US3065368A - Cathode ray device - Google Patents

Description

E. ATT] 3,065,368

CATHODE RAY DEVICE Nov. 20, 1962 Filed Dec. 27, 1957 2 Sheets-Sheet 1 wrmssszs:

I v R b Eros At ZM cilia? ATTORNEY United States 3,065,363 CATHODE RAY DEVHCE Eros Atti, Breeseport, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed Dec. 27, 1957, Ser. No. 705,583 9 Claims. (Ci. 313-711) This invention relates to electron devices and, more specifically, to those of the cathode ray type which make use of intensity modulated electron beams.

The electron gun of this invention concerns a high transconductance, high resolution, broadband gun. There are many applications for such a device. One, for instance, is within a television picture tube in which an electron gun is utilized to generate an electron beam which is scanned across a phosphor screen to display a light image thereon by modulation of the electron beams current intensity.

Various types of electron guns have been so far employed by the picture tube industries. One particular structure of an electron gun is disclosed in U.S. Patent 2,773,212 entitled: Electron Gun by J. H. Hall and issued December 4, 1956. The normal electron gun is constituted basically of two axially arranged systems: the beam forming and modulating system and the principal lens system. The purpose of the beam forming and modulating system is to provide a well-defined electron beam of controlled intensity to the principal lens, which provides for the focusing of the beam into a small spot upon the viewing screen. The principal lens of the gun may be an electrostatic, magnetic or electromagnetic lens. Most electron guns, including the ones used in present picture tubes, possess a beam forming and modulating system which comprehends a lens, the socalled immersion lens.

This invention concerns basically the beam forming and modulating system of the gun and for this reason the descriptions and drawings will refer principally to this portion of the gun. For the television display systems now used in commercial television, the electron beam must be amplitude modulated within quite wide limits in order to reproduce the pictures highlights, low lights and all the intermediate values of brightness which are required for an adequate picture presentation. For instance, in a direct viewing picture tube, the beam may reach peak values as high as 1500 microamperes in the pictures peak highlights, and it must go as low as microamperes for beam cutoff in the blacks. To modulate the beam throughout this current range requires the application of relatively large picture signals to the beam modulating electrode of the gun. A typical value of video drive voltage required to change the beam from 0 to 1500 microamperes is about 5070 volts in a modern television gun. Such a high drive is required because of the guns low transconductance or gm, that is, the change in beam current per unit voltage change of the voltage applied to the modulating electrode. The gm is usually only a few microamperes per Volt, well below the gm figure normally found in typical radio receiving tubes.

The low amplitude picture signal supplied by the conventional receivers detector, usually less than volts peak to peak, cannot be directly fed to the picture tube, but has to be amplified to a much higher level of the order of 25 times higher. This calls for a video amplifier capable of providing a relatively large gain throughout the entire 3.5 megacycles/second bandwidth of the conventional picture signal, that is, an amplifier having a gain X bandwidth product of about 25 3.5=87.5 megacycles. The video amplifier in a conventional television set is a relatively high-powered and expensive amplifier because of the high output drive voltage and the large total shunt capacitance Ct==C0+Ci+Cs associated with the picture tube drive circuit, namely: the output capacitance C0 of the video amplifier tube, the input capacitance Ci of the cathode ray tube, and the addi tional capacitance Cs due to wiring, sockets and other components associated with the picture tubes drive circuit. The insistent demand of the television industry for a reliable high transconductance cathode ray gun of good performance and low cost, suitable for low video drive picture tubes, has provided incentive for a large amount of work to improve the gm of the present television guns.

The average gm of a typical television gun throughout the 01500 micramperes beam current range is only 20-30 microamperes per volt, which is about two orders smaller than the average gm of a typical radio receiving tube. The low gm figure, characteristic of all electron guns, is due to the extremely stringent electronoptical requirements which must be met by guns in order to be able to reproduce sharp, bright pictures. Picture sharpness, or high resolution pictures, can be obtained only if the electrons leaving the gun are brought to land upon the viewing screen concentrated in a small area or spot, while picture brightness can be obtained only if the gun can provide sufficient beam current into the spot. Unfortunately, the high resolution and high brightness requirements are mutually conflicting, because the size of the spot increases with the beam current.

