US1894503A - Radio frequency amplifier - Google Patents

Description

Jan. 17, 1933. Y. WALSH 1,894,503

$111310 FREQUENCY AIPLIFIER Filed Sept. 26, 1927 5 Sheets-Sheet 1 INVENTOR z/mzn Walsh ATTORNEYS Jan.17,1933. AL 1,894,503

RADIO FREQUENCY AMPLIFIER Filed'Sept. 2a. 1927 5 sheets-sheet 2 INVENTOR 2' (:2 5m lf a 25/1 ATTORNEY$ Jan. 17, 1933. I L. WALSH nmxo FREQUENCY AMPLIFIER Filed Sept. 26. 1927 5 Sheets-Sheet 4 ATTORNEYS Jan 17, 1933. L. WALSH 1,394,503

RADIO FREQUENCY AMPLIFIER Filed Sept. 26. 1927 5 Sheets-Sheet 5 W0 ve/e/rgfb med: r

eco 5 o I .500 Fhyuengf- /7//0 qyc/es INVENTOR LII/coin Walsh ATTORNEY 5 Patented Jan. 17, 1933 UNITED STATES PATENT OFFICE v I LINCOLN WALSH, OF ELIZABETH, NEW JERSEY, ASSIGNOR TO HAZELTINE CORPORA- TION, OF JERSEY CITY, NEW JERSEY, A CORPORATION OF DELAWARE RADIO FREQUENCY AMPLIFIER Application filed September 26, 1927, Serial No. 222,009, and in Canada December 14, 1927.

The present invention relates to radio-frequency vacuum tube amplifiers of the transformer-coupled type and, more particularly, tuned amplifiers intended to operate over a fairly wide range of frequencies, as for example, in broadcast receivers.

In the design of tuned radio-frequency amplifiers one of the controlling factors to be dealt with is the tendency toward excessive regeneration and consequent oscillation which is due mainly to the coupling capacity between the grid and plate of each vacuum tube. Some provision must always be made against this tendency, preferably by neutralization. But since neutralization involves the use of a fixed neutralizing capacity which is balanced against the coupling capacity of a "acuum tube and since individual vacuum tubes, even of the same type and manufacture, are bound to vary to a certain extent, it follows as a direct result that there is a limit to the amplification per stage at any given frequency beyond which it is impracticable to go. Since the inter-electrode (plate-grid) capacitive reactance decreases with rising frequency, thus allowing a greater amount of high-frequency energy to be fed back from the plate circuit to the grid circuit of each tube, the permissible amplification per stage also decreases with rising frequency. As a matter of experience, the permissible amplification per stage at 1500 kilocycles per second is very much less than at 500 kilocycles per second. These fre quencies are the limits of the broadcasting band as now established. Theory and experience both indicate that the amplification should vary inversely as the square root of the frequency in order that adequate manufacturing tolerance in the vacuum tube coupling capacity may be allowed.

On the other hand, the amplification per stage obtainable with coupling transformers of the conventional type, that is, transformers having fixed coils of high stepup ratio and low losses, is less atthe iower than at the higher frequencies. is between the frequent-cs 500 kilocycles per second and 1500 kilocyclcs per second the actual a1nplitication per stage is very much less at the lower frequency. Thus, while the permissible amplification perstage at the lowerfrequency end of the broadcast band is much greater than at the high frequency end, the characteristics of the conventional radiofrequency transformer are such that the actual amplification obtainable is very much less at the low frequencies than at the high frequencies.

It has sometimes been attempted to modify the amplification characteristics of the conventional transformer to increase the amplification at low frequencies without also increasing the amplification at high frequencies. This can be done by introducing sources of energy loss which are mainly effective at high frequencies, but this expedient has the serious disadvantage of impairing the selectivity at high frequencies.

Notonly is the lower-frequency amplification with conventional transformers limited by the instability which would occur at higher frequencies, but the conventional transformer arrangement does not permit of high amplification at low frequencies'irresoective of instability. The reason is that in order to be able to tune to all frequencies within the broadcast band it is necessary to employ a tuning condenser having relatively great capacity at the lower frequencies. Such a large capacity in combination with transformer coils of conventional small dimensions and such as will give satisfactory fidelity, introduces an excessive secondary conductance at low frequencies. The high secondary conductance thus introduced has the effect of reducing the amplification at low frequencies, as may be easily demonstrated both mathematically and by actual performance tests.

