US1966910A

US1966910A - Triode apparatus

Info

Publication number: US1966910A
Application number: US355101A
Authority: US
Inventors: David E Sparks
Original assignee: Individual
Current assignee: Individual
Priority date: 1929-04-15
Filing date: 1929-04-15
Publication date: 1934-07-17
Anticipated expiration: 1951-07-17

Description

July 17, 1934. SPARKS 1,966,910

TRIODE APPARATUS Filed April 15. 1929 4 Sheets-Sheet l INPUT OUTPUT DYNHMIC SPEAKER /8000 Ohms J3 PAC f J5 @W M July 17, 1934.

D. E. SPARKS TRIODE APPARATUS- Filed April 15, 1929 4 Shee'bs-Shet 2 Zow/ aged/Fer T. T i

WA fizz/6T2??? flaz/zd ffparha,

Wdwww Juiy 17, 1934. D, E SPARKS 1,966,910

TRIODE APPARATUS Filed April 15, 1929 4 Sheets-Sheet 5 In Puf iftour' w1 July 17, 1934. D E P RK; 1,966,910

' TRIODE APPARATUS Filed April 15, 1929 4 Sheets-Sheet 4 3 fave/275 f. 1752003 1?. jam-la Patented July 17, 193 1- UN ITED L STAT ES ii F'FI QE 1.966.310 ram s APPARATUS David a. Sparks, Chicagoflll;

Application April 15, 1929, Serial-'No.'" 355,1(li

10 Claims.

' This invention relates to-triode apparatus and it'has particular-relation to direct-coupled triode-amplifiers, although some of the features of the invention are not limited thereto.

Amongthe-objects of the invention is the provision of a novel triodeapparatus, such as amplifiers, permitting efficient utilization of a plurality of direct-coupled triode stages with a minimum of current sources and simplified control.

The objects of the invention'will be best understood from the following description of exemplifications-thereof, reference being made to the accompanying drawings, wherein Fig. 1 is a diagrammatic view of a three-stage direct-coupled triode amplifier of the prior art; Fig. 2 is a diagrammatic'view ofan improved direct-coupled three-stage triode amplifier exemplifying some of the principles of my: invention;

Fig. 3 is a-view similar to Fig. 2, illustrating an improved modification of my invention as applied to a practical case;

Fig. 4 is a diagrammatic viewof an exempli-- fication of my invention as applied to a radio receiver;

Fig. 5 is a diagrammatic view ofan exemplifi cation of my invention as embodied in a radio receiver utilizing screen grid tubes in the amplifier stages;

Fig. 6 is a diagrammatic view of'an exemplification of my invention of'a still more improved form;

Fig. '7- is a view similar to Fig. 6, of a still further improved modification of my invention;

Fig. 8 is a diagrammatic view of a further modification of my invention embodying certain features of the exemplification shown in Fig. 3 and the exemplification shown in Fig. 6;

Fig. 9 is a diagrammatic view of a television receiver embodying the principle of the invention shown in Fig. 8.

My invention will be described in its application to direct-coupled amplifiers wherein it is particularly useful, but it will be understood that it-has advantages in other applications that will suggest themselves to those skilled in the art.

Direct-coupled amplifiers are principally used where it is desirable to exclude transformers or other reactive impedances from the circuit. Such is the case in circuitsin which the frequency is verylow so thatefiicient-transformers are costly-and difficult to design, as in telegraph circuits, submarine work, and the like. In such and similar applications' where undistorted amplificationjmust'be secured-over a wide range of frequencies, it is particularly desirable to exclude transformers because it is important" that the amplifier shall reproduce the exact wave form of the input.

Direct-coupled 'triode' amplifiers have-been used heretofore for suchapplicationsyresistors or Sim-"- -ilar impedance elements'whiclr-are not-affected -by-the variations of the frequency of-the-current the'input terminals of the triode, and the plate 7 with the cathode 5 'forni"the'outputterminals of thetriodef The cathode is shown in the-form of an incandescent filament that is suitably heated by means of the filament: supply source 8, al-

though any other form of' electrode=emitting -cathode maybe used. The terms cathode,

grid and plate as usedherein are-not intended to designate a-particular electrode shape but are used to 'designate theelectrodes performing the functions of the respective elements" of the triodes as used in practice.

The input-electrodes Band 6 of the first triode stage 1 are connected across the terminals Man input resistor Ri upon which there isimpressed the input voltage derived from an'input source, I. The output circuit of the first stage comprises a resistor R1 connected between the plate '7 and the positive terminal of a voltage source 9 andnegative terminal of which is connected to the cathode 5. The plate terminal'of theoutput resistor R1 of the first stage is connected to the grid 6 of the second stage thereby impressing the amplified output of the first stageon the input-terminals of the second stage; In a similar way, the-output voltage developed across the plate resistorR2 of the second stage is impressed-upon theinput terminals of the-third stage, 3. The output of-the third stage is shown by way of exemplification utilized in an output resistor R0' of the output device 0. The output resistors R2-and R0 of the second and third triode stages are each shown connected between the plateterminals of the respective triodes and the-positive terminal of the common-plate voltage source 9.

Amplifiers of the foregoing type'are commonly designated in the art as direct-coupled amplifiers because the plate of one stage isdireotly connected to the grid or'input'terminal of thenext stage, as distinguished from the indirect connections by means of transformers in the transformer-coupled amplifiers.

One of the complications and the troublesome difiiculties encountered in the practical use of direct-coupled amplifiers of the foregoing type resides in the necessity for introducing into the grid circuit of most or all of the higher amplifier stages, a compensating biasing voltage source in order to bring the voltage on the grid within the desirable operative range of the tube. In most of the tubes generally used in practice, the voltage drop produced in the plate circuit of the individual stages is not best suited for satisfactory operation of the grid of the next higher tube. In order to compensate for the incorrect value of the grid voltage, a compensating voltage source, in the form of a battery 16 for instance, has to be introduced in series with the grid 6 of most or all of the higher triode stages. Such additional voltage sources require maintenance and introduce very serious complications in the practical operation of such amplifiers. They present particular difiiculties where direct current for operating the amplifier stages is derived by rectifying alternating current, because of the complications and expenses involved in providing the special direct current sources for each of the grid input circuits.

