US2048528A - Low frequency amplifier - Google Patents

Description

' July 21, 1936. H. A. WHEELER LOW FREQUENCY AMPLIFIER Original Filed Feb. 17, 1927 ATTORNEYS Patented July 21, 1936 UNITED STATES LOW FREQUENCY AMPIHYIEB.

Harold A. Wheeler, Great Neck, N. Y., assignor to Hazeltine Corporation,

ware

a corporation of Dela Original application February 17, 1927, Serial No. 168,925, now Patent No. 1,904,185, dated April Divided and this application March 6, 1933, Serial No. 659,669

10 Claims.

This invention relates to wave signaling apparatus and more particularly concerns a system of lowor audio-frequency amplification for use in connection with radio, telephone, and other signaling systems.

This is a division of my copending application Serial No. 168,925, filed February 17, 1927, which became Patent 1,904,185, April 18, 1933.

In the transmission and reception of radio communications such as speech and instrument or vocal music, it is customary to employ a vacuum. tube of the well known three-element type, and to amplify the audior low-frequency signal by means of an audio-frequency amplifier comprising one or more vacuum tubes connected in some suitable manner. For example in one well known' and highly efiicient type of audio-frequency amplifier, the vacuum tubes are connected in cascade through transformers, the input energy being supplied from the detector tube in a radio receiver, or from a microphone, in the case of a transmitter. An amplifier of this type can be used in a public address system, foramplifying the speech input of transmitters or in other systoms of the type in which the amplification of low-frequency signals is desired.

The audio-frequency amplifiers now in general use in radio receiving sets leave much to be desired when amplification is required over the ranges of frequency and amplitude. encountered in the loud speaker reproduction of voice, music and other sounds. The introduction of the cone type loud speaker, combined with an increasingly exacting public demand for higher quality, often regardless of cost, makes it necessary to greatly improve the quality of reproduction of the present amplifiers.

Inan audio-frequency interstage transformer of the type usually empldyed in audio-frequency amplifiers, the secondary winding is placed over the primary winding, and is therefore removed a substantial distance from the transformer core. This construction gives rise to a rather high socalled leakage reactance in the secondary winding, which reactance with the internal capacity comprises the equivalent of a series connected inductance and capacity in parallel with this winding, and gives rise to a resonance peak in the voltage amplification curve of the transformer, which peak usually occurs at some frequency between three thousand and ten thousand cycles per second. This resonance peak or rise in amplification is objectionable, as it results in non-.

uniform amplification at the higher frequencies, and may also cause oscillations which are fed back and may be sustained throughout the amplifier as awhole.

Further distortion in audio-frequency amplifiers arises from the fact that the grid of a vacuum tube draws a. certain amount of energy from the input circuit of the tube,that is, the secondary winding of the input transformer; and, under certain conditions,'the maximum primary energy which can be supplied to the primary "inding of the input transformer by the plate circuit of the preceding or driver tube is not sumcient to supply both the required secondary or grid energy and the degree of grid voltage variation necessary for satisfactory operation of the driven tube under consideration. Distortions arising from this cause may be reduced by the use of a grid, or C, battery to put a negativepotential bias on the grid, but this expedient reduces the plate current obtainable from the tube for a given plate voltage, and is for this and other reasons undesirable.

In audio-frequency amplifiers the plate voltages for the various tubes are usually supplied by a single plate, or B, battery. The internal resistance of this battery has been found to increase rapidly with use, and long before the useful life of the battery is at an end, this resistance reaches a value which results in an interstage feedback of audio-frequency voltages and consequent oscillations and distortion of the signals being amplified.

With the above and other considerations in mind, it is proposed in accordance with the present invention to provide a system of audio-frequency amplification embodying means for giving a comparatively large power output with a minimum amount of distortion, and, more specifically, it is proposed to provide a system of this. type embodying certain devices, circuits and othei expedients by means of which distortion and oscillation arising from the leakage reactance of the interstage transformers, the energy drawn by thevacuum tube grid circuits, and the resistance of the plate or B" battery, are effectively reduced or eliminated.