, The primary object of the present invention is to provide a radio-frequency amplifier from which may be obtained, at all frequencies within its range, the highest amplification consistent with stability, as limited by the customary tolerance in coupling capacities of vacuum tubes, without impairing the selectivity at high frequencies where this is naturally thepoorest), or the fidelity at low frequencies (where the latter characteristic is naturally the poorest).

Another object, in furtherance of the primary invention, is to provide a structural and electrical arrangement which is adequately simple and trouble-proof in operation. The present invention accomplishes these ends by providing means adapted to change vacuum tube and whose secondary winding together with a tuning condenser is connected in the grid-filament (input) circuit of a sueceeding vacuum tube, the greatest amplification would occur at the highest frequency to which the receiver may be tuned, and the least amplification at the lowest frequency.

Therefore, it is ordinarily necessary to design such transformer so as to avoid exceeding the permissible amplification per stage at the highest frequency, as limited by the tendency toward instability.

In so far as the transformer per se is concerned, the factors'which ordinarily determine the degree of amplification are the selfinductance of the secondary winding, the mutual inductance between primary and secondary winding and the conductance (or resistance) of the secondary winding. The voltage ratio, or simply the ratio of a transformer the secondary circuit of which is tuned is substantially equal to the ratio of the secondary self-inductance to the mutual inductance. In the conventional radio-frequency transformer now referred to, both the secondary self-inductance and the mutual inductance remain constant irrespective of the frequency, and these values must, therefore, be determined by the requirement. that the maximum permissible amplification must'not be exceeded at the highest operating frequency. If the ratio of secondary self-inductance to the mutual inductance is lowered as the operating frequency is lowered then desired amplification while maintaining fi-' delity and compactness of coil construction.

the amplification per stage will in general be increased. Furthermore, if the secondary self-inductance is increased as the frequency is lowered, it will be possible to obtain the The present invention involves the proper adjustment of the inductanees and the tuning capacity so as to obtain the maximum permissible amplification at each frequency within the band of frequencies for which the receiver is designed.

Theoretically there are a variety of ways be called, are preferably made of a non-magnetic metal of high conductivity, such as copper or aluminum. Each cup is arranged to slide longitudinally with respect to its associated transformer. The larger cup is adapted to variably envelop the outer winding, which is the secondary, while the smaller cup is adapted to move longitudinally within the smaller or primary winding. The effect of moving the metallic shields or cups with respect to the transformer windings is to vary both the secondary self-inductance and the mutual inductance. each at the desired rate. The effect of the metal cups or shields disposed closely adjacent the windings of the transformers is to restrict the cross section and thereby to increase the reluctances of the magnetic paths, thus reducing the inductances.

The transformers may accordingly be designcd with a view to obtaining the maximum permissible amplification per stage at the lowest operating frequencythis value being much higher than that of the permissible amplification at the highest operating frequency. Then by providing means for moving the metal shields or cups so as to increasingly envelop the transformer windings as the operating frequency is increased the amplification may be decreased to the proper value for each operating frequency.

In the preferred form of this invention the movement of the metal shield or shields relatively to the transformer windings is accomplished automatically by means of suitable mcchanical connections with the tuning control or controls, and preferably the several stages of amplification are adjusted simultaneously by a single manual control. However, some of the advantages of the invention together with greater flexibility may be obtained by independently operating the shields and the tuning condensers.

With reference to the drawings which accompany this specification,

Fig. l is a cross sectional view of a coupling unit comprising a radio-frequency transformer with its associated metal shield together with a variable tuning condenser and a neutralizing condenser, all of WlllCll are assembled within a sheet metal receptacle; n

Fig. 2 is an elevational view of the coupling unit shown in-Fig. 1 and is taken as i/ iewed from the same observation point as Fig. 3 is a partial elevational view taken along the line 3-3 of Fig. 1 and looking toward the front of a radio receiver. This figure illustrates the operating mechanism for the. shields and tuning condensers;

Fig. at is a sectional view taken along the line 44 of Fig. 1;

Fig. 5 is a plan view of a condenser plate;

Fig. 6 is a comparative amplification graph representing by appropriate curves the amplification obtainable at different frequencies with a conventional tuned amplifier and with an amplifier in accordance with this invention; and

Fig. 7 is a partial circuit diagram of a neutralized radio-frequency amplifier adapted for use in conjunction with this invention.