My invention as shown in the exemplifications described hereinafter, avoids the foregoing difliculties of direct-coupled amplifiers. In accordance with my invention, the several amplifier stages of a cascade or chain are supplied from adjacent sections of a plurality of serially connected current sources, or in general, a battery of sources, the term battery as used herein not being limited to dry cells or storage batteries, but intending to designate broadly a combination of serially arranged elements from each of which it is possible to derive a current or voltage. The required biasing voltage for the grids of the several stages is obtained by suitably adjusting or overlapping the connections from the plate terminals of one stage and the cathode terminals of the next successive stage, without the complications of introducing special voltage sources in the grid circuits of the individual stages.

Such arrangement is shown in Fig. 2, as applied to a three-stage amplifier although the invention is of course applicable to amplifiers of any number of stages.

As shown in Fig. 2, three amplifier stages, 1 to 3, having triodes such as shown in Fig. 1, are connected in cascade to amplify the input received from an input device I and supply the amplified output to output device 0. The cathodes of the several tubes of Fig. 2 are shown to be heated by alternating current, the tubes, for instance, being of the familiar type now generally used in which alternating current heated filaments serve to amplify impulses without introducing any hum from the alternating current source. Each of the cathodes is shown supplied by the secondary winding 11 of a filament trans former 12, the primary windings of which are supplied from an alternating current supply line 13.

The plate circuits of the several outputs are supplied from a special high voltage battery 15 so arranged that the successive amplifier stages are supplied by battery sections of successively higher voltages. In the exemplification shown in Fig. 2, the battery 15 for the plate circuits of the several triode stages is made in the form of a potentiometer resistor through which direct current is sent in the direction of the arrow 16 from the positive terminal 17 to the negative terminal 18. The direct current for the potentiometer resistor 15 may be supplied from any suitable direct current source and in ordinary practice, it will usually be from an A, C. pack 19, comprising a rectifier unit 20 and a filter 21 of familiar construction.

The plate circuits of the tubes of the successive triode stages are supplied from consecutive sections of the battery or resistor 15. As shown in the drawings, the conductors leading to the several plate circuits have adjustable tap contacts permitting adjustment of the voltage supplied to the several terminals. The cathode of the first tube is connected to the resistor through a tap Cl which leads to the mid-point of the secondary transformer winding 11, energizing the filament constituting the cathode of the first triode. The plate of the first triode is connected to the resistor through a tap P1 leading from the plate resistor R1 of the first triode. The cathode and the plate of the second and third triodes are similarly connected to the potentiometer resistor 15 through taps C2, P2, C3, and P3, respectively. There may be also provided an additional tap contact G1 for controlling the bias of the grid of the first triode.

While the several stages, 1, 2, 3, of the amplifier are all supplied from a common battery 15, the successively higher stages are connected to sections of the battery of successively higher potentials. By varying the points of connection of the plate terminals P1, P2 and the cathode terminals C2, C3 of the adjacent triodes, I am able to apply to the grids of the several triodes any desired biasing potential without introducing any separate voltage sources in the grid circuits of the individual stages. Were taps P1 and C2 brought into direct contact, the potential of the grid of the second triode 2, would have a potential determined by the voltage drop produced by the current flowing through the plate resistor R1 of the first triode, this being equivalent to the condition of the arrangement shown in Fig. 1, if no grid biasing battery were provided in the circuit of the grid of the second tube. By moving the tap P1 to the right or to the left of the tap C2, the

biasing potential of the grid against the cathode may be varied in one or another direction by an amount corresponding to the voltage drop in the section of the battery 15 between the taps P1 and C2.

The cathode tap C1 of the first tube is connected to a point of the potentiometer resistor 15 near its negative end. The grid contact G1 of the first tube is suitably placed so as to give the required negative grid bias to the grid of the first tube. cuit of the first stage 1, produces across the coupling resistor R1 a voltage drop which, with the ordinary tube now on the market, causes to be impressed on the grid 6 of the next stage a negative potential in excess of the value required for the satisfactory operation of the second tube. In order to compensate for this excess negative voltage, the set-back tap P1 is so adjusted on the pi'itentiometer as to inject a positive voltage in The plate current flowing in the plate (-irthe circuit of the grid of the second tube so as to connected to points on the battery 15 having a ILL potential that is higher than the potential of the respective cathode taps, C2 and C3. In this way, each of the plate circuits is provided with a source of proper plate voltage and at the same time a suitable biasing voltage is applied to the grids of the several tubes without requiring additional voltage sources distinct from the plate voltage sources. I designate the taps P1, P2, by means of which the voltage of the grids of the several tubes is set back against the cathod volt age as set-back taps, and the voltages injected by means of said set-backs in the grid circuits as set-back voltages.

The tap G1 in the circuit of the grid of the first stage 1, enables ready adjustment of the biasing voltage of the first grid, the tap G1 being shown soconnected as to apply a negative biasing volt-- age to the first grid. As will be seen by comparing the arrangement shown in Fig. 1 with my improved arrangement shown in Fig. 2, the independent sources of grid voltage required in the prior art direct-coupled amplifiers are eliminated and instead, a straight connection between the plate of one tube and the grid of the next tube is provided without any batteries or sources in between. The plate circuits of all of the stages are energized from one battery, and at the same time, the grids of the several stages are biased so as to have the proper operating voltage required by the characteristics of the tubes.

In using a potentiometer resistor 15 as a battery for energizing the several amplifier stages, it is important that the several elements of the circuit be so designed that the currents taken by the plates of the several stages, particularly of the lower stages, 1,2, shall be negligible in comparison with the current flowing in the potentiometer resistor 15. In practice, I have found that satisfactory results are obtained with arrangement such as shown in Fig. 2 if the potentiometer resistor 15 is made or" such value that the current fiowing therethrough is about ten times as large as the plate current taken by any one tube in the cascade. It will be noted that the voltages derived from the tap sections of the potentiometer resistor 15 are applied to the individual tubes of the cascade consecutively from the negative end 18 to the positive end 17 in the same sequence as the tubes are arranged in the cascade. In order to efficiently use such arrangement, it desirable to use independent sources of filament or cathode energizing current for each tube in the cascade. This may be done in a simple and convenient manner inasmuch as the cathodes may be heated by alternating current, the tubes using either indirectly or directly A. C. heated cathodes such as are now on the market. The power consumption of the cathodes of is very small, and transformers for supplying the cathode heating current of such tubes from the commercial alternating lines are cheap, eflicient and very convenient to apply.

Where a three-stage amplifier is used, the tube of the third stage is usually a power tube requir ing an unusually high grid voltage. Where the voltage drop in the coupling resistor R2 of the second stage is not sufiicient to supply the required high negative bias for the grid of the third stage required grid potential may be obtained by pi cing the set-back tap P2 on a point of the potent neter resistor 15 to the left of the cathode tap C3, injecting a negative biasing voltage in the grid circuit of the third tube.