The eiforts of the industry to obtain high brightness and resolution have led to the development of the presently used television guns possessing beam forming and modulating systems utilizing an extremely small cathode emissive area, of the order of about 0.5 square millimeter, which is less than l%5% of the total useful cathode area available in the gun. This factor accounts in a large degree for the guns low transconductance. Another factor also contributing in lowering the guns gm is that the beam forming and modulating system operates as a variable mu type tube chiefiy because of the fact that the electric fields of the system must have an axially symmetric configuration.

It should be noted however, that if the transconductance per square centimeter of utilized cathode is considered instead, a television gun has an average gm about 200 times larger than the 20-30 microamperes per volt figure mentioned above, or an average gm of 4000- 6000 microamperes per volt per centimeter square, which is about the same gm of a good radio receiving tube possessing the same cathode area. Thus the gm of the present electron gun may be considered to be not too far away from the peak value that the present design allows. Any attempt to improve it by a factor of 25, without providing for a substantial change in the utilized cathode area may well prove to be a very difiicult task as the many attempts of the past seem to clearly bear out.

It has been proposed that a higher gm may be obtained by applying fine wires or meshes across the beam modulator grid aperture. The idea is to break up the beam into a multitude of smaller, weaker beams of much lower cutofi. This method of paralleling many low cutoff beams is very effective in increasing the gm. Unfortunately, however, it seriously deteriorates the resolution capabilities of the gun because of the distortion introduced in the axially symmetric fields associated with the beam modulator grid aperture. In other words, the means chosen to boost the guns gm interfere with the guns immersion lens and degrade the optics of the system. An additional serious complication is the very critical close spacing required between cathode and wires or mesh when negatively biased beam modulating electrodes are used, as generally occurs.

An entirely different approach to solving the problem is by making use of a larger cathode area. The current emitted by a large cathode area is controlled by a fine array of thin wires, closely spaced to the cathode. The electron current, subdivided in many electron beams, is then focused upon a very small limiting aperture which acts as an electron point source, supplying the principal lens of the gun with a divergent electron beam, of controlled intensity. The aperture has to be small enough that the enlarged image of it upon the viewing screen, provided by the principal lens, is still small enough to satisfy the resolution requirements of the gun. The real problem here is to concentrate most of the cathode current upon the tiny limiting aperture to avoid an excessive current interception by the electrode containing said aperture. This problem, together with the increased gun length, are but two of the weak points of this approach. Still other workers have tried to increase the gm by using einzel lenses in the beam forming and modulating system but have only partially succeeded in their objective.

This invention provides means to greatly increase the guns transconductance at no cost to the guns resolution or brightness performance. It permits obtaining the very desirable objective of substantially separating the two distinct functions of beam formation and beam modulation. The guns drive characteristic can be designed now without interfering with the guns resolution and brightness characteristics, and thus one of the major stumbling blocks in which many of the past high gm guns have incurred may be overcome.

This high gm min has other important characteristics such as a very low capacitance associated with the beam modulation system. Therefore, very wide bandwidth guns requiring small driving power are made possible. In addition, the gun has a short length, appreciably shorter than standard guns of identical electron optical performance. There are still other important advantages such as manufacturing simplicity, non-critical operation, low cost, uniformity of characteristics and reliability.

It is accordingly an object of this invention to provide an electron gun with a greatly increased transconductance.

Another object is to provide an electron gun with a greatly improved bandwidth performance.

Still another object of this invention is to provide a highly efficient and reliable electron gun incorporating an amplifier therein requiring a minimum number of additional electrodes.

A further object of this invention is to provide an electron gun substantially shorter than the presently available electron guns.

Another object still is to provide an improved electron gun of more uniform and reproducible characteristics.