In Figs. 1 and 2 there are illustrated in section and elevation, respectively, a radiofrequeucy transforn'ler with its associated metallic cup-like shields together with a tuning condenser and a neutralizing condenser-the arrangement shown being in accordance with one of the preferred embodiments of the invention. The transformer per se comprises two coaxial tubes 1 and 2 of dielectric material such as formica. The tube 1 is mounted inside the tube 2, being spaced therefrom by means of a suitable spacing ring 3 and secured by bolts 4. In this particular case there is, in addition to the prij mary winding, a neutralizing winding having an equal number of turns, wound on the tube 1. The turns of the primary and neutralizing windings are interleaved. The secondary is wound on the tube 2. The metal-- lie shield 5 comprises two coaxial cylindrical cup-like members 6 and 7. The two cups are secured together forming a unit. The tuning condenser comprises two identical elements, namely, a stator 8 and a movable element 9. Each of these elements has four plates, as shown. The plates'of one element are interleaved with those of theother. The movable element 9 is rigidly secured to the shield 5 through the medium of a connecting member 10 of insulating material. The shield 5 together with the movable condenser element 9 is adapted to slide axially with respect to the transformer. Thus it will be seen that as the plates of the tuning condenser move togetherthe capacity of the condenser thereby increasing-the shield 5 is moved to the right. as shown in Fig. 2, whereby it decreasingly affects the magnetic field of the transformer causing both the secondary self-inductance and the mutual inductance to increase. lVhen the cup '6 envelops the secondary winding to the maximum extent the reluctance of the magnetic field is a maximum, and the secondary selfinductance is consequently reduced to a minimum. Likewise the mutual inductance between primary and secondary windings is decreased as the cup 7 is moved into closer relation with the primary winding, that is, to the left as viewed in Fig. 1. While the two shields interact in their effects the outer one 6 aflects mainly the secondary self-inductance, and the inner one 7 the mutual inductance. lVith the proportions indicated'in Fig. 1of which specific dimensions will be given-the mutual inductance varies at a faster rate than the secondary self-inductance, giving the desired variation in the ratio. Although the arrangement illustrated comprising two coaxial cups constitutes the preferred embodiment of the invention, it has been found that similar but less ideal results may be obtained by omitting either one or the other of the cups 6 and 7 Since the function of the shields is to vary the reluctance of .the magnetic field it will be apparent that the shield or shields may take a variety of forms besides the specific arrangement illustrated.

As a part of the tuning condenser structure illustrated in Figs. 1 and 2 there is shown a flexible conducting element 11 which is adapted to function as one plate of a neutralizing condenser. This flexible plate may be adjusted toward and away from the adjacent fixed plate of the stator 8 by means of the adjusting screw 12. The capacity between the element 11 and the stator of the tuning condenser may thus be adjusted to a proper value for efiecting neutralization.

In order to clarify the illustration, the operating mechanism for the two tuning condensers and shields has been omitted from Figs. 1 and 2. A suitable mechanism for this purpose is illustrated in Figs. 3 and 4. It will be realized, however, that the operating mechanism illustrated in Figs. 3 and 4 constitutes only one of a great variety of ways in which the same results may be obtained. and that the invention is in nowise dependent upon the particular mechanism by which the operation of the tuning condensers and shields is brought about.

The operating mechanism of Figs. 3 and 4 is arranged for the unitary operation of two sets of tuning condensers and shields. It may obviously-be extended to take care of as many stages of amplification as may be desired.