As seen in the drawings, the entire arrangement is very flexible and permits adjustment of the potentials applied to the various elements of the circuit in a way to secure the most efficient operation.

In the practical operation of the amplifiers of the type shown in Fig. 2, there are encountered certain limitations which tend to reduce the efficiency of the amplifying action. These limitations are of a two-fold nature. First, not all of the output of the amplifier is available for use in the output device 0 and in its impedance R0, but a part of it is consumed in the external resistance of the circuit of the battery 15; and second, a certain amount of the power of the higher stages is fed back in the wrong direction into the input of the lower stages cutting down amplification.

I have provided very effective means for overcomhig the foregoing limitations and for deriving the full benefit of the advantages resulting from the novel arrangement of the triodes in accordance with the principles illustrated in Fig. 2.

Let us consider the first limitation, namely, the reduction of the usefui output by the loss in the external impedance of the circuit outside the coupling impedance, or the impedance of the output device. If a periodical electro-motive force of small magnitude is impressed upon the input of he first amplifier stage, there will be available in the plate circuit of the final stage a periodical electro-motive force similar to that applied to the input, but of greatly increased magnitude. Assuming for simplification, a purel non-reactive circuit, the resistance of the plate circuit the last stage will be the sum of the output resistor R0 of the output device and of an additional resistance which we may designate here as external resistance R3 in series therewith. This additional resistance represents the impedance of the elements in the connection between the cathode of the last stage and the cathode terminal of the output resistor R0, and includes the resistance of the section of the potentiometer resistor 15 between the taps C3 and P3 and its parallel shunting circuit. According to my invention, the useful output is increased by reducing to a minimum the resistance or inipedance which is in series with the useful output impedance R0 in such way as to avoid interference with the operation of the amplifiers.

One way of reducing the impedance in series with the useful output impedance R0 is to make the potentiometer resistance 15 small. However, there is a serious limitation imposed in this direction because for practical reasons, it is necessary to keep down the current flow in the potentiometer resistor to a low value. This current is ordinarily supplied from C. packs with rectifier and filter systems, which become very expensive it the currents derived from the packs are materially increased. In the present stage of the art, 100 milliamperes is the practical limit the current that may be derived from A. C. packs without going to too much expense. If possible, the current demand should be held at a lower value.

In considering the second limitation, namely, that imposed by the feed-back of power to the lower stages it is important to differentiate between the odd and even tubes of the cascade. Let us consider an amplifier, such as shown in Fig. 2, having two odd amplifier stages 1 and 3, and one even stage, 2. The same considerations would apply to an amplifier cascade having more tubes, the

stages

1, 3, 5, being th odd stages rec complications.

and 2, 4, etc., being the even stages. If a positive voltage is impressed upon the grid of the first stage, this will cause an increase of the current fiowing through the associated plate circuit in the direction from the plate '7 to the cathode 5, the current through the plate circuit dividing at the tapping points 01 and P1, one part flowing over the resistor sections C1, and P1, and the other part flowing through the remaining part of the closed potentiometer circuit which forms a parallel path to the resistor sections Cl, and P1. The increase of the current flow in the plate circuit of the first stage impresses through the coupling resistor R1 a negative potential on the grid of the second stage, reducing the plate current of the second tube or in other words, producing a flow of current in the direction from the cathode to the plate, this current flowing through the resistor section 02, P2, and the parallel path of the closed potentiometer circuit, in the direction towards the terminal C2. The flow of plate current in the second amplifier stage increases the positive potential of the grid of the third stage, which in turn causes the plate current in the third stage to increase in the direction from the plate to the cathode, producing a current flow through the potentiometer resistor section C3, P3, and the parallel path through the closed circuit of the potentiometer circuit in the direction away from the tap C3.

The flow of the plate or output currents of the higher stages react back on the inputs of the lower stages inasmuch as a part of the current flows across the resistor section of the potentiorneter 15 which determines the potential impressed upon the grids of the individual tubes.

It is clear from the foregoing that the currents flowing in the plate circuits of the even stages are in opposite direction or of opposite phase to the currents flowing in the plate circuits of the odd amplifier stages. Where the plate current of the higher stages is flowing through a section of the potentiometer resistor 15 which determines the potential of the grid of the lower stage with respect to its associated cathode, the higher stage may feed back power into the grid of the lower stages either in the right or in the wrong direction depending on the position of the set-back tap and on whether both stages are odd, or one stage is odd and the other even. If the setback of the lower stage is set at a point on the potentiometer resistor having a lower voltage than the cathode tap, then the power feed-back from a higher stage of like character (that is. both even or both odd) will be out-of-phase, and the power feed-back from a higher stage of opposite character (one odd, the other even) will be in-phase. The opposite is the case where the set-back of the lower stage is at a point of the potentiometer resistor that has a higher voltage than the cathode tap.

According to my invention, the same means which is eifective to increase the useful part of the output power is also eifective to reduce the amount of feed-back from the higher stages to the lower stages. In practice, it is desirable to eliminate feed-back whether in-phase or out-ofphase because, while out-of-phase feed-back reduces the amplification, in-phase feed-back tends to produce self-oscillation and other undesirable This is particularly true because of the fact that there are no entirely non-reactive circuits so that as a mater of fact, there is a certain phase displacement even between the inphase feed-back and the input of the lower stages. Reduction of the feed-back is accord ingly of advantage both in case of in-phase as well as out-of-phase feed-back.

In the practical amplifiers such as shown in Fig. 2, the amount of feed-back from the second stage is relatively small so that it may be practically ignored and the only feed-back that gives concern is that obtained from the third or higher stages. Accordingly, in practical use, only the elimination of the feed-back from the third stage to the first stage is of importance. Likewise, the principal loss of power is that resulting from the external resistance in the output circuit of the power stage, that is, the third stage on the arrangement shown in Fig. 2, this power loss being relatively negligible in the lower amplifier stages.

In accordance with my invention, both the power loss by reason of the resistance in series with the useful output resistance and the undesirable feed-bacl to the lower stages are eliminated by completing the circuit between the cathode terminal of the output resistor of a given stage and its cathode by an element of such character as to supply the proper voltage to said elements while at the same time constituting a relatively low impedance for the changing current in the plate circuit.