These and other objects are effected by my invention as will be apparent from the following description taken in accordance with the accompanying drawing throughout which like reference characters indicate like parts, and in which:

FIGURE 1 is a schematic view of a cathode ray tube embodying the prinicples of my invention;

FIG. 2 is a side view in section of a portion of the gun shown in FIG. 1;

FIG. 3 is a top view in section of the structure shown in FIG. 2;

FIG. 4 is a schematic view of FIGS. 2 and 3 to explain the invention;

FIG. 5 is a side view in section of a modification of the structure shown in FIG. 2;

FIG. 6 is a top view in section of the structure shown in FIG. 5;

FIG. 7 is a side view in section of a modification 0f the structure shown in FIG. 2; and

FIG. 8 is a top view in section of the structure shown in FIG. 7.

Referring in detail to FIG. 1, there is shown a cathode ray tube embodying my invention. The tube is comprised of an envelope 12 having a tubular neck portion 14 a flared bulb portion 16 and a face plate 18. The face plate of the envelope has a suitable fluorescent coating 20 of phosphor material deposited on the inside surface thereof and may have an electron permeable electrical conductive coating 22 deposited on the surface of the fluorescent coating remote from face plate 18. The flared portion is of the bulb is also provided with an electrically conductive coating 24 of a suitable electrical conductive material such as carbon or aluminum, which extends into the neck portion 14 of the envelope.

An electron gun 3th is mounted within the neck portion 14 of the envelope. The gun 3!} consists essentially of a beam forming and modulating system and a principal focusing electron lens electrode system. The conductive coating 24' on the interior surface of the flared bulb portion 16 serves as an anode of the cathode ray tube to which suitable voltage is supplied by means of an exterior terminal 26. V

The general gun structure described and shown herein is of the electron gun type known as electrostatic focus type. There is a suitable deflection system, such as electromagnetic, provided on the exterior of the neck portion of the envelope and illustrated by the deflection yoke 28. The yoke 28 by application of suitable signals provides means of deflecting the electron beam generated by the gun 30 to scan a raster on the screen 20.

The beam forming and modulating system of the gun 30 is illustrated in enlarged detail in FIGS. 2 and 3. The system is comprised of an indirectly heated cathode 36, a modulator grid 40 and a first accelerator anode or screen grid 46. The beam forming and modulating system incorporates also a triode amplifier structure consisting of the indirectly heated cathode 36, a control grid 50 and an anode 56. In the specific embodiment in FIGS. 2 and 3, the screen grid 46 consists of a cup-shaped member 49 of electrically conductive material with an open end on the side facing the fluorescent screen 20. The open end, which may be either totally or partially open, has its opposite end 45 closed by a flat rectangular sheet or plate portion 47 having a centrally located aperture 48 therein. The flat sheet or diaphragm 47 is of electrically conductive material and is substantially normal to the axis of the electron gun. The screen grid 46 may of course consist of just the apertured diaphragm 47 as shown in FIGS. 5, 6, 7, 8. The tubular skirt portion 49 of the member 46 is not required in all applications. The member 56 may be in the form of a U-shaped member positioned on the opposite side of the diaphragm 47 with respect to the tubular skirt portion 4-9. The ends of the legs on the member 56 are attached to the apertured diaphragm 47 to form a tubular enclosure perpendicular to the axis of the gun. The middle portion of the U-shaped member has a reentrant centrally located portion which constitutes the active anode portion 57 of the anode member 56 of the triode amplifier.

Positioned within the tubular enclosure formed by the apertured diaphragm 47 and the U-shaped member 56 is the cathode 36. The cathode 36 consists of a tubular sleeve 35 of rectangular cross section having a heater 34 disposed therein. The sleeve 35 has its larger area sides substantially parallel to the apertured diaphragm 47 and to the active portion 57 of the triode anode 56 which is of similar area as the surface of the sleeve 35 facing said surface 57. The surface of the sleeve 35 facing the anode active area 57 has a coating 39 of a suitable electron emissive material. A coating 37 of electron emissive material is also placed on the surface of the sleeve 35 facing the apertured diaphragm 47. The coating 37 is aligned with the aperture 48 in the diaphragm 47 and has an area larger than that of the adjacent aperture 41 of the modulator grid 40. The modulator grid 40 is positioned between the cathode 36 and the diaphragm 47 of the screen grid 46. The modulator grid 40 is of electrically conductive material. The aperture 41 is in alignment with the gun axis and with respect to the aperture 48 in the screen grid diaphragm. Positioned on the opposite side of the cathode 36 with respect to the modulator grid 40 is a conventional planar type control grid 50, as utilized in receiving tubes for instance, and in the form of a mesh or plurality of parallel wires supported by a frame. The various electrode elements of the beam forming and modulating system are positioned by means of suitable insulating spacer members 68 on each end thereof. This technique of assembly is well known in the receiving tube art.