Figs. 3 and 4; are views taken along the lines 33 and 4-4, respectively, of Fig. 1

and drawn to a somewhat smaller scale than- Fig. 1. As previously stated, the operating mechanism shown in Figs. 3 and 4, and

meshes with a toothed quadrant carried by a shaft 16. A graduated dial 15a is attached to and moves withthe quadrant 15. A leyer arm 17 which for convenience is made in the form of an L, is secured to shaft ,16 and is rotatable therewith. Tov the lever 17 is pivotally secured a link 18 which is connected to the member 10 (shown also in Figs. 1 and 2). Since both the shield 5 and movable condenser element 9 are attached to the member 10, it is apparent that these parts all move together in a direction parallel to the axis of the transformer in response to rotation of the tuning control knob. As shown and shields indicated in Fig. 4 are operated from the same control knob, and that as many additional sets of condensers and shields as might be desired could be added and operated through the one control. Three stages of tuned radio-frequenc amplification have been found very suita le.

Incidentally, the present invention lends itself very advantageously to unitary tuning control. tuning condensers are preferabl of small maximum capacity as compared with thetuning condensers ordinarily used in broadcast receivers of the conventional type. The reason wh a particularly small tuning condenser can e used to cover a wide band of frequencies is that the tuning is accomplished by varying the secondary self-inductance at the same time that the tuning capacity is varied. Since the tuning condensers may be of unusually small maximum capacity the plates may be heavier and spaced farther apart without excessive bulkincss. For these reasons it is possible to maintain a consid erably greater precision in manufacturin the condensers; and the tuning of the severa stages of amplification under a single control may, therefore, be carried out with greater accuracy or with less difliculty.

As a specific example, the dimensions of the transformer, shield and tuning condenser shown in Figs. 1 and 2 will now be given. The tube 1 is made of natural formica 1% inch outside diameter, 3 inches long, and 1/32 inch thick. On this tube are wound a primary coil and a neutralizing coil. The turns of these coils are interleaved, each coil having 36 turns of number 38 double-silkcovered copper wire, 16 double turns per inch.

This is because of the fact that the tween about 550 kilocycles The tube 2 is made of the same material, 2 inches outside diameter, 3% inches long and 1/16 inch thick. On this tube is wound a secondary coil consisting of 120 turns of number 26 enameled copper wire, 48 turnsper inch. The cup 6 which forms part of the metal shield is made of sheet copper 0.031 inch thick. Referring to Fig. 1, dimension A is 2 11/16 inches and B is 2.95 inches. Cup 7 is also made of sheet copper 0.031 inch thick. Dimension C is 2 inches and D is 1% inches.

With the above dimensions, the step-up ratio of the transformer is at all frequencies substantially higher than that giving greatest amplification; or, in other words,the input conductance at resonance is substantially higher than the plate conductance of the preceding vacuum tube. This is for the purpose of increasing selectivity and stability,

as explained in the co-pending application of Louis A. Hazeltine', Serial No. 12,000, filed February 27, 1925.

- The tuning condenser elements 8 and 9 are identical-one being fixed and the other movable; Each consists of four aluminum plates 0.031 inch thick and spaced 0.187 inch apart. A plan view of one condenser plate is shown in i 5. Suitable values for the dimensions indicated in Fig. 5 are as follows:

E 3 inches F 2% inches G 1 inch H 1 inch J inch K= inch This particular condenser was designed with a view to obtain tunmg characteristics lying approximately midway between- 1 and 2 employed in a multistage ampli er using vacuum tubes of the 201A type having an amplification factor of about 8, the results actually obtained are indicated by curve B ofFig. 6., On this graph sheet the curve A shows the calculated maximum permissible amplification at all frequencies be- Ker second and 1550 kilocycles per second. ccordin g to the theory on which curve A is based, the amplification should vary inversely as the square root of the frequency in order that a practical manufacturing tolerance may be allowed with respect to the plate-grid coupling capacity. This is well confirmed by actual experience. It will be observed that curve B closely follows the form of curve A and,

therefore, indicates an amplification which varies inversely as the square root of the frequency. In calculating the permissible amplification at the various frequencies from which curve A was plotted a tube capacity tolerance of 0.5 micromicrotarad was allowed. This tolerance represents the permissible manufacturing deviation of the internal capacity between the plate and grid. It is this unavoidable deviation which exercises the greatest influence in limiting the permissible amplification. Curve C, Fig. 6, indicates the radio-frequency amplification obtainable in a good representative neutralized receiver having radio-frequency transformers of the general form shown in Fig. l but without the metallic shields or equivalent means for accomplishing the same purpose. From an examination of curve (J it will be at once apparent that the amplification at the highfrequency end of the band is much greater than at the low-frequency end. Also by comparison of curve C with curve A it will be seen that the amplification at the low-frequency end is very much less than the permissible amplification, whereas it closely approaches the maximum permissible amplification at the high-frequency end.