As pointed out before, the series resistance between the cathode terminal and the output resistor R0 and the cathode must be made relatively high because of the necessity of limiting the current flow through the potentiometer resistor and the requirement that the flow of current to the resistor shall produce the voltage necessary for the plate. In accordance with my invention I use a special bridging element between the cathode terminal of the output resistor and the cathode, of such character as to leave substantially unaffected the operation of the potentiometer resistor as far as the direct current flow is concerned but changing its characteristics for the variable currents in such manner as to make the resultant resistance to variable currents negligible. An ideal device would be one that has a definite impedance to direct currents and zero or negligible impedance to alternating or changing currents. A device of such nature would eliminate the voltage drop for alternating currents in the part of the circuit which is in series with the output resistor, thus eliminating the power loss. Such a device would also eliminate the feed-back because substantially all of the alternating plate current would flow along the short-circuited path between the output resister and the cathode, substantially no current fiowing along the path through which feed-back could be effected.

In accordance with my invention I use as a low A. C. impedance bridging device, glow discharge tubes of the type used for the past few years as constant voltage regulators. These tubes generally consist of a hermetically sealed vessel filled with rare gas with a pair electrodes within the vessel. The spacing of the electrodes and the gas pressure are such that at a predetermined voltage a discharge occurs in the tube between the electrodes, the discharge voltage being practically constant and independent of the current within the operating range of the tube. The design of these tubes varies quite greatly, some of them being so arranged as to have a much higher discharge voltage in one direction than in the other by the use of electrodes of different areas, the tubes acting as rectifiers. In these asymmetrical tubes, the large-area electrode acts as a cathode and the small-area electrode acts as an anode permitting a glow discharge for current in the direction from the anode to the cathode but preventing current flow in. the opposite direction. In other words, such tube acts: as a very high impedance for the currents flowing in the direction from the cathode to the anode but its impedance is substantially negligible for any variations of current in the direction from. the anode to the cathode.

Tubes of the foregoing character have what: is called a negative resistance characteristic. By properly designing the tube it is possible to obtain current voltage characteristics such that the voltage drop across the tube is nearly constant for any variation of the current within a given range. This means that the effective resistance or impedance of the tube varies within the given range inversely as the current flow.

If such glow tube is placed in shunt to the external. or series resistance of the output circuit of a. triode it will leave substantially unafiected the direct current flow conditions through the circuit inasmuch as the glow discharge through I. the tube may be made very little, between 10 to 50 milliamperes. This tube will maintain a constant voltage D. C. drop across the terminals of the section of the potentiometer resistor 15 to which the cathode tap C3 and the anode tap P3 are connected. It will present however, a practically zero resistance or impedance to the current variations in the circuit inasmuch as these current variations will not at all affect the. voltage drop across the potentiometer sections 03,

- P3, because of the negative resistance characteristic of the tube.

A specific example of this type of tube suitable for-use in a triode amplifier of my invention is that known at present in the trade as UK-874.. This particular type of tube has a substantially constant voltage drop of volts over a range of discharge currents from 10 to 50: milliamperes, this constant voltage drop being substantially independent of the frequency of the currents over a frequency range from several hundred thousand cycles per second to one or two cycles a minute or even lower.

Such tubes may be made with various characteristics and by suitable choice of the electrodes and gas filling the constant voltage drop across the tube may be made to lie between the value of 90 volts of the UX-874 tube or several hundred volts. In the practical construction of amplifiers embodying my invention I use the commercially available Uri-874 tube and since I usually desire more than so volts across the plate of the output stage I usually employ twosuch tubes in series, having a constant'voltage drop of about 180 volts.

In Fig. 3, there is shown a practical embodiment of my invention as applied to a three-stage direct-coupled amplifier for amplifying sound record impulses. The triodes 34 used in this amplificr are shown to be of the indirectly heated cathode type familiar in the art. The arrange-- merit of the potentiometer circuit 15 for supplythe resistor. The voltage drop across the third stage triode is made to be about 180 volts.

In order to reduce feed-back and also the losses of the third amplifier stage, a pair of glow tubes 36 such as L X-874 tubes are connected in series c oss the section of the potentiometer resistor tween the taps C3, P3, from which current is lied to the plate circuit of the third stage. By leason of the characteristics of the glow tubes connect d across the external impedance of the e circuit of the power stage, substantially the re output will be delivered to the dynamic s a her or other output device in the plate circuit, and there will be substantially no feedback into the lower stages of the amplifier. I have found that in practice it is not necessary to provide such glow tubes across the external resistances of the lower stages of such amplifier because the coupling impedance in the lower stages is much higher than the impedance of the external circuit so that the loss in the external circuit of the lower stages and the feed back are so small that they do not, in most cases, require to be eliminated as. in the caseof the power stages of the amplifier. The principle of eliminating the loss and the feed-back by connecting acrossthe external resistance of the plate circuit a bridging element that would make the value of the external resistance substantially negligible for varying currents is of course appli- 1 cable to each of the stages of the, amplifier where such elimination may be of value.

In the drawings, I have shown two asymmetrical glow tubes 36 connected across the external impedance of the output circuit. Although such 1 tubes permit current fiow only in one direction, they are nevertheless efiective for current variations in both directions inasmuch as there is a certain continuous current flow through the tube, and current variations produced in the plate circuit merely produce an increase or decrease of the current through the tube above and below the normal value with the voltage across the tube constant all the time. Because of this fact, the desirable negative impedance characteristics are eiiectivefor current variations in both directions although the tubes are uni-directionally conducting. y

In the exemplification of the invention shown in Fig. 3, there is also provided an adjustable tap 37 across the input resistor R1 of the first stage, permitting a variation of the magnitude of the output, and an adjustable rheostat 38 in series with the potentiometer resistor 15 to permit variation and adjustment of the current flow in the resistor 15 and the glow tubes 36 bridging a portion thereof. In a practical amplifier according to Fig. 3 built by me, I used in the

first stages

1 and 2, indirectly heated cathode tubes known as type UY22'7 and for the third stage 1 I used a power tube known as type UX-l'll-A. For purposes of illustration I shall give another set of values for the various constants of a practical circuit with which'I obtained very satisfactory operation. Using the same type of tubes as in the amplifier described above, I employed a potentiometer resistor having a resistance of 990 ohms between G1 and C1; 1300 ohms between Cl and C2; 2400 ohms between 02- and C3; 16,000 ohms between C3 and P2; and 2,000

ohms between P2 and P3. The coupling resistors R1 and R2 of the first two stages had 100,000 ohms each. The direct current flow in the potentiometer circuit was adjusted to 50 milliamperes.