Other specific objects, advantages and characteristic features of the invention will become apparent as the description thereof progresses.

In describing the invention in detail, reference through the wires Fig. 2 represents a two-stage audio-frequency amplifier of slightly modified form; and

Fig. 3 represents a three-stage audio-frequency amplifier constructed in accordance with the present invention.

Referring to the drawing and more particularly to Fig. 1, an amplifier embodying the present invention has been illustrated in connection with the detector audion tube V of a radio receiving set, the input energy for this tube being supplied from any suitable source such as a radiofrequency amplifier of any suitable type. The grid I .of the detector tube V is connected to a source of radio-frequency current supplied 2 and 3 and the usual grid condenser C1, the filament 4 of this tube being connected with the same supply through wire 5; and a grid leak or high resistance R1 is connected between the grid I and the filament 4' through a circuit which is obvious from the drawing. The output or plate circuit of the detector tube V is connected in series with the primary winding 6 of an audio-frequency interstage transformer T1 through a circuit which includes the plate I of the tube V, thewires 8 and 9, primary winding 6, wires I0 and II, the plate or B battery I2, wire I3, filament resistance I4, and wires'l5 andIG to the filament 4 of the tube V.

A condenser C2 is preferably connected between the plate I and the filament 4 of the detector tube V as shown, this condenser acting as a shunt for the radio-frequency voltages which may flow in this plate circuit, and thereby prevent possible radio frequency feedback due to these voltages. The condenser C2 is of a capacity such that its presence has no appreciable effect on the flow of the audio-frequency current in the plate circuit of the tube V.

The transformer T1 may take any suitable form, but is preferably of the iron core type, and preferably has a step-up voltage transformation ratio of about two-to-one, although other suitable ratios of voltage transformation may be employed. The secondary winding ll of the transformer T1 is connected between the grid 68 and the filament It of the first audio-frequency amplifying tube V1, this circuit including a resistance R4 connected in series therewith as shown. A condenser C4 is connected directly between the grid l8 and the filament IQ of the tube V1. The constants of the resistance R4 and the condenser C4, as well as the purpose of the particular connection of these devices, will be hereinafter described.

The plate circuit of the tube V1 is connected in series with the primary winding 20 of a second audio-frequency transformer T2, this circuit being obvious from the drawing. The transformer T2 preferably has a voltage transformation ratio of one-to-one, and its windings are so arranged that the relative polarities of its adjacent terminals 2! and 22 are the same at any given instant. This relation of relative polarity at the transformer terminals may be obtained by winding the primary coil 20 and secondary coil ,23 in the same direction. This particular arrangement comprises an important element of the present invention, as will be hereinafter explained. A condenser (15 is connected between the plate terminal -2i of the primary winding 20 and the grid terminal 22 of the secondary winding 23 of the transformer T2 as shown.

The secondary winding 23 of the transformer T2 is connected between the grid. 34 and the plifying tube any suitable type. The filaments the tubes V, V1 andIVa are heated from a filament, or A", battery 26 through circuits which are obvious from the drawing, the currents fiowing ments being controlled by the adjustable filamenresistance I4. The plate voltage-of the tubes V, V1 and V1, is supplied by a plate, or B, battery l2 through obvious circuits.

includes in series therewith a loud in these fila- W In the operation of the audio-frequency amplifier illustrated in Fig. 1, the radio-frequency energy impressed between the grid I and filament 4 of the detector tube V is rectified, or detected, in the usual and well known manner, and results in a current having an audio-frequency component flowing in the plate circuit of this tube. As pointed out above, the condenser C2 connected between the plate I and the filament 4 of the tube V, eliminates or effectively reducesv radio-frequency voltages in this plate circuit, and thereby prevents possible feedback of such voltages to the grid circuits of the radio-frequency amplifying tubes, or the detector tube V.