Curve B indicates how the large discrepancy between permissible amplification and actual amplification at low frequencies and, in fact, over substantially the entire band of frequencies except for the immediate region of the upper end, has been corrected by the use of metal shields as herein described.

The specific structure which has been described as an example representative of the preferred embodiment of the invention is applicable for use in the radio-frequency portion of a neutralized broadcast receiver of which a partial circuit is shown diagrammatically in Fig. 7 This figure illustrates diagrammaticallytwo stages of neutralized ra (ho-frequency amplification the output of which may be passed through one or more additional stages of radio-frequency amplification and thence to adetector or it may be passed directly to the detector and thence to an audio-frequency amplifier.

In Fig. 7 the three-electrode vacuum tube amplifiers 20 and 21, respectively, are coupled in cascade through the medium of a radio-frequency transformer '22 which may be the transformer shown in Figs. 1 and 2. The primary Winding 23 and the neutralizing winding 24 of this transformer are interleaved on the same tube, as-previously described in connection with Fig. 1, and may be in all respects-identical. One end of the neutralizing coil 24 is connected to a neutralizing condenser 25 the other terminal of which is connected to the grid of the tube 20. This is in accordance with one form of the well known Hazeltine method of neutralization. The secondary winding 26 of the transformer 22, Fig. 7, may be the same as specified hereinbefore in connection with the description of Fig. 1. The tuning condenser 27 may be identical with that shown in Figs.

1, 2 and 5. The transformer 28 which couples the output side of tube 21 with the next succeeding tube (not shown) may be identical with transformer 22 and, of course, the tuning condenser 29 may be identical with the condenser 27. Likewise the neutralizing condenser 30 may be in accordance with the disclosure of Fig. 1 wherein the flexible element 11 comprises one plate of a neutralizing condenser of which the stator of the tuning condenser forms the other plate.

The present invention has been developed in conjunction with neutralized amplifiers of the type indicated in general by Big. 7 and is particularly well adapted for use in connection with that type of neutralization, but it may be used effectively with other methods of neutralization and oscillation suppression.

What is claimed is:

1. In a vacuum tube amplifier, a plurality of vacuum tubes, a tunable coupling means interconnecting the output side of one of said tubes with the input side of another of said tubes, said coupling means comprising a transformer having a primary winding and a secondary winding and a variable tuning condenser, and means operable to vary, simultaneously, the tuning capacity, the secondary self-inductance, the mutual inductance and the ratio of secondary to mutual inductance of said transformer whereby the degree of amplification may be increased as the frequency to which the amplifier is tuned is decreased.

2. In a vacuum tube amplifier comprising a plurality of vacuum tubes, a coupling transformer interconnecting the output side of one ofsaid tubes with the input side of another of said tubes, said transformer comprising a primary winding and a secondary winding, a variable tuning condenser connected across said secondary winding, and means operable conjointly with said condenser to decrease the self-inductance of said primary winding and of said secondary Winding and decrease the mutual inductance coupling system is tuned is decreased andvice-versa.

4; A multistage vacuum tube amplifier comprising a pair of vacuum tubes, a coupling'transformer interconnecting the output side of one of said vacuum tubes withthe input side of the other of said vacuum tubes, said transformer comprising a primary winding and a secondary winding, a variable tuning condenser connected across said secondary winding, said amplifier being intended to selectively amplify waves of all frequencies within a predetermined band of substantial width, said primary and secondar windings being so designed that when said condenser is adjusted to its maximum capacity, theresultant amplification will at least approximate the maximum permissible amplification as determined by the limit of stability, and a shield adjustable in position relative to said transformer and operable to variably restrict the magnetic path of said transformer, said shield being increasingly effective to decrease the self-inductance of said secondary winding and the mutual inductance of said transformer as it is moved into increasingly intimate relation to said windings.