The use of the glow tubes for bridging the section of the potentiometer resistor which is applied to the power stage of the amplifier has, besides the prevention of iced-back and loss of power, the additional advantage of reducing the amount of direct current which has to be drawn from the A. C. power pack 19. When using the glow tubes in the way shown in Fig. 3, it is no longer necessary to keep the value of the potentiometer resistor so small compared to the coupling resistors as in the case where no glow tube shunt is used, but much higher potentiometer resistors may be used. The magnitude of the current which it is necessary to circulate from the power pack depends on the range over which the glow tube will operate with the desired characteristics and on the current requirements of the power tubes of the third stage. When using an arrangement employing a UK- l'll-A power tube and the UX-B'I glow tube, I have found it convenient to design the amplifier so that it takes about milliamperes from the power pack although satisfactory operation can be obtained with a still smaller drain from the pack.

In Fig. 4, an amplifier of the type described. in Fig. 3, is shown applied to a radio receiver. The radio signals are received from a radio frequency amplifier 41 and amplified at radio frequency and the amplified radio frequency signals are applied to a detector tube 42 which constitutes the first stage of a thre6-Stage amplifier 43 of the type shown in Fig. The detector tube 42 is shown to be of the indirectly-heated cath- T ode type while the triodes of the second and third audio-frequency stages are of the directly-heated cathode type. The circuit connections of the various elements are otherwise the same as in the amplifier shown in Fig. 3.

In Fig. 5, there is shown a radio receiver with a direct-coupled audio-frequency amplifier 51 similar in general arrangement to that shown in Fig. 4, except that screen grid tubes 52 are used in the first and second stages of the audio-irequency amplifier.

The screen grid tubes of this amplifier comprise each a cathode 55, a control grid 56 and a plate 57, and an additional screen grid 58. The control grid 56 of each tube is connected to the '1 input conductor while the screen grids are connected to special taps S1, S2, on the potentiometer resistor 15 for applying to the screen grid a proper screening potential. The arrangement is otherwise like that in the amplifier shown in Fig. 3.

In the various figures exemplifying my invention, I have indicated the connections of the terminals of the plate circuits of the individual amplifier triodes as being of the adjustable tap type to permit adjustment of the voltages and potentials of the various elements to the correct value. I shall now describe a practical way for making the various adjustments in a simple way. Taking for instance, the three-stage amplifier 1 i shown in Fig. 2 or in Fig. 3, I first remove tube 1 from the socket and place all grid set-backs in contact with the cathode tap of the respective tubes, that is, I bring P1 in contact with C2, and P2 in contact with C3. When this adjustment is made, the grid of the second tube will be at the potential of the cathode and the grid of the third tube will be at a negative potential equal to the voltage drop produced by the flow of the plate current of the second tube through the coupling l resistor R2.

With the various contacts so arranged, I read the current in the output of the third tube. This current will be either zero or it will have a value in excess of the normal value at which the tube is to' operate. Generally, the value of the current will be Zero since with most of the commercial power tubes the drop of the plate cur rent of the second tube when. its grid potential against the cathode is zero is such that it reduces the plate current of the third tube to zero.

If the plate current is zero, the plate voltage of the second tube is decreased by moving the cathode tap C2 towards the positive end of the voltage supply potentiometer until the plate current of the" third tube reads about one milliampere. If the plate current is excessive, the cathode tap C2 of the second tube is moved towards the negative end of the potentiometer resistor. until the plate current is brought down to a value of about one milliampere.

After the plate current of the third tube has been thus brought to the correct value, the first tube is replaced in the socket and the grid voltage of this tube is adjusted by means of the tap G1 until the plate current of the third tube has been increased to the normal value of about 20 milliamperes.

I new measure the voltage between the grid and the filament of the second tube, using any suit able vacuum-tube volt-meter for this purpose. If the measured value of the negative grid voltage of the second tube is greater than the desired value, which is usually the case, I move the grid set-back P2 of the third tube towards the positive end of the voltage supply. This will increase the plate current above the normal value of 20 milliamperes. I then bring this current back to normal by making the grid of the second tube less negative, suitably shifting the set-back P1 of the second tube. If the grid of the second tube is still too negative the operation is repeated, i. e., the grid bias of the third tube is made still more positive and the plate current of this tube is again brought back to its normal value by decreasing the negative bias of the second tube and repeating this operation until the grid of tube 2 has the correct negative potential.

After the grid voltages of

tubes

2 and 3 have thus been adjusted, the grid voltage of the first tube is next brought to the correct value. This is done by adjusting the grid set-back sliders G1 and P1 of the first two tubes, the set-back P2 of the third tube being left in the previously stated position. The set-back sliders G1 and P1 of

tubes

1 and 2 may be set in any desired position, and as long as they are adjusted relative to each other so that the plate current of the third tube is kept constant, the grid voltages of

tubes

2 and 3 will always have the same values. Accordingly, the grid voltage of the first tube may be adjusted to the correct value by suitably setting the slider G1 and then the set-back on the second tube is adjusted so as to give the desired normal plate current of 20 milliamperes for the final tube. The grid voltages of

tubes

2 and 3 will then have the correct values at which they were set in the previous operations.

By following the procedure described above, the correct points of connections of the various elements of the amplifier are arrived at. When I mention the values of the currents in describing the steps of the various operations I do it in order to give a concrete illustration of the procedure as applied to an amplifier having the constants given in the description of a practical amplifier i set referred to before. It is clear that when tubes and circuits of other constants are used, theadjustments will be made in a waycorresponding to the operating constants of the particular design.

While the method and means for reducing the feed-back of the higher stages to the lower stages by means of bridging elements of negligible impedance to varying currents across the external impedance of the higher stages is very effective, and may initself be sufiicient in many or" the practical applications, it'is in many cases desirable to avoid the use of glow tubes. I have devised an arrangement and a system for reducing feed-back from one stage to the other which seures the desired effect without glow tubes or impedance elements having the characteristics of glow tubes, relying merely on neutralizing circuits so'arranged as to eliminate the eiiect of current changes in the plate circuits of the higher stages on the input of the lower stages. In other words, a distinctive feature of my present invention is the provision of a feed-back neutralizing arrangement of such character as to permit the supply of the several plate circuits of a triode cascade from a common battery or voltage divider while at the same time preventing the current from one stage from reacting on the input of another stage.