As explained above, the inherent leakage reactance of the secondary winding ll of the transformer T1 is-very high, this being due to the disposition of this winding outside of the primarywinding 6 and therefore removed a considerable distance from the transformer core. The circuit through the secondary winding H has a point of resonance due to this'reactance, which point occurs at thehigher audible frequencies, that is, from two thousand to. ten. thousand cycles per second. In order to eliminate this resonance peak or to reduce it to a negligible value, the resistance R4, which may be the resistance of the transformer secondary winding I1, and the condenser C4 are introduced into this secondary circuit in the manner shown. The value of the resistance R; employed is so chosen that the power consumed due to the passage of the grid current therethrough is so small as to be negligible, this grid current being very small in the first stage of audio-frequency amplification. The capacity of the condenser C4 is of a value such that this condenser does not pass any appreciable amount of current below a given frequency, about two thousand or three thousand cycles, but readily passes current at frequencies above three thousand cycles. Wnen the frequency of the voltage in the secondary winding ll of the transformer T1 is below a given value, say about two thousand cycles, energy is delivered to the grid I8 of the tube V1 in the usual reduces the voltage applied to the grid circuit at these higher frequencies. Also, the increased current through the secondary winding I1, due to the current passed by the condenser C4, increases the load on the transformer T1 and thereby reduces the secondary terminal voltage thereof in the well known manner; Thus at the higher frequencies, that is, frequencies above two thousand or three thousand cycles per second, at which frequencies the grid circuit voltage is inclined to increase to an undesirable peak due to the leakage reactance of the secondary winding l1, the combination of resistance R4 and condenser C4 act to reduce this grid voltage, with the result that the grid voltage remains substantially constant over the audible range of frequencies, and no undesirable peak occurs in the amplification curve.

As noted above, the grid current in the grid circuit of the first stage tube V1 is very small, so that the effect of the resistance R4 on the average grid voltage is negligible. In the grid circuit of the second stage tube V2, however, this is not the case. The interstage transformer T2 used in this amplifier preferably has a one-toone ratio of voltage transformation, for reasons which will hereinafter appear, and due to the amplification obtained in the first stage, the current flowing in the grid circuit of the tube V: is large as compared with that flowing in the rid circuit or the tube Vi. For this and other reasons, it has been found that the combination of the resistance R4 and the condenser C4 as employed in the first audio-frequency stage is not, as satisfactory for use in the second stage, and other more efficient means are employed to prevent distortion due to the leakage of the transformer T2.

The condenser C5, connected between the primary terminal 2| and the secondary terminal 22 of the transformer T2, is employed to prevent distortion at the higher frequencies in the stage or stages subsequent to the first. this condenser may be explained in various ways. According to one theory, the condenser C5,- together with the tube V1, form a-shunt including a capacity and resistance across the secondary winding 23 of the transformer T2, this shunt path starting at the terminal 22 and including the condenser C5, wire 21, the plate 28, and filament IQ of the tube V1, and wires 29 and 30 to the secondary winding 23 ofthe transformer T2. The condenser C5 has a capacity such that at least the higher frequencies, that is, the frequencies at and ,above which the undesirable resonance peak occurs, are passed thereby, and hence a load is placed on the secondary winding 23 through the shunt path just traced, at these higher frequencies. This load reduces the terminal voltage of the secondary winding 23 in the well known manner, and so counteracts or nullifies the effect of the resonance peak in a manner somewhat similar to that described above in connection with the grid circuit of the tube V1.

The theory of operation of the condenser C5 may be explained in another and probably more accurate manner. Since the primary and secondary windings 20 and 23 of the transformer T2 are so arranged that the terminals 2| and 22 thereof are always at the same relative polarity and since the ratio of voltage transformation of this transformer is one-to-one, the instantaneous voltages of the terminals 2! and 22 should always be the same. This would be true if the transformer T2 had what is known as unity coupling, that is, if the ratio of transformation remained unchanged at all frequencies.-

As noted above, however, due to the leakage reactance of the secondary winding 23, the transformer T2 does not have the same transformaergy which can be supplied to the primary wind- The action of tion ratio at all frequencies, but has an increase in its voltage amplification ratio which reaches a peak at some frequency between four thousand and ten thousand cycles per second. The condenser Cs, being of a capacity such that it passes it currents of these frequencies, acts to increase or tighten the coupling of the transformer T: at these frequencies, thereby effectively reducing or limiting the amplification peak referred to. Thus the condenser C5 effectively eliminates distortion arising from the leakage reactance of thesecondary winding 23 in the last audio stage or stages.