The principal focusing electrode lens system consists of a first anode 60 comprised of a tubular or skirt member and a second anode 70, also a tubular member, spaced along the axis of the tube from the first anode 60. The end of the skirt portion of the first anode nearest the screen, is closed by a diaphragm perpendicular to the axis of the tube and having a centrally located aperture therein. The end of the second anode facing the first anode also has a diaphragm aperture provided therein. Means in the form of contact members are also provided on the end of the second anode nearest the screen for making electrical contact to the coating 24 and also positioning the electron gun within the tube neck. The first and second anodes 60 and 70 are also connected together electrically and are supplied with voltage from the coating 24.

A sleeve or focusing electrode 64 with a larger diameter than the skirt portions of the first and second anodes 60 and 70 surrounds the space between the first and second anodes and is coaxial with said anodes. The principal focusing electrode systemis of the conventional univoltage type structure and it is not intended to be illustrated here in detail. The electrodes of the electron gun are normally spaced by providing radial projecting anchor pins from the electrodes and embedding the pins within longitudinal glass support rods.

Referring in detail to FIG. 4, a schematic view of the beam forming and modulating system of the cathode ray gun illustrated in FIGS. 2 and 3 is shown to describe the operation. The cathode 36 is connected to the point 101 of a suitable voltage source shown as a battery 72. The screen grid 46 and the anode member 56 which are mechanically and electrically connected are biased positively with respect to the cathode 36 by being connected to a point 100 of the battery 72 through a load resistor 76. The control grid 50 of the amplifier triode is biased at a negative potential E with respect to the cathode 36 by being connected to a point 102 of the battery 72. The modulator grid 40 is biased at a variable negative potential E by being connected to a variable point between 101 and 103 of the battery 72 through a resistor 82. The structure which forms the first accelerator anode or screen grid 46 of the gun and the anode 56 of the triode amplifier is connected to the modulator grid 40 by means of a coupling capacitor 84. The value of the capacitance of capacitor 84 is such that, throughout the frequency range of the video signals bandwidth obtained from a suitable television receiver, the modulator grid 40 may be considered to be substantially operating at the same A.C. voltage as the grid 46 and anode 56.

The operation of the beam forming and modulating system may be described as follows. In well-designed guns the electron beam current i flowing through the aperture 41 of the negatively biased modulator grid 40 is essentially a function of the voltage applied to the modulator grid 40 and screen grid 46 of the gun. The electron beam current 1' increases or decreases when the voltage applied to either one or both grids becomes respectively more or less positive. For the high transconductance electron gun object of this invention, the voltage 6 V of the screen grid 46 of the gun is identical with the voltage V of plate 56 of the triode amplifier. But:

F is the voltage applied between the cathode 36 and the load resistor 76;

I is the triodes plate current;

i0 is the cathode ray beam current intercepted by the screen grid 46.

(The above relationship holds at all frequencies at which the shunt capacitance effect may be neglected.) Since the beam current i0 intercepted by the screen grid in present guns is usually very small, it necessarily follows that the voltage V of the guns screen grid 46 is determined essentially by the electron current I flowing from the cathode 36 to the plate 56 of the triode. V decreases with increasing plate current I and vice versa. Since I increases or decreases according to whether the signal E applied to the control grid 50 becomes more or less positive, the beam current i is modulated by the plate voltage variations caused by the video signal E positive variations of l3 causing negative variations of beam current 1'. To improve the high frequency response of the amplifier an inductor of suitable value may be used in series with the load resistor 76. V