Such arrangement is shown in Fig. 6. In this figure, there is shown a three-stage direct-coupled amplifier having the several elements thereof arranged essentially in the same way as the amplifier shown in Fig. 3. There is omitted, however, the glow tube bridging element 36 across the section of the potentiometer resistor which is connected in the plate circuit of the third stage between the taps P2 and P3. Instead, the feed-back is. prevented by the use of a special neutralising potentiometer bridge 61 formed of a resistor like the resistor constituting the main potentiometer resistor 15 and so connected across the two

points

63 and 64 of the main potentiometer 15 that the plate current flow in the third stage amplifier does not react on the operating conditions of the first or second stages.

The principle underlying this neutralizing arrangement shown in Fig. 6, may be best understood by comparing the operating conditions of the'arrangement shown in Fig. 3 with those of Fig. 6. Let us consider the case where an increase of the positive potential of the grid of the first tube of Fig. 3 has resulted in an increase of the current in the plate circuit of the third tube in the direction from the plate to the cathode. A part i this plate circuit current will flow through the potentiometer resistor 15 in the direction from the tap C3 towards the negative end of the resistor, producing between the taps Cl and G1 of the input of the first tube a voltage drop that is opposite in phase to the main input into the first tube. It is this out-of-phase flow of feed-back current across the resistor sections Cl, G1, that is to be eliminated. The current in the plate circuit of the third tube on reaching the tap C3, divides into two parts, one flowing from the tapping point C3 towards the positive end of the potentiometer resistor and the other part flowing towards the negative end of the potentiometer resistor. These two current .parts how in the potentiometer resistor 15 in opposite directions and produce across said potentiometer resistor oppositely directed voltage drops. Accordingly, there will be in the section of the potentiometer resistor between the tap C3 and the positive end of the resistor apoint hav- *resistor 61.

ing a voltage equal and opposite to the voltage between the tap C3 and the tap G1.

I may accordingly connect a neutralizing bridging resistor 61 as shown in Fig. 6 to two

points

63 and 64 on the potentiometer resistor 15 on both sides of the tap C3 that have the same voltage with respect to the current flow in the plate circuit of the third stage, so that there is no tendency for any of the current in the plate circuit of the third stage to flow by way of the bridging As far as the operation of the tubes of the first and second stage of the amplifier is concerned, this bridging resistor 61 will act exactly like the potentiometer resistor 15 in the arrangement shown in Fig. 3 so that the tap contacts G1, C1, C2 and P1 may be connected to this bridging resistor 61 the same way as they were connected in Fig. 3 directly to the main potentiometer resistor 15. In other words, the

potentiometer circuit of the arrangement shown in Fig. 6 is provided with a bridging potentiometer'fil which, as far as the operation of the tube of the first and second stage is concerned, acts in the same way, and is in the same relationship, as the main potentiometer resistor 15. The points of connection of this bridging resistor 61 to the main potentiometer resistor 15 are so arranged however, that they have zero voltage for any current flow through the plate circuit of the third tube. In this way, none of the variations of this plate current of the third tube is reflected in the voltage conditions of the first two tubes and feed-back from the third tube to the first and second tubes is eliminated.

The principle underlying this method of utilizing the feed-back is thus based on the idea that the current of the plate circuit of each stage divides into two branch currents flowing in opposite directions thrcugh the potentiometer circuit and that a parallel potentiometer path may be provided between two points on the main potentiometer circuit so arranged that none of the current of the plate circuit of the particular stage flows by way of the branch circuit. This branch potentiometer circuit in turn is used for supplying the operating voltages to the tubes which are to be free from feed-back from the stage under consideration.

As in'the case or the arrangement shown in Fig. 3, it is in most practical applications sufficient to take care of the feed-bacl of the third or power stage of amplifier neutralizing only this feed-hack. In most cases it is not necessary to neutralize the feed-back from the second tube, first because this feed-back is very small, and second, because this feed-back may be so arranged as to be in phase with the input of the first stage.

A simple way for practically determining the points of connection of the bridging potentiometer 61 or neutralising element to the main potentiometer resistor 15, is to connect a voltmeter between the

points

63 and 64 to which the neutralizing potentiometer resistor is connected and change the point of connection 64 until a point is reached where variation of the voltage applied to the grid of the first stage does not produce any changes in the indication of the voltmeter.

In. applying the foregoing arrangement for not) impedance being a part of the circuit traversed by the plate current flowing in the third stage or in general, the stage of which the feed-back is to be neutralized. Because of this fact, a part of the feed-back current flowing from the point of connection C3 of the cathode of the third stage in the direction to the negative terminal 63 will have a reactive component the magnitude of which will vary with the frequency of the amplified current impulses. However, for frequencies above the cut-off frequency of the power pack, the phase angle so introduced is very small and may be neglected. The cut-off frequency of filters used in B-eliminators and power packs that are now generally used is quite low, being of the order of 30 cycles. In general, where the frequency amplified is above '30 cycles, the amplification and the neutralization will be practically independent of the frequency since we can neglect the phase angle introduced by the reactance of the filter net-Works having such cut-01f frequency. For frequencies ranging both above and below the cut-01f frequency, the neutralizing effect will be somewhat aifected by the variation in the frequencies.

Where an extremely wide band of frequencies, say from 10 to 100,000 cycles, is to be handled, it is necessary to take special precaution to obtain complete neutralization over the entire range. In such cases, it is not commercially expedient to reduce the cut-off frequency of the filter as low as or lower than 10 cycles because of the expense involved in providing the large inductances and capacitances that are required. In such cases it is better to secure the complete neutralization of the feed-back irrespective of the frequency by shunting a glow tube or device of similar characteristics across the filter. The glow tube then functions as a non-reactive bypassing device across the filter circuit as explained in connection with Fig. 3.

In some cases, it is desirable to neutralize not only the feed-back of the third stage but also the feed-back of the lower stages, or in general, the feed-back of all the stages of the amplifier. In Fig. '7 there is shown an exemplification of an arrangement for securing such effect. The amplifier comprises three triode stages having the elements arranged in exactly the same way as the arrangement shown in Fig. 6. The third triode is connected to the main potentiometer resistor 15 at the taps C3 and P3, in the same way as shown in the arrangements in Fig. 3 and Fig. 6. Connected between point 63 and point 64 on the main potentiometer resistor 15 there is a first neutralizing resistor 61 arranged in the same way as described in connection with Fig. 6, i. e., the

points

63 and 64 are so chosen that they have no voltage difference for current flowing in the plate circuit of the third stage by way of the contact tap C3. In this Way, the current fiow from the plate circuit is kept away from the circuit of the first neutralizing resistor 61, thereby eliminating any feed-back action from the third plate circuit to the lower-stage circuits that are energized from the potentiometer resistor.

