conductor 35 to conductor 19.`

The respectiveplates 36v and 38 of the tubes 1 and 3 are connected by conductors 41 and43 tothe end terminals of an out ut circuit "which may be afwinding 46 o a transformer or output coupling `means 47. Likewise the respective'plates 37 and 39 of supply output portion of the power-sup-4 ply device 29. It will thus be seen that potentials derived from the alternating current source terminals 31 are therefore applied to the plates o6 and 38 through the intermediate terminal of the primary winding46 and to the plates 37 and 39 through the intermediate terminal of the primary winding 49.

The secondary winding 55 supplying the plate current for the several tubes is centertapped, and the output or plate circuits of the tubes l, 2, 3 and 4 are completed connecting the mid-point or intermediate terminal of the secondary winding 55 through a conductor 5G to the conductor 19. In said conductor 5G I place an inductance or current-steadying means 57 Which together with the grid-bias resistance 34 is common to all of said plate circuits. As Will be understood and further explained hereinafter, the i gridbias voltage supplied to thel grids of the tubes is substantially constant. The effect of the pulsating voltage supplied Ato the plates of the four tubes causes a fluctuating change in the impedance and space current of the individual tubes. Supply-voltage fluctuations at the plates 36 and 38 of t e tubes l and 3 are at any instant equal and opposite in direction to the supply-voltage fluctuations at the plates 37 and 39 of the tubes 2 and 4. Thus the sum of the normal space `currents in the case of tubes l and'2 is at any instant substantially constant, and the sum of the normal space currents in the case of tubes 3 and 4 is also at any instant substantially constant. It will also be seen that fluctuations in the individual normal space currents are not transmitted to the circuit of the output winding 50, i. e., in the case of tubes l and 3 the normal space currents in the portions 46a and 4Gb of the primary winding 46 are opposite and are therefore not transmitted, and in the caseof ltubes 2 and 4 the normal space currents in the portions 49a and 496 ofthe primary winding 49 are opposite and are therefore not transmitted. Since the sum of the normal space currents for each of the pairs of tubes (l and 2) and and 4,)l is substantially. constant, the effective plate impedance of each of said pairs of tubes, the grids of each of which are' connected in parallel, is

substantially constant, except as varied by the signal voltage impressed on said grids. Hence, each of said pairs of tubes function to properly transmit a signal therethrough Without undue distortion.

From the above description it becomes apparent that a signal proceeding through the input transformer 14 will produce equal and opposite potential 'changes in the grids 5 and 7 of the tubes 1 and 3 and hence equal and eo-directional changes inthe ampere turns of the -two portions 46a and 4Gb of the primary winding 46 to which the respective plates of said tubes l and 3 are connected. Also, the signal will produce equal and opposite potential changes in the grids G and 8 of the tubes 2 and 4 and hence co-directional changes in tbe ampere-turns of the two portions 49a and 496 ofthe primary winding 49 to. which said tubes 2 and 4 are connected. Thus the'tubes (l and 3) and (2 and 4) function simultaneously in a manner to assure balanced amplification. rl`wo tubes,

i. e. one tube of one pair (l and and one tube of the other pair (2 and 4), are always functioning, and at times, when the voltage in winding 55 is nodal, all the tubes are functioning. Stated otherwise, the tubes l and 3 may at a given instance be operating 1 to transmit a signal and are assisted by the tubes 2 and 4. As tubes 2 and 4 take up the signal, tubes l and 3 gradually cease to function.

'Ihe novel circuit arrangement of the four tubes provides the effect of two amplifying stages of balanced amplification placed in parallel, the signal being repeated from both primary windings 46 and 49 into the secondary winding 50 and into the load (not' shown) attached to the output terminals `The four tubes may each be operated approximately full rated average plate current, allowing only for 4forni-factor (the ratio of heating to average current) to obtain the same effective heating current as when similar tubes are operated in a conventional manner on steady supply-potentials. Thus, the power output of the tour tubes so ein- Aployed closely a} proXimates the power output which would be available from four similar tubes employed in a balanced or symmetrical amplifier, sometimes referred to conventionally as a push-pull amplifier, having in each side two tubes connected in parallel.