5. A multistage vacuum tube amplifier comprising at least two vacuum tubes, a coupling transformer interconnecting the output side of one of said vacuum tubes with the input side of another of-said vacuum tubes, said transformer comprising a primary winding and a secondary winding, a variable tuning condenser connected across said secondary winding, said amplifier being intended to selectively amplify waves of all frequencies within a predetremined band of substantial width, said primary and secondary windings being so designed that when said conenser is adjusted to its maximum capacity the resultant amplification will at least approximate the maximum permissible amplification as determined by the limit of stability and an adjustable shield operable to variably restrict the magnetic path of said transformer,,said shield being increasingly effective to decrease the self-inductance of said secondary winding and the mutual inductance of said transformer as it is moved into increasingly intimate relation to said windings, an driving means operable to adjust the position of said shield conjointly with tuning adjustments of said condenser, whereby the secondary self-inductance and the mutual inductance of said transformer are maintained at'such values as will result in at least an approximation to the maximum input side of the other of said tubes, sail I coupling system comprising a high frequency transformer having a primary winding and a secondary winding, a variable tuning condenser connected in circuit with said secondary winding, a metal shield operatively associated with said transformer, said shield and said transformer being movable relative to each other, said shield being operable to vary the self-inductance of said secondary winding and the mutual inductance between said windings and also to vary the ratio of said self-inductance to said mutual inductance, said transformer and tuning condenser being designed to provide high amplifi cation with stability at low frequencies, and means for bringing said shield into increasingly intimate relation with said transformer as the operating frequency is increased whereby'the amplification is decreased with increasing frequency and'thereby maintained within the limit of stability at all frequencies.

7. A radio-frequency vacuum tube amplifier comprising a plurality of vacuum tubes, the output terminals of each tube being coupled with the input terminal of the succeeding tube through the medium of a tunable coupling system, means for neuiralizing the capacity coupling of each of said tubes. each of said coupllng systems comprising a transformer having a primary winding and a secondary winding, a variable tuning condenser connected in circuit with said secondary winding, said transformer being designed to permit of obtaining at least an approximation to the maximum permissible amplification at the lowest frequency to be amplified, a metal shield operable to restrict the magnetic path of said transformer to a variable extent dependent upon its position with respect to said transformer and means operable to adjust said tuning condenser and said shield conjointly whereby the magnetic path of said transformer is increasingly restricted as the effective capacity of said luning condenser is increased, the arrangement being such that. the secondary self-inductanee and the mutual inductance of said transformer are both reduced, as the frequency is increased, so that the amplification at each frequency is near to but not in excess of the maximum permissible amplification at that frequency.

8. In a vacuum tube coupling system, a radio-frequency transformer comprising a primary winding and a secondary winding, a tuning condenser connected in circuit with one of said windings, a conductive shield adapted to variably restrict the magnetic path of said transformer, and means operable to conjointly vary the capacity of said condenser and the position of said shield relative to said transformer.

9. A vacuum tube coupling system, comprising a transformer having a primary winding and a secondary Winding, a tuning condenser connected in circuit with said transformer, and a metallic shield for said transformer, said shield comprising two cuplike members, one of which is more particularly assocated with said secondary winding, said members being operative to vary the mutual inductance and the secondary selfinductance of said transformer, respectively, and driving means for adjusting the position of said shield relative to said transformer and the capacity of said condenser conjointly.

10. In a radio-frequency amplifier, a plurality of vacuum tubes and a plurality of tunable coupling systems interconnecting said vacuum tubes, each of said coupling systems comprising a transformer, a variable tuning condenser and a shield operable to vary the electrical characteristics of said transformer, and control means for-simultaneously varying the capacities of said condensers and the positions of said shields relative to said transformers.

11. The combination with a high-frequency transformer comprising a primary winding and a secondary winding of a variable condenser in circuit with said secondary winding, a metallic shield comprising two co-axial cup-like members, said shield being movable axially with respect to said windings and operable to variably restrict the magnetic paths thereof, and a mechanism for operating said condenser and said shield conjointly whereby said magnetic paths are restricted to the desired extent for each operating position of said condenser.