The second plate circuit is energized from the voltage drop produced between the taps C2, P2, that are connected to the first neutralizing resistor at points having a voltage corresponding approximately to the voltage of the points of connection of the taps C2, P2, to the main potentiometer resistor 15 in the arrangement shown in Fig. 3, although there is a wide latitude of adjustments making it possible to shift the voltages to a different value.

The feed-back from the plate current of the second stage to the first stage is eliminated by providing a second neutralizing or bridging element '71 connected between two points 72 and 73 on the first neutralizing resistor 61 so as to eliminate any flow of current from the plate circuit of the second amplifier stage through the second neutralizing resistor while at the same time, providing a direct current potential drop through said resistor sufficient to energize the circuits of the first stage of the amplifier. In practice, the points of connection are so chosen that the point 72 is approximately of the potential of the negative end of the main potentiometer resistor 15 and the point 73 is disposed on the neutralizing resistor 61 on the side opposite to the tapping point C2 so that the voltage drop produced by the plate current of the second triode across the section C2-72 is equal and opposite to the voltage drop produced by the branch of the current flowing through section 02-73.

The plate circuit of the first triode is energized from the second neutralizing resistor '71, the tap contacts C1 and P1 of the first triode being connected to points on said second resistor '71 in a way analogous to the connections of the taps C2, P2 of the second triode to the first neutralizing resistor 61.

In order to make the neutralizing arrangement independent of the frequency, there may also be connected a glow tube 78, or device having the characteristics of such tubes as explained above,

across the direct current source supplying the potentiometer resistor 15 thereby eliminating any reactive current components from the potentiometer circuits, as explained before.

The neutralizing arrangement of Fig. 7, is thus based on the principle of eliminating the effect of the current flow in the potentiometer circuit or supply battery of the plate circuit upon the input of the triode and the circuits of any triodes back of the input by providing a bridging battery or potentiometer so arranged that the current flow in the plate circuit of the triode from which feed-back is to be eliminated does not flow by way of the bridging potentiometer or battery. The input voltage of the triode and the current fiow conditions in the triodes back of it may be readily controlled from this neutralizing potentiometer or battery in the same way as this would be done directly from the main potentiometer or battery if the feed-back were disregarded.

In Fig. 3, is shown an exemplification of my invention embodying both the feed-back neutralizing arrangement like that shown in Fig. 6, and also a glow-tube bridge across the output of the power stage thereby combining the advantages of the arrangement in Fig. 3 with those of the arrangement in Fig. 6.

The arrangement of my invention may be modified by using as a part of the potentiometer resistance the impedance of the triode itself. For instance, in Fig. 6, the resistance section 61 may be so arranged that the potentiometer resistor section R2 between the contact point C3 and the contact point 64 is left out, so that the triode 3 is itself used as a part of the potenticmeter circuit. Such use of the triodes is applicable wherever low impedance triodes are utilized as a part of the arrangement. It is of course understood that when utilizing the triode as a part of the potentiometer circuit, the resistance or impedance in the other elements of the circuits aresuitably adjusted in accordance with the principles of operation described hereinbefore.

A particular feature of my invention is an amplifier. arrangement of the type using glow-tubesv for bridging a part. of the plate circuit of the output stage and wherein the glow tube is at the same time used as a signalling device, for instance, in a television application. The light source :at the reception end of television sets as used today consists in many cases of a rare gas glow tube of the type which I described as suitable for reducing the loss external to the outputresistor and for. reducing the feed-back from the output circuit. The principal difference between the ordinary glow tube of the type that I mentioned before and the glow tubes used in television sets lies in the shaping of the electrodes. the cathode in the television tubes being-of rectangular shape. These television tubes are so designed that the part of the surface of the oathode that is covered by a glow varies in direct proportion to the current flow through the tube, this being also the condition for obtaining a constant voltage drop across the tube for the varying current such as required for the purposes described before in connection with Fig.3. According to my invention, I use such a television light source for bridging the external resistance of the output circuits or still better I connect such television glow tubes directly between the plate and the cathode as a shunt across the entire output circuit.

This is shown in the arrangement illustrated in Fig. 9 where a familiar type of radio receiver 81 is used for impressing television signals upon a three-stage direct-coupled amplifier 82. This amplifier is arranged in the way shown in Fig. 8, combining the advantages of feed-back neutralizing resistors with bridging efiectof the glow tube 83 connected across the output of the last amplifier stage. This glow tube 83 reduces the feedback and power loss in the way explained in connection with Fig. 3. It serves further in cooperation with the scanning disk 84 and the other usual television appurtenances for receiving television signals. As an additional function this glow tube 83 serves to maintain a constant voltage across the last tube of the amplifier.

The foregoing arrangement thus secures the advantages of the direct-coupled amplifiers while at the same time utilizing the glow tube as (a) an indicating light source for television signals (b) for increasing the amplification by reducing the feed back and (c) for regulating the D. C. voltage supplied to the amplifier tube, besides the many other advantages that such arrangements have from the standpoint of economy, efiiciency and simplification. For example, television sets now in use require a separate energizing source for the indicating lamps. In my arrangement as shown in Fig. 9, no such separate source for the indicating lamp or light source 83 is necessary.

It is not necessary to describe here in detail the operation of the scanning disk and the arrangement for properly synchronizing it with the operation of the scanning disk at the transmitting station, any practical system now in use being suitable for combination with the amplifier system described above.

In describing and illustrating the various arrangements, I have employed conventional designations for the various parts but I do not intend to limit myself to any particular arrangement of such parts, as the invention may be embodied in j many other modifications that will suggest themance including said grid potential point and havselves to those skilled in the art. For instance, in making amplifiers of thetype described hereinabove for commercial production, the various taps for connecting the circuits of the several stages to the potentiometer will not alwaysbe made in the form of adjustable sliding taps, but once the magnitude of the several resistor elements to be used in the circuit has been determined, more or less permanent connections between the potentiometer and the various circuits of the triode stages may be used.

The term triode is used in this application in its broad sense and is not intended to limit the invention totuoes having only three electrodes, namely, an electron-emitting electrode, an anode, and a control electrode, but is intendedto embrace tubes. which have in addition to such three electrodes other electrodes as for instance, in the screen grid tubes referred to'in the application.