In Fig. l, the arrows shown in full lines indicate the path of the amplified output alternating signal current derived from 'tubes l and 3 at an instant when all of the tubes are operating. The arrows shown in dotted lines are an indication of the path at the same instant of the amplified output alternat-ing signal current derived from tubes 2 and 4. These two output currents are additive in `effect when passing through the "iii or distortion currents as may occur.

v'primary windings 46 and- 49. Thus, the sum of the signal currents in the primaries 46 and 49 iat any instant isZ the true amplified signal, which `1s 7transmitted vto 'theout ut windingO. "Since each of the pairs of tu s (l and 3) and (2 and 4) are arranged in a balanced or push-pull amplifying Circuit, the resultant effect `produced by the -four tubes will be to smooth out any irregularities that may occur due either to the amplification of the signal inthe tubes per se, or to the operation ofy said tubes by'reason of the-plate and filament vcurrent being derived vfrom a source of alternating current." It is noted that thesealternatinfr output currents or Vtheir sum do not flow throughthat portion of .the plate circuits common to all four tubes (including the inductance 57 and the grid-biasjresistance 34), said common plate' circuit' portion being traversed :nolr- `mally onl by such unbalanced, out of hase is 1s afeature-of balancing'the amplification from a plurality of tubes whichI am able toobv tain and is an vadvantage in that the inductance 57 maybe made high with no detrimental effects and because, by reason of the introduction of ahi h inductance, undesirable currents may e materially reduced. Said circuit portion ycommon to all four plate circuits is also traversed by the normal space currents of the four tubes, and it is to `be particularly noted that, since' the sum of the normal space currents of tubes 1l and`3 and thev sum of the normal space currents of tubes 2 and 4 are substantially constant at any instant, the sum of the normal space currents of all four'tubes in said common plate circuit portion is at any instant substantially constant. Therefore, observing that the inductance 57 functionsto revent any tendency toward pulsationonv uctuations of current therethrough, the current in said common. plate circuitportion which includes the grld-bias resistance 34 `is maintained substantiallysteady and the grid-bias potential impressed on the grids ofthe four tubes, or the supply-potential applied thereto relatively to the potentials of the cathodes of the f tubes, is maintained substantially constant.

It will be observed that I "obtain a balanced amplification in an am lifier by utilizing' the general scheme o connection of tubes l, 2, 3- and 4. If theprimary windings 46 and 49 be accurately center-tapped and the tubes on each of said primary windings be accurately `matched, there is a complete cancellation of such portions vof the normal space currents as would otherwise niagnetize the core of the output transformer 47. This lnay be understood by observing that the same supply-voltage value is applied at any instant to the plates of the tubes 1 and 3'and that likewlse thesame grid-voltage r'signal voltage. 'inherent in the 'operation of vacuum tubes supply-voltage value is applied at any instant to the plates of tubes2 and 4.l Therefore, the normal space currents in the plate` circuits of tubes 1 and 3 are at any instant 1 equal and opposite in direction when flowing, respectively, through portions 46a and 4Gb of the primary winding F46, and their magnetizing effect on the core of the output transformer `47 is neutralized. Likewise, the

normal space currents in the plateV circuits '5 of tubes 2 and 4 are at any-instant'equal and opposite in direction when flowing, re.- spectively, through portions 49a and 496.013 the j primary Winding 49, and their magnetizingeect onthe core of the output transance 34, the best results inV the amplification 7 of the received'signal is obtained. Since tubesl and 2.operate on one portion kof the plate-currenty characteristic curve -j(common to ally four tubes) onl one side-ofthe point `on said` curve determined by thegrid-bias potential, While tubes 3 and 4 operate at -the sameV instant on the other portion of that curve on the other side of saidpoint the apparatus,lwhen`employed as yan amplifier, will by reason of saidfour tubes substantially eliminate the distortion arising through the action of the tubes and taking the formof even yharmonics of the This form of distortion is used singly.

The vcircuit arrangements :illustrated in Figs.f2, 3 and 4 are modiications fof the apparatus shown in Fig. 1,*and 'manyV of their parts are similar'or identical to parts" of the apparatus which has been described. Forfthe vsake of brevity, the similar parts shown in these figures will be designated by correspondin numerals, primed in Fig. 2, double prime in Fig. 3, and provided with the suix a in Fig. 4.