12. In a stage of tuned radio-frequency amplification employing a tuning system coupling two vacuum tubes said system including inductance and capacity, with means for neutralizing the capacity coupling of at least one of said vacuum tubes, the method of attaining high amplification, high selectivity and freedom from oscillations at all radiofrequencics within a wide range, with a single tuning control'nieans, which method comprises simultaneously decreasing the selfinductance of the tuning inductance, decreasing the tuning capacity and raising the stepup voltage ratio as the frequency is raised, at such respective rates that the voltage amplification varies approximately inversely as the square root of the frequency, whereby the limiting permissible variation of said coupling capacity is approximately the same at all frequencies.

13. The method of improving the effectiveness of a transformer-coupled high-frequency vacuum tube amplifier having a tuning condenser in circuit with the secondary winding of the coupling transformer, said method comprising :-automatically progressively decreasing the primary sclf-inductanc e, the secondary self-inductance and the mutual inductance of said transformer as the effective capacity of the tuning condenser is decreased, and progressively increasing the secondary self-inductance and the mutual inductance as the effective capacity of the turn ing condenser is increased.

14. The method of operating a tuned radio frequency vacuum tube amplifier having a tuning capacity, wherein vacuum tubes are interconnected through the medium of transformers, said method conslsting 1n ncreasing the reluctance of the magnetic path of the transformer as the tuning capacity is decreased.

15. In the operation of a vacuum tube amplifier of the tuned transformer-coupled type, the method of increasing the amplification a the lower frequencies witho'ut impairing the selectivity at the higher frequencies and with out exceeding the limit of stability at any frequency, said method consisting in automatically varying the primary self-inductance, the secondary self-inductance and the mutual inductance of the transformer simultaneous 151 while tuning.

16. In a radio-frequency coupling system an inductance coil, means for tuning said system over a relatively wide range of frequencies, a conductive shield comprising a cuplike member, said shield being movable axially with respect to said coil and adapted to variably restrict the magnetic paths thereof, and means operable to vary the position of said shield conjointly with said tuning means.

17. In a radio frequency coupling system, an inductance coil. means for tuning said system over a relatively wide range of frequencies, a conductive shield comprising a plurality of cup-like members, said shield being movahle axially with respect to said coil and adapted to variably restrict the magnetic paths thereof, and means operable to vary the position of said shield conjointly with said tuning means.

18. In a vacuum tube amplifier system ineluding a radio-frequency coupling inductance, means for tuning said system over a relatively wide range of frequencies, wherein a voltage built up across said inductance tends to produce oscillations at the higher frequencies of said range, a conductive shield positioned in the magnetic paths of said inductance and movable relatively thereto, and means for effecting relative movement between said shield and said inductance and interrupting said paths to decrease said voltage proportionately as said system is tuned to higher frequencies.

19. In a vacuum tube amplifier system including a radio-frequency coupling inductance, means for tunlng said system over a relatively wide range of frequencies, whereln voltage built up across said inductance ned to higher frequencies. 20. In a Vacuum tube amplifier system including a radio-frequency coupling inductance, means for tuning said system over a relatively wide range of frequencies, wherein voltage built up across said inductance tends to produce oscillations at the higher frequencies of said range, a conductive shield positioned in themagnetic paths of said inductance and movable relatively thereto, and means for effecting relative movement between said shield and said inductance and interrupting said paths to decrease said oscillation producing voltage automatically as without impairing the selectivityat id system is tuned to higher frequencies. In testimony whereof I aflix my signature.

LINCOLN WALSH.

place-AMER 1,894,503r-I1i7g00ln I I dated'January 17, 1933.. v Disclaimer filed May 29, 1934,-by the patentee, 7 the assignee,]Ha; zeltine Corporation, consenting. I Hereby 'enters this disclaimer ofclaim 15 in said specification, which is in the following words, to wit:

f15. In the operation of a vacuum tube amplifier-of the tuned transformercoupled type,-,. the method of increasing-the amplification at the lower frequencies the higher frequencies and without exceeding the limit of stabilit at any frequency, said; method consisting in automaticallyvarymg the primary se -inductanc.e, the secondary self-inductance and the mutual mductanoe ofthe transformer simultaneously whiletuning.

- [Qfidiul Gazette June19, 1934.]

Walsh,oElizabeth, Nj. J. RAmo Fnnounnc rifittrmrina; "Patent'

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