The beam modulation process just described is accompanied by a second one due to the influence upon the beam current i of the same plate voltage variations simultaneously applied to the modulator grid 40 by means of the coupling capacitor 84. Because of the'closer cathode 36 to modulator grid 40 spacing the electron beam current i modulation due to this latter modulation is generally much larger than that caused by the screen grid 46 modulation process previously described. .As a result of this double modulation mechanism, the beam current i change for a given voltage change of the control signal E is greatly enhanced. Consequently, if E is the control voltage swing required by a prior art gun to produce, for instance, a 0-1500 microamperes swing of the beam current i, the high gm gun of this invention will require a control voltage swing G times smaller, G being the voltage gain provided by the triode amplifier incorporated in the gun. It can be shown that the voltage gain G can attain unusually high values in this gun, thus making possible guns requiring only l-2 volts drive instead of the 50 volts drive demanded, for instance, by present guns of identical performance. A picture tube drive of the order of only 1-2 volts requires that the gun-amplifier system be capable of providing a gain G bandwidth B product of about:

G B=25 3.5 mc.50 3.5 mc.=87.5- me.

Since said product is given by the relation:

where gm is the transconductance of the amplifier triode and C =C0+Ci+C is the total shunt capacitance earlier mentioned, the attainment of the high GXB values above indicated becomes quite a simple matter with this gun because of the very small shunt capacitance C: value inherent to the gun, as compared with the C associated with the systems in present use wherein C and gm may have typical values of 15 micromicrofarads and 7000 microamperes per volt respectively. The resulting gain bandwidth product G B is therefore only:

and video drive signals in excess of 2 v. are therefore required by present systems. The high gm gun has a shunt capacitance C considerably smaller than 15 micromicrofarads because of several features which drastically re duce it. For example, because of the interpenetration of the amplifier section and of the beam forming and modulating section most electrodes of these two sections are common or double-purpose electrodes. Consequently, a duplication of the capacitances, associated with these electrodes and their respective leads, is in large degree avoided. The total capacitance C, terms, due to the amplifier output capacitance C and to the modulator grid input capacitance C are therefore considerably reduced. Furthermore, for the same reason, there is no need for the usual lOng connector going from the present final video amplifier stage to the input of the picture tube. In addition to all this, the gun requires only one socket and one socket wiring instead of the two required for the systems in present use. Both these two latter features produce a drastic cut in the shunt capacitance term C Guns have been made with a gm of the amplifier triode of 6000 microamperes per volt and a C +C capacitance of only 3 micromicrofarads. Assuming a C of 2 micromicrofarads, we find that C =Co+Ci+C =5 micromicrofarads and the gain bandwidth product of such a gun becomes:

which is almost three times higher than the 75 mc. value typical of present-day systems.

The low shunt capacitance value O, of the gun has other favorable efiects besides that of vastly increasing the gain-bandwidth product. It also results in a large cut in the amplifiers anode power requirement which becomes only a fraction of that presently required. From the standpoint of physical dimension the high gm broad band gun of this invention has appreciably shorter length than the present low gm gun.

The factors contributing to such an improvement are the following:

(1) The traditional Wehnelt cylinder or modulator grid #1, /2 inch long usually, has been replaced by a planar grid #1 only .010-020 inch thick.

(2) The cathode is horizontal instead of vertical as in present guns.

(3) The heater is introduced from the side of the gun rather than from underneath the gun. This permits reducing the relatively large spacing between the gun and the glass stern presently required in order to insert the heater inside the cathode after the gun has been assembled to the stem.

A typical beam forming and modulating system of the high gm broadband type is only about A inch long, while the corresponding system of a present low gm gun is about twice this length. Picture tubes about /2 inch shorter are made possible by this invention.

The use of mica spacers 68, as supporting and spacing elements, permits a. spacing of the various electrodes of the beam forming and modulating system of the gun more accurate than that allowed by the present gun structure, wherein individually selected cathode-modulator grid spacers are required because of the variable cylindrical cathode height. This advantage derives from the fact that the mica spacers, used to secured the cathode, the modulator grid, and the control grid to the first accelerator anode of the gun, may be punched with very close tolerances. Consequently, beam forming and modulating systems of more reproducible characteristics are made possible With improved gun performance not only, but also at a smaller cost.