As pointed out above, a further source of distortion arises from the fact that the grid circuit of the vacuum tube draws a certain amount of energy during its operation, and the input ap-. paratus for supplying such grid circuit is some times incapable of supplying this energy and at the same time supplying the necessary degree of grid voltage variation required for the satisfactory operation of the tube. To explain this phenomenon more in detail; when the grid of a vacuum tube is supplied with a positive potential with respect to the filamentthereof, a cer- 25 tain amount of current flows from the grid to the filament, due. to the passage of electrons from the filament to the grid. This flow of current draws energy from the input transformer supplying the grid circuit, and if the current is comparatively large, the voltage supplied by the transformer is appreciably reduced, this reduction being due to the characteristic behavior of a loaded transformer; that is, the amount of ening of the transformer from the plate circuit of the preceding tube is limited by the plate resistance. It should be noted that when a negative voltage is applied to the grid with respect to the filament, no current flows in the grid circuit, and, for this reason, the grid circuit voltage is not reduced. Hence the voltage reduction in the grid circuit occurs during one-half of the grid voltage cycle only, and results in a very marked distortion of the signals. The grid current drawn by the tube during the positive half of the voltage cycle increases very rapidly as the grid voltage increases, and the power which can be supplied from the preceding plate circuit decreases as the transformer ratio is increased above a certain value.

Distortion due to the grid current as described above is objectionable only when comparatively large variation of grid voltage is used, and for this reason, very little, if any, objectionable distortion arises from this source in the first audio stage where the grid voltage variations are comparatively small. 1

In the amplifier of the present invention, distortion due to grid circuit current is avoided by the use of a transformer having a one-to-one ratio of voltage transformation to supply the grid circuit energy for all except the tube of the first audio amplifying stage. Accordingly, in the amplifier of Fig. 1, the transformer T2 has a oneto-0ne voltage transformation ratio. The use of a one-to-one ratio transformer gives undistorted amplification for several reasons. In a one-toone ratio transformer, an increase in current in the secondary winding does not decrease the secondary voltage to the extent that such voltage would be decreased by such current if a transformer having a higher ratio of voltage transformation-were employed. A comparatively small increase in secondary current in a transformer of this ratio. such as the increase in current occurring when the voltage of the grid of a vacuum tube is positive with respect to its filament, does not ordinarily require an amount of power from the plate circuit of the preceding tube in excess of the amount that this plate circuit can supply. In other words, the ratio of the amount of power dissipated in the grid circuit of a given tube to the amount of power which can be supplied by the plate circuit of the preceding tube is much greater as the ratio of voltage transformation of the transformer connecting these tubes is increased, and for this reason the positive voltage to which the grid of the tube under consideration can be raised is limited by the transformer ratio, and is greater for transformers having low ratios of transformation. 'Ihus, by the use of the one-to-one ratio transformer T2 in the amplifier of Fig. 1, the plate circuit of the driver tube V1 supplies sufficient power through this transformer to supply the current drawn by the grid circuit of the driver tube V: during the half of each voltage cycle when the grid 24 is positive with respect to the filament 25, and at the same time supplies sufficient voltage to raise this grid M to the desired positive potential.

Referring now more particularly to Figs. 2 and 3, two audio-frequency amplification systems have been disclosed in which distortions due to the leakage reactance of the secondaries of the transformers T1, T2 and T3, have been effectively eliminated by the same means as those disclosed and described in connection with the amplifier of Fig. 1, namely, the combined resistance and condenser R4 and C4 in condenser C5 in the remaining stages of amplification. These amplifiers also include transformers T2 and T3 having one-to-one ratios of voltage transformation in all but the first audiofrequency stage, thereby effectively eliminating distortion due to grid circuit current in the same manner as that described in connection with Fig. 1. In addition to these means for preventing ob-. jectionable distortions, the amplifiers of Figs. 2 and 3 include means for preventing distortions or oscillations arising from the feedback of audiofrequency voltages due to high resistance in the plate, or B-, battery.