It is accordingly desired that the appended claims be given a broad construction-commensurate with the scope of the invention within the art. 4

I claim:

1. In a triode apparatus, a triode having a cathode, a grid and a plate, anoutput circuit connected to said plate and said cathode, an input circuit connected to said grid andsaid cathode, a first impedance, means for producing a voltage drop along said impedance, means for connecting an element of said impedance into said output circuit to produce a flow of current therethrough, a second impedance connected between two spaced points on said first impedance, said two spaced points being substantially of equal potential with respect to the current flowing in said output circuit, and means for connecting said input circuit to a pointon said second impedance.

2. In a triode apparatus, a triode having a plate, a cathode and a grid, an input circuit connected to said grid, an output circuit connected to said plate, a first impedance, means for producing a voltage drop along said first impedance, means for connecting said plate circuit and said cathode to two spaced points on said first impedance for producing a current flow between said cathode andsaid plate, a second impedance connected to two spaced points on said first impedance, said two spaced points being of substantially equal potential with respect to current fiow through said output circuit so that substantially no current flowing in said output circuit passes through said second impedance, and means for connecting the input circuit leading to said grid to a point on said second impedance.

3. In a triode apparatus, a triode having a plate, a grid and a cathode, an output circuit I connected to said plate, an input circuit connected to said. grid, a first. impedance, means for producing a voltage drop along said impedance, meansfor connecting two spaced points on said impedance to said output, circuit to impress on 'saidplate circuit a voltage required for operate ing said triode, said impedance having a grid control potential point suitable for impressing on said input circuit a biasing, potential for said grid with respect to said cathode, a second impedance bridginga portion of said first impeding a point ofthe same potential as said grid potential .point on said first impedance, said bridging impedance being sc -connected to said first impedance that substantiallynone of the current flowing in said output circuit is diverted into said bridging means, and means for connecting the input circuit to a point on said bridging impedance having a potential of said grid control potential point on said first impedance.

4. In a triode apparatus, a triode having a plate, a cathode and a grid, a source of input energy having one terminal connected to said grid, an output device having one terminal connected to said plate, an impedance, means for producing voltage drop along said impedance, the free terminal of said output device being so connected to one point on said impedance, said cathode being connected to another point of said impedance to include in the plate circuit of said triode a voltage source of sufficient magnitude to produce the required operating plate current, a bridging impedance so connected to two spaced points on said first impedance that substantially none of the plate current flowing through said plate circuit is diverted into said bridging impedance,

and means for connecting the free terminal of said input source to a point on said bridging impedance.

5. In a triode apparatus, a triode having a plate, a cathode and a grid, a first impedance, means for producing a voltage drop across said impedance, a second impedance connected to said first impedance and arranged to have therealong a voltage drop corresponding to the voltage drop on said first impedance, an output device, means for connecting the cathode, the plate and said output device to two spaced points on said first impedance to produce a current flow between the plate and the cathode of said triode by way of said output device, said second impedance being so arranged as to be outside of the path of the current through said triode, and

an input device connected between said grid and a point on said second impedance for determining the potential of said grid with respect to said cathode.

' 6. In a triode apparatus, a triode having a cathode, a grid and a plate, a first impedance, means for producing a flow of the current through said impedance, an output device connected between said plate and a point of positive potential on said impedance, a connection between said cathode and a point of relatively negative potential on said impedance, a bridging impedance, bridging a portion of said first impedance including the point of connection of said cathode, and input means connected between said grid and a point on said bridging impedance, said bridging impedance including a point on said first impedance that would produce the proper biasing potential on said grid with respect to said cathode, the points of connection of said bridging impedance being so arranged as to reduce feed-back from the output circuit to said input circuit.

7 '7. In an amplifying apparatus, a low amplifying stage, a high amplifying stage, each of said stages comprising an amplifier tube having a cathode, an anode and a grid, input circuits between the grids and the cathodes and output circuits between the anodes and the cathodes of said tubes, respectively, means for coupling the circuits of said tubes into a cascade to amplify in said high amplifying stage amplified impulses from the low stage in response to impulses impressedon the input circuit of the low stage, a first impedance, means for producing a voltage drop across said impedance, the output circuit of said high stage tube bridging a portion of said 1 first:imp'edance to energize said output circuit, a second impedance connected to two spaced points of said first impedance that are of substantially equal potential with respect to the current flowing in the output circuit of said higher stage tube, and means connecting the input circuit of said low stage tube to said second impedance.

8. In an amplifying apparatus, a low amplifying stage, a high amplifying stage, each of said stages comprising an amplifier tube having a cathode, an anode and a grid, input circuits between the grids and the cathodes and output circuits between the anodes and the cathodes of said tubes, respectively, means for coupling the circuits of said tubes into a cascade to amplify in said high amplifying stage amplified impulses from the low stage in response to impulses impressed on the input circuit of the low stage, a first impedance, means for producing a voltage drop across said impedance, the output circuit of said high stage tube being connected across a portion of said first impedance to energize said output circuit, a second impedance connected to two spaced points of said first impedance positioned to prevent diversion of the current fiowing in the output circuit of said higher stage tube into said second impedance, and means connecting the input circuit of said low stage tube to said second impedance.

9. In an amplifying apparatus, a low amplifying stage, a high amplifying stage, each of said stages comprising an amplifier tube having a cathode, an anode and a grid, input circuits between the grids and the cathodes and output circuits between the anodes and the cathodes of said tubes, respectively, means for coupling the circuits of said tubes into a cascade to amplify in said high amplifying stage amplified impulses from the low stage in response to impulses impressed on the input circuit of the low stage, a first impedance, means for producing a voltage drop across said impedance, the output circuit of said high stage tube bridging a portion of said first impedance to energize said output circuit, and second impedance means connecting a circuit element of said lower stage to two spaced points on said first impedance positioned to pre-- vent diversion of current fiowing in the highstage output circuit into the circuit of said low stage.

10. In an amplifying apparatus, a low amplifying stage, a high amplifying stage, each of said stages comprising an amplifier tube having a cathode, an anode and a grid, input circuits between the grids and the cathodes and output circuits between the anodes and the cathodes of said tubes, respectively, means for coupling the circuits of said tubes into a cascade to amplify in said high amplifying stage amplified impulses from the low stage in response to impulses impressed on the input circuit of the low stage, a first impedance, means for producing a voltage drop across said impedance, the output circuit of said high stage tube bridging a portion of said first impedance to energize said output circuit, and second impedance means connecting a circuit element of said lower stage to two spaced points on said first impedance that are of substantially the same potential with respect to the current flowing in the high-stage output circuit.

DAVID E. SPARKS.