This invention has been described in a few typical embodiments. Other ones may be easily devised. For instance, more than one side of the cathode may be used to provide current to the amplifiers plate. This allows for an increased transconductance of the amplifier section and allows also for a fuller and better use of the existing electrodes, electrodes capacitances, heater power supplied to the cathode, with the benefit of an enhanced overall gun performance. One of these highly efiicient gun structures is shown in FIGS. 5 and 6. Most of the cathode area 36 of both wide sides and partially of both narrow sides is being utilized by the triode amplifier, while only a small portion of the cathode side facing the aperture of the modulator grid 40 is used to form the electron beam i. The modulator grid 40 may be fastened to the screen grid 46 by means of an insulating spacer which may be bonded to members 46 and 40 by suitable materials.

Other obvious embodiments of my invention may also be mentioned here as those wherein amplifier structures of the well-known tetrode, beam tetrode, or pentode type are utilized in the beam forming and modulating system. These differ from the triode structure so far described because of the addition of one or two extra grids @2 and 94 inserted between the control grid 50 and the anode 56. The grids illustrated in FIGS. '7 and 8 are of the socalled frame type, while the one illustrated in FIGS. 5 and 6 is of the more conventional type. A tetrode or pentode type of amplifier permits greatly decreasing the feedback between output and input of the amplifier. Among the advantages gained is a higher input impedance of the electron gun which allows for a large reduction in input driving power requirement.

The suppressor grid 94 may be tied directly to the cathode 36 of the amplifier, while the screen gird so may have to be brought out through a separate lead in order to be biased at a fixed potential. It could, for instance, be tied to point of the battery 72 or to an intermediate point between 100 and 101 as shown in FIG. 4.

If the load resistor 76 is inside the picture tube, then it is possible to dispense with a separate lead for the screen grid 92 because it may be tied to the extremity of load resistor 76 and thus improve the shunt capacitance situation at the same time.

In still another embodiment the aperture of the first accelerator anode 46 may be obtained directly from the amplifiers anode as shown in FIG. 5. Finally, instead of the grid 40 being A.C. coupled to the anode 56 of the beam forming and modulation system, by means of the coupling capacitor 84 as hitherto described, the beam modulator grid 40 may be D.C. coupled to the anode 56 so as to maintain the DC. component of the input modulating signal in the modulation of the beam.

While I have described my invention in only a few forms, it will be obvious to those skilled in the art that it is not so limited, but is susceptible to various other changes, and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. An electron beam forming and modulating system, comprising a first and second electron discharge path, said first path comprising a cathode, an intensity control grid, and a multipurpose accelerating electrode having an aperture therein from which emerges an intensity modulate-d electron beam, said second path comprising a cathode, an amplifier grid having a plurality of apertures, and an anode, said cathode being a common electrode, said multipurpose electrode being formed to provide the acceleration electrode and said anode, said cathode electrode and said multipurpose electrode tubular in form with their axis perpendicular to the axis of the electron beam emerging from the anode of said first path.

2. An electron beam forming and modulating electrode system, said electrode system comprising a multipurpose anode electrode, a cathode member and two control electrode members to provide a first and a second electron beam path, said multipurpose electrode comprising a tubular member position substantially perpendicular to the paths of said electron beams, said cathode member also a tubular member and positioned within said anode member and having its axis perpendicular to the paths of said electron beams, a portion of said multipurpose electrode having an aperture therein forming the anode for said first path, a control grid having an aperture therein positioned between said anode portion of said first path and said cathode to form said first path, the remaining portion of said multipurpose electrode forming the anode of said second path and a control grid having a plurality of apertures between said remaining portion of said anode member and said cathode to form said second path.

3. A cathode ray tube comprising a target electrode, an electron gun for forming and directing a small cross sectional electron beam along a first path to said target, said electron gun comprising a sleeve member having on its outer surface a first electron emissive surface facing said target and a plurality of beam forming electrodes and focusing electrodes spaced along said first path, said beam forming electrodes including a control electrode and a screen electrode in the order named between said electron emissive surface and said focusing electrodes, each of said control and screen electrodes including a planar portion having an aperture provided therein in said beam path, an amplifying system associated with said gun including a second electron emissive surface on the opposite surface of said sleeve member with respect to said target, an amplifier control electrode adjacent said second electron emitting surface having a plurality of apertures therein, an amplifier anode for collecting the electrons emitted from said second electron emissive coating, said amplifier anode electrically connected to said screen electrode and operating at the same potential as said screen electrode.