It is desirable from the standpoint of economy to employ a common plate, or B, battery for supplying the plate circuit voltages of all of the tubes of the amplifying apparatus. This battery is designated at 3| in Figs. 2 and 3; and the platefilament circuits of all of the tubes V, V1 and V2, are connected in multiple with this battery as clearly shown in the drawing. It has been found that the internal resistance of a plate battery of the usual dry cell type increases very rapidly with use, and often reaches a very high value, such as one hundred ohms or more, before the useful life'of the battery is at an end. Since the plate currents from all of the audio-frequency tubes, including the detector tube V, flow through the battery 31, this high internal resistance often gives rise to a feedback of audio-frequency voltages, which affects the amplification and may cause sustained oscillations in the amplifier as a whole.

In order to avoid oscillation, distortion or impaired amplification arising from the high internal resistance of the plate battery 3|, each of the amplifiers of Figs. 2 and 3 is provided with a filter circuit comprising an inductive reactance coil L3 and a condenser C3 in the plate circuit of the detector tube V. The reactance coil L3 is the first stage, and the preferably provided with an iron core and has an inductance suchrthat the'fifiow of audio-frequency currentsitherethrough is substantially prevented. The condenser f}: ,has a capacity such that audio-frequency alternating currents are readily passed thereby. In operation, the audiofrequency component of the plate current in the detector tube V is passed by the/condenser C3,

traversing a-path including the wires 32 and 33,

the primary winding M of the transformer T1, 'wire 3t, condenser Cs,

I and wires :38, 31 and 38 to the filament 32 of the tube V. The direct current component of the detector tube plate circuit cannot pass through the condenser C3, and as the coil La offers but little resistance to the fiow of this direct current, its path is from the plate 40, through wires 32 and 33, primary winding 3, wires 35 and M. coil L3,, wires 42. and 43, plate battery 3!, wire 44, filament resistance and wires Ni and 41 to the filament 39 of the tube V. The audio-frequency currents in the.

plate circuits of the tubes V1 and V: of Fig. 2 and of'the tubes V1, V2 and V3 of Fig. 3 fiow to the plate battery through the wire 58, and due to the internal resistance of this battery, audiofrequency voltages are built up across the terminals thereof. blocked from the plate circuit of the dgtector tube V by the coil L3 and hence no undesirable audio-frequency feedback to the tube V can occur.

Although the filter comprising the coil L3 and condenser G3 has been shown only in the plate circuit of the detector tube V, a similar filter may be employed in the plate circuit of the tube V1 of the amplifier shown in Fig. 2 and in the plate circuits of the tubes V1 and V2 of the amplifier of Fig. 3, if desired. In this manner, feedback from the plate circuit of the audio-frequency amplifying tube in the last stage to all of the preceding tubes may be prevented. Obviously, it is most important to prevent audio-frequency feedback to the detector tube V, as any undesired audio-frequency currents flowing in the plate circuit of this tubewill be amplified by the several audio-frequency amplifying stages, and will, therefore, be reproduced at a greatly increased volume, sometimes resulting in the production of sustained oscillations.

The amplifier disclosed in Fig. 2 is identical with the amplifier of Fig. 1 except for the addition of the filter circuit La and C3 described above. In Fig. 3, a detector tube V with a threestage audio-frequency amplifier has been disclosed, this amplifier embodying the plate circuit filter La and C3 and the combined resistance and capacity R4 and C4 in the first stage, and the condenser C5 in the second and third stages, to prevent distortion due to the resonance amplification peak of the transformers T1, T2 and T3. The amplifier of Fig. 3 also employs transformers T: and T3 having one-to-one ratios of voltage transformation in the second and third stages, therevby preventing distortion due to grid circuit current as described above in connection with the amplifier of Fig, 1. The amplifier of Fig. 3 is desirable for use where greater amplification is desired than that which can be obtained by the use of the two stage amplifiers of Figs. 1 and 2.