' In the embodiment of the invention shown inFig. 2, 'the resistance u of the inductance common toV the plate circuits of all four 'fino tubes is utilized to obtain the grid-bias po-'-2 tential to be applied to their respective grids. Thus an inductance 60 is connected at-one end bythe conductor to the center-tap -of lthe secondary winding'o the power ysupply transformer 29. The center-tap 0513-3 the secondary winding 27 serving to supply current to theiilaments of the tubes, is 'connected directly by a conductor 61 to the ground 20. The center-tap of the secondary winding l2 of the input transformer 14 `is connected by aconductor 62 to one end of al resist-ance 64, the other end of the latter being connected by a conductor 65 to the conductor 56. A capacity 66 is connected at one end to the ground 20 and at the other end to the conductor 62. Owing to the inherent resistance of the indu'ctance unit 60 there is a D. C. potential across this inductance, which potential, if the resistance is high enough, may be utilized for grid bias purposes. This voltage exists vin addition to the high pulsating voltage across the inductance 60. vThe resistance 64 and the capacity 66 act as a resistance-capacity filter which reduces the effect of the high pulsating voltage across the inductance 60 before the grid-bias potential is applied to the grids. It is evident that the inductance 60 may be provided, as indicated by dotted lines, with various taps7 so that the gridbias voltage may be adjusted without altering the current steadying effect kof the inductance.

The embodiment of the invention shown-in Fig. 3 is particularly advantageous in that it permits adjustment ofthe grid-bias voltage in the two sides of the apparatus in which the two pairs of tubes are respectively connected. A power supply transformer 67 is provided with a primary winding 69 connected to the terminals 7() of a source of alternating current, and with a secondary winding or voltage-output portion, the end terminals of which are connected to the conductors 52 and 54 leading tothe respective mid-points of theprimary windings 46l and 49 and the intermediate terminal of which is connected to the conductor 56"-l leading to the ind'uctance 57 The transformer 67 is also provided with two addit-ional secondary windings or separate current-output portions 72 and 74. One winding 72 serves to supply current through the conductors 75 and Y76 to the filaments 21 and 22" of the tubes lfand 2, and ,the other winding 74 serves to supply current through the conductors 77 and 79 to the filaments 23 and 24 of the tubes 3 and 4.. The mid-point of the winding 72 is connected by a conductor 8() to one end of a resistance 81 the other end of which' is connected to one vend of a voltage divider or potentiometer 82, and the mid-point of the winding 74 is connected by a conductor 84 to one end of a resistance 85 the other end of which is connected to the other end of the potentiometer7 82. The intermediate or adjustable terminal 86 ofthe potentiolneter 82 is in electrical connection through a conductor 87 with the grid return connection of the several tubes. Thus the resistance 81 and that portion of the potentiometer 82`located between its lower end and theadjustable contact 86 constitute a gridbias resistance which permits the proper grid bias potential to be applied to the grids of tubes 1" and 2, and the resistance 85 and that portion of the potentiometer 82 located between its upper end and the adjustable Contact 86 constitutes a separate grid-bias potential to be applied to the er grid-bias potential to be applied to the tubes 3 and 4". A variation of the adjustable contact 86 permits the grid-bias potential on the grids of the tubes of one of said pairs of tubes to be decreased while that on the grids of the tubes of the other of said pairs of tubes is increased. This adjustment provides a means for correcting for such differences as occur in the characteristics of commercially available vacuum tubes, which frequently vary as much as twenty per cent, and it has been found to be of particular value in the reduction of hum in the load attached to the output terminals 51, such for instance as a loud-speaker.

In the description of the circuits of Figs. l, 2 and 3 the arranging of the tubes in pairs has for convenience been explained by reference to the manner in which the plates and the two output circuits are connected. That is, one terminal of each of the output coils is connected to the plates of the first tube of each pair and the other terminals of said coils are attached to the plates of the other remaining tubes. This arrangement calls for a crossing of two of the plate conductors and the tubes will bc paired in the order heretofore referred to. The same result will be accomplished by the arrangement depicted in Fig. 4 Vwhich shows two of the grid conductors 10a and 15ay crossed. Hence, using the previous nomenclature and following the order of the tubesv shown in the former figures (and reading from the top of Fig. 3 downwardly), the disposition of the tubes in pairs is (1 and and (2 and 4) and, as previously described, the separate output circuits are each connected at one terminal to the anodes of one pair of tubes and at their other terminals to the anodes of the vtwo remaining tubes. Therefore, whichever atefthe use of some of the filtering devices which have heretofore been consideredfessential for maintaining steady potentialsat electrodes of thermionic devices. ,l v I The rform of amplifier I have devised may be constructed in units, each embracing four tubes, and may be used separately, or as va final stage of power amplificatiomfor as an intermediate stage in a radio receiving set, orL as a telephone repeater.