4. A cathode ray tube comprising a target electrode, an electron gun for forming and directing a small cross sectional electron beam along a first path to said target, said electron gun comprising a tubular member having a first electron emissive coating on the outer surface of said tubular member facing said target, a plurality of beam forming and focusing electrodes spaced along said first path, said beam forming electrodes including a control electrode and a screen electrode in the order named between said first electron emissive surface and said focusing electrodes, said control and screen electrode including a plate portion having an aperture positioned therein and lying in said beam path, an amplifying system associated with said gun including a second electron emissive coating of larger area than said first coating on the opposite surface of said tubular member with respect to said target, an amplifier control electrode adjacent said second electron emissive coating having a plurality of apertures therein, an amplifier anode for collecting the electrons emitted from said second electron emissive coating, said amplifier anode electrically connected to said screen electrode and operating at the same potential as said screen electrode.

5. A cathode ray tube comprising a target electrode, an electron gun for forming and directing a small cross sectional electron beam along a first path to said target, said electron gun having a first electron emissive surface facing said target and a plurality of beam forming and focusing electrodes spaced along said first path, said beam forming electrodes including a control electrode and a screen electrode in the order named between said first electron emissive surface and said focusing electrodes, said control and screen electrode including a plate portion having an aperture therethrough in said beam path, an amplifying system associated with said gun including a second electron emissive surface of larger area than said first surface, an amplifier control electrode adjacent said second electron emissive surface and having a plurality of apertures therein, an amplifier anode for collecting the electron emitted from said second electron emissive surface, said amplifier anode electrically connected to said screen electrode and operating at the same potential as said screen electrode.

6. A high transconductance cathode ray tube comprising an envelope and having therein an electron gun structure to provide an electron beam directed to impinge upon an image display screen in said envelope, said gun comprising an amplifier including a cathode, an amplifier control grid and an amplifier anode electrode, a control grid and an accelerating anode for focusing the electrons within said electron beam, said amplifier anode electrically connected to said accelerating anode.

7. A high transconductance cathode ray tube comprising an envelope and having therein an electron gun structure to provide an electron beam directed to impinge upon an image display screen in said envelope, said gun comprising an amplifier including a cathode, an amplifier control grid and an amplifier anode electrode, an electrode assembly having an electron beam source, a control grid and an accelerating anodefor focusing the electron within said electron beam, said amplifier anode electrode electrically c onnected to said accelerating anode and operating at the same potential.

8. A high transconductance cathode ray tube comprising an envelope and having therein an electron gun structure to provide an electron beam directed to impinge upon an image display screen in said envelope, said gun comprising an amplifier including a cathode, an amplifier control grid and an amplifier anode, an electrode assernbly having an electron beam source, a control grid and an accelerating anode for focusing the electron within said electron beam, said amplifier electrode and said accelerating anode electrically connected together and operating at the same potential, said control grid of said electrode assembly electrically coupled to said acceleration electrode to follow the voltage on said accelerating anode.

9. A high transconductance cathode ray tube employing an electron gun disposed Within the envelope thereof formed to provide an electron beam directed to impinge upon an image display screen, said gun comprising a cathode, an amplifier grid, an amplifier anode formed with an aperture therein, a control grid for the electron beam originating from said cathode and passing through said aperture, and an electrode assembly formed to provide acceleration and focusing of said beam.

References Cited in the file of this patent UNITED STATES PATENTS 2,055,174 Kwartin Sept. 22, 1936 2,163,210 Wienecke June 20, 1939 2,173,498 Schlesinger Sept. 19, 1939 2,175,690 Happe Oct. 10*, 1939 2,180,957 Hollmann Nov. 21, 1939 2,449,339 Sziklai Sept. 14, 1948 2,454,204 Raymond Nov. 16, 1948 2,661,436 Van Ormer Dec. 1, 1953 2,806,163 Benway Sept. 10, 1957 FOREIGN PATENTS 949,187 Germany Sept. 13, 1956 495,732 Great Britain Nov. 18, 1938

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