In the amplifiers of the present invention, the constants of the various coils, condensers, transformers and batteries used may be yaried within certain limits, and the constants of these means which may advantageously be employed in any given amplifying circuit depends on the characteristics of the-tubes used in such circuit, on the These voltages are effectively w former winding connected thereto.

character of the power supply available and on various other factors. One set of constants which has been found to give excellent results will be given, it being clearly understood that the values set forth represent only one specific embodiment of the invention disclosed, and that many other suitable values of these constants may be employed within the scope of the invention. In the particular arrangement referred to, the capacities of the condensers are as follows: C1, .0001 microfarad; C2, .002 microfarad; C3, 1 microfarad; C4, .0001 microfarad; C5, .006 microfarad. The inductances of the various coils and windings are as follows: L3, henries (5,000 ohms D. C.) primary winding of the transformer T1, 25 henries; the secondary winding of the transformer T1, 100 henries; primary and secondary windings of transformers T: and Ta, 50 henries. The resistance unit R1 has a resistance of 5 megohms, and the resistance R4, .1 megohm. The transformer T1 has a two-to-one ratio of voltage transformation, and has about 10,000 turns on the primary winding and 20,000 turns on the secondary winding, the primary winding being wound directly on a figure 8 laminated magnetic core and the secondary winding being wound over the primary winding. The transformers T2 and T3 are also wound on figure 8 laminated magnetic cores and these transformers have a transformation ratio of one-to-one, their windings comprising each about 15,000 turns. The wire used in all of the transformer windings is about No. 40 B. & S. The choke coil Ls may comprise about 20,000 turns of No. 36 B. 8: S. wire, wound on a figure'B core or a plain closed core. The values of the constants of the various devices given above, are particularly adaptable when vacuum tubes of the 201A type are em ployed.

Since the 201A type of tube has a normal anode circuit impedance of about 10,000 ohms, it follows that the impedances appearing across the secondary windings of the one-to-one ratio transformers T2 and T3 are also about 10,000 ohms plus the resistance of the transformer windings. The input impedance of the succeeding tube is much greater than this value when its grid is driven negative by the signals, and is not reduced to this value until the grid is driven considerably positive. Hence throughout a considerable range of applied signal intensities the grid circuit impedances of tubes V2 and V3 are greater than the impedance appearing across the secondary trans- It follows from this that the impedances looking into the primary windings of transformers T: and T: are greater than the plate impedances of .the tubes connected thereto.

The amplifying system of the present invention has many advantageous features. Distortions arising from the inherent leakage reactance of the transformers, and oscillations resulting from the amplification peak of the transformers are effectively prevented by the arrangement of the resistance R4 and the condensers C4. and C5 which act to oppose or counteract the effects of thisleakage reactance as described above. With the use of the one-to-one ratio transformers in the last audio-frequency stages, amplification of excellent quality and volume is obtained without the use of grid circuit, or C, batteries. The use of the filter circuit including the choke coil La and the condenser C3 prevents feedback and oscillations due to plate, or B, battery resistance, and.

is also useful in preventing fluctuations in the plate circuit current when a generator or a rectifier is used to supply the plate voltage. By reason of the combined action of the various features enumerated, the amplifier as a whole acts to am- 5 plify the signals received with a minimum of distortion and without sustained oscillations, and gives a very satisfactory volume as compared with the audio-frequency amplifying systems previously employed. 1.

Although the present invention has been described in connection with certain rather specific amplifying systems, as employed in receivers, it should be clearly understood that the invention is not limited to radio receivers or to the exact 15 details of the systems disclosed. For example, certain features of the invention, such as the combined'resistance and condenser R4 and C4 might be used in connection with a system of amplification wherein the feature including the condenser 20 I C5 was not employed, or the one-to-one ratio transformer with the condenser connected between its primary and secondary windings might be employed independently of the other features disclosed. Furthermore it will be obvious that the 25 features introduced by the invention may be advantageously employed in any low-frequency system, such as public address systems and speech amplifiers in transmitters, merelyby substituting the systems herein described for those previously 30 known.