While I have illustrated the inventionby using standard three element thermionic devices, it is to be understood that otherforms of vacuum tubes. may be employed possessing a larger number of elements with appropriatecircuit connections. The term three element thermionic device as employedherein is therefore not to be considered as a limitation, butas descriptive of any vacuum tube structure embodying the necessary input and output electrodes vand means forfcreating an electron stream, whether or not aplurality of one .or more ofy such elements is included in the assembly of a singleV envelope or bulb.

I claim: l 1. VIn an amplifier, the combination with two pairs of thermionic ,devices each comprising a cathode, a grid and an anode, and an input circuit connected to the grids of each pair of tubes, of two output; circuits each connected at one sideto the anodeof a v tube of 011e pair and at its other side to the anode of a tube of the other pair,V a heating circuit for the cathodes of allthe tubes, return conductors for the input circuit common to the two pairs of tubes and return conductors for the two output circuits, vsaid conductors being arranged 1n electrical communication with each otherY and vwith the filament, heating circuit.

2. In an amplifier, the combination withV from said heating circuit to theinput circuit andftothey output circuits, and means for supplying a source' of potential to said anode circuitsj f s 3. Inanamplifier, .the combination with two pairsof thermionic vacuum tubes each `comprising a filament, a grid and a plate, and, an` input circuit arrangedv tox'feed the signal waves in phase: opposition to each pair of tubes, of two output circuits eachA comprising. two terminals,-loneof each of whichis connected to a plate'of one of the lpairs of tubes, each, of the otherl terminals being connected to the plate of the remainf two pairs ,offthermionic vacuum tubes each` comprising a filament, a' grid and a plate, and an input circuit arranged to feed the signal wavesy in phase opposition to each pair of tubes, ofl two koutput circuits each comprising two terminals, one of each `of which is connected'to a. plate of one of the pairs of` tubes,l each of the other terminals being connected tothe `plate ofthe remaining tube of the other pair, a source of alternating current including a transformer arranged` `to supply current to heat the filaments of the several tubes and'potential for the late circuits, circuit` connections from the laments to the input circuit, and `circuit connections `fromthe filamentsto the output circuits including current steadying im.- pedance means and grid bias resistance means. y

5. In lan amplifier, the combination with an input circuit, two `output circuits, and two pairs of thermionic vacuumtubes each comprising a grid, a filament and a plate,

the two grids of `each pair of tubes being connected to thek opposite terminals of the input circuit, the two plates of one of said pair V.of tubes being connected to a terminal of each output circuit and the two remain-fv ing plates being connected respectively to the remaining lterminal of each output circuit, of acircuit connection between the midpoint of the input and the midpoints of each Vof said outputs including an impedance, a

circuit for heatingthe laments of the several tubes, a connection between said last mentioned circuit and said inputincluding aresistance, and a source of plate potential for the several tubes. v

6. A signal transmitting apparatus, comprising two pairs/of `threefelementthermionic vacuum tubes, Vmeans-for 5 impressing signal'potentia'l changes in thejgrids of one 'of said pairs and for simultaneously im-f pressing yopposite signal potential changesV in the grids of the other of saidv pairs of tubes, an output circuit lconnected. to the vplates of the first tubes of each pair of tubes, vanother output circuit connected to the plates of the remainingtubes, and a source of current supplying plate currentv potential and cathode heating` current for all of said tubes.- y Y 7.- In a signal rtransmitting apparatus, two,

pairs of thermionic vacuum tubes, means for impressing signal potential changes in the grids of one `of said pairs oftubes and for simultaneously impressing opposite signal potential changes in thegrids of the other f current potential and cathode heating current for all of said tubes, connections for completing the plate circuits of all of said tubes, and a current steadying means located in said plate circuit connections.

8. In a signal transmitting apparatus, two pairs of three element thermionic vacuum Y tubes, means for impressing signal potential f' changes in the grids rof the tubes of one of said pairs and for simultaneously impressing opposite signal potential changes in the grids of the tubes of the other pair, an output circuit connected to the plates of the first tubes of each pair of tubes, another outputv circuitV connected to the plates of the remaining tubes, center taps onY said output circuits, and a source of alternating current, a transformer therefor, connections from the latter to said center taps for supplying plate current potential to` said tubes, and means for heating the cathodes of the tubes.