I claim:

1. A coupling system adapted to connect a first to a second stage of an amplifier, said second stage comprising an electron discharge device 35 having a cathode and a control grid upon which there is applied zero biasing voltage, said coupling system being provided with primary and secondary windings, said secondary winding being connected between said grid and cathode, the impedance appearing across said secondary winding being lower than the grid-cathode impedance of said second stage when said grid is driven positive and negative during operation.

2. A coupling transformer adapted to couple the output of a first vacuum tube to the input of a second vacuum tube, said transformer having a low resistance secondary winding connected to the input of said second tube, the impedance appearing across said secondary winding being less than the input impedance of said second tube when the control element of said second tube is driven both negative and positive due to signal waves applied thereto, said control element having no biasing voltage applied thereto.

3. In an audio-frequency amplifier, comprising a plurality of vacuum tube stages, an impedance adjusting transformer connected from the output circuit of a first of said stages to the input circuit 60 of a second of said stages, said second stage having no negative grid-biasing voltage applied thereto, said transformer having such an impedance ratio that the impedance which is effectively connected to said output circuit is greater than the impedance of said output circuit over a range of audio frequency signal intensities;

4. In an electrical system for transmitting alternating current energy, a pair of electron discharge devices each having a control grid, a cathode and an anode, a transformer having its primary winding connected to the anode circuit of one of said, devices and its secondary winding connected to the'grid circuit of the other of said devices, the grid of said second device having 15 applied thereto no biasing voltage, the impedance across said secondary winding being substantially less than the impedance of said grid circuit over a range of'negative and positive voltage applied to the grid of'said second device by said alternating current energy.

5. In an audio-frequency thermionic amplifier characterized by the absence of negative biasing potential applied between the cathode and control electrode of the thermionic elements, a transformer serving to couple the output section of one tube with the input section of another tube, said transformer having primary and secondary windings proportioned to have such a low ratio of transformation that the impedance appearing acrosSthe secondary winding is lower than the input section impedance of the second tube when the grid of said second tube is driven through a range of positive and negative voltages by the signal waves impressed thereon.

6. In an audio-frequency signaling system comprising a plurality of vacuum tubes, a transformer serving to couple the output section of one tube with the input section of a second tube, said second tube having a cathode and control electrode and being characterized by the absence of negative biasing potential applied between said cathode and control electrode, said transformer having primary and secondary windings proportioned to a sufficiently low ratio of transforma-- tion that the impedance appearing across the secondary winding is lower than the input section impedance of the second tube when the grid of said second tube is driven through a considerable range of positive and negative voltages by the audio-frequency signals applied thereto.

'1. A coupling device adapted to connect a driver and a driven stage of an amplifier, said driven stage comprising a discharge device having a grid, said coupling device being provided with primary and secondary windings, said secondary winding having an impedance lower than 5 the input impedance of the driven stage when the grid of the said driven stage becomes positive during operation.

8. A coupling transformer adapted to connect a voltage amplifier vacuum tube to a power out- 10 put vacuum tube, said transformer having a low resistance secondary winding whose impedance is less than the input impedance of said power output tube when the grid of said power output tube is driven into the positive range of grid potentials 15 by the peak of the wave to be amplified.

9. In an electron tube circuit, a power output electron tube, an electron tube driver for said power output tube, an impedance adjusting transformer connected to couple said driver tube to 20 said output tube, said transformer having an impedance ratio whereby the load impedance reflected back into the anode circuit of said driver tube is substantially greater than the anode impedance of the driver tube.

10. In an electrical system adapted to transmit alternating current energy, an electron discharge device adapted to transmit alternating current energy, a second electron discharge de-' vice, a grid, a cathode and an anode for each of said devices, a transformer connected between said devices, said transformer having a secondary impedance substantially less than. the impedance of the grid circuit of said second device under the condition that the grid of the second device is at a positive potential.

g HAROLD A.

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