9. In a signal transmitting apparatus, two pairs of three element thermionic vacuum tubes, means for impressing signal potential changes in the grids ofthe tubesof" one of said pairs and for simultaneously impressingl opposite signal potential changes in the grids of the tubes of the other pair, an output circuit connected to the plates of the first tubes of each pair of tubes, another output circuit connected to the plates of the remaining tubes, center taps on said output circuits, a source of alternating current, a transformer therefor having two secondary windings, connections from one of said windings to the aforementioned center taps for supplying plate current otentiai to the severall tubes, (ponnections rom the other seconc ary win in@ su y l vin heatinor current for the cathidespfysaii tubesleads from intermediate points of the two secondary windings to the grid circuits, and a resistance and an inductance included in said leads. Y p

10. In a signal transmitting apparatus, two pairs of three element thermionic vacuum tubes, means for impressing signal potential ychanges in the grids of the tubes of one of said pairs and for simultaneously impressing opposite signal potential changes in the grids of the other of said pairs, an output circuit winding provided with an i11- termediate tap and having its end terminals connected to the plates ofV the first tubes of each pair of tubes, a second similar output circuit havingl its end terminals connectedA to the plates of the remaining tubes, a fluctuating source of current supply feeding plate current potential to the intermediate taps of said two output circuits, connections from said current supply to the filaments of said tubes, and a current steadying means in said connections.

11. In a signal transmitting apparatus,

two pairs of three element thermionic vacu um tubes, means for impressing signal potential changes in the grids of the tubes ofy one of said pairs and for simultaneously impressing opposite signal potential changes in the grids of the other of said pairs, an output circuit winding provided with an intermediate tap and having its vend' terminals connected to the plates of the first tubes of each pair of tubes, a second similar output circuit having its end terminals connected to the plates of the remaining tubes, a iiuctuating source of current, supply feeding heating current to the cathodes of the tubes and plate current potential to the intermediate taps `of said output circuits, and connections from said current supply to the grid circuits of said tubes comprising an adjustable grid bias resistance.

12. In a signal transmitting apparatus, two pairs of three element thermionic vacuum tubes, means for impressing signal potential changes in the grids of the tubes of one of said pairs and for' simultaneously impressing opposite signal potential changes in the grids of the other of said pairs, an output circuit Winding provided with an intermediate tap and having its end terminals connected to the plates ofA the first tubes of each pair of tubes, a second similar output circuit having its end terminals connected to the plates of the remaining tubes, a fluctuating source ofA current supply feeding plate current potential' to the intermediate taps of said two output circuits, said source of supply comprising two windings the terminals of which are connected to the catliodes of said pairs of tubes respectively, connections from intermediate points on said4 windings tothe grids of all the tubes, and an adjustable grid bias means in said connections. Y

13. A thermionic amplifier and full wave rectifier comprising an input transformer comprising a secondary winding having an intermediate tap, two pairs of three element thermionic vacuum tubes,y the grids of each of said pairs being connected to the end terminals of said secondary, anoutput.

transformer comprising two primary windings, one of which has its terminals respec- Vtively connected to the plate of one tube of one pair and to the plate of one tube of the other pair and the other ofA which has its terminals connected respectively to the plates of the remaining tubes, a `source of alternating current supply, a power transformer therefor having two secondary windings, connections from one of said windings.

to intermediate points on the output primary windings for supplying plate current potential to the tubes, connections from the other secondary winding to the filaments, leads from the two secondaries ofthe power transf, former to the intermediate tap ofthe input secondary winding, a grid bias resistance in one of said leads, and a current steadying impedance in the other of said leads.

14. A thermionic amplifier and full wave rectifier comprising two pairs of three element thermionic vacuum tubes, means for supplying signal voltage in phase opposition to the grids of each of said pairs, an output transformer comprising two primary windings, one of which has its terminals respectively connected to the plate of one tube of one pair. and to the plate of one tube of the other pair and the other of which has its terminals connected respectively to the plates of the remaining tubes, a source of alternating current supply, a power transformer therefor having three secondary windings each provided with an intermediate tap, connections from one of said windings to the intermediate tap points on the two output primary windings for supplying plate current potential to the tubes, separate connections from each of the other two windings to the cathodes of the respective pairs of tubes for supplying heating current to said cathodes, conductors joining the intermediate taps of the two heating current seconda-ries to the signal supply means, grid biasing resistors in said conductors, and a circuit connection from the intermediate tap of the plate current potential secondary to a common point of said signal supply means, said circuit connection being also joined to the aforementioned conductors at a point between said resistors.

In testimony whereof, I have signed this specification.

GILBERT SMILEY.

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