US3065376A

US3065376A - Electron beam device

Info

Publication number: US3065376A
Application number: US748338A
Authority: US
Inventors: Atti Eros
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1958-07-14
Filing date: 1958-07-14
Publication date: 1962-11-20
Anticipated expiration: 1979-11-20

Description

Nov. 20, 1962 E. ATTI 3,065,376

ELECTRON BEAM DEVICE Filed July 14, 1958 3 Sheets Sheet 1 i 20 I8 g 22 24 I2 12 l- 60 :21 :III' "a Three H II- 90 Fig.l. 70 I4 WITNESSES. INVENTOR Nov. 20, 1962 E. ATTl ELECTRON BEAM DEVICE 3 Sheets-Sheet 2 Filed July 14, 1958 Fig.

Fig.5.

2 s M T T O 7 6 3 3 I w .lHl l m n v m h T 1.3;. T m l l l Fig. 6.

Nov. 20, 1962 E. ATTI ELECTRON BEAM DEVICE 3 Sheets-Sheet 3 Filed July 14, 1958 Fig.9.

United States Patent 3,065,376 ELECTRON BEAM DEVICE Eros Atti, Horseheads, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., :1 corporation of Pennsylvania Filed July 14, 1958, Ser. No. 748,338

Claims. (Cl. 315-30) This invention relates to electron devices and more specifically, to those of the cathode ray tube type which make use of intensity modulated electron beams.

In these devices, an electron gun is normally provided to generate and form an electron beam of pencil like form. One particular application of such a device is within a television picture tube in which an electron gun is utilized to generate an electron beam of small cross sectional area which is scanned across a screen which emits light in response to electron bombardment and thereby displays a light image corresponding to the intensity modulation of the electron beam.

Various types of electron guns have been employed in the'picture tube industry and one particular structure which is representative of the common type gun structure is disclosed in US. Patent 2,773,212, entitled Electron Gun, by J. H. Hall, issued December 4, 1956, and assigned to the same assignee. The normal electron gun is constituted basically of at least two axially arranged systems; the beam forming and modulation system and the principal lens system. The purpose of the beam forming and modulation system is to provide a well defined electron beam of controlled intensity to the principal lens. The principal or main lens provides the focusing of the electron beam into a small spot on the viewing screen. The principal lens of the gun may be electrostatic or electromagnetic. Most electron guns, including the ones used in present television tubes, possess a beam forming and modulation system which comprehends a lens known as an immersion lens.

This invention concerns basically the beam forming and modulation system of the electron gun and the description and drawings will refer principally to this portion of the gun. In the present type television display systems used in commercial applications, the electron beam must be intensity or amplitude modulated within wide limits in order to reproduce the pictures high lights, low lights and all the intermediate values of brightness which are required for adequatepicture presentation. For example, in a direct viewing tube, the beam may reach peak values as high as 1500 microamperes in the pictures peak high lights and it must go as low as zero microamperes for beam cutoff in the blacks. To modulate the beam throughout this current range, it is required that a relatively large picture or video signal be applied to the beam modulating electrode of the gun. A typical amplitude of video voltage required to drive the beam from its cutoff level up to 1500 microamperes peak level is about 60 volts in the modern television tube. Such a high amplitude driving signal is required primarily because of the electron guns low transconductance, that is, change in beam current per unit voltage change of the voltage applied to the modulating electrode. In conventional television receivers, the video detector supplies a signal which is less than 5 volts peak topeak and of course, this signal cannot be utilized to directly, drive the electron gun. It is necessary that this signal be amplified to a much higher level, usually of the. order of 25 ,times. This requires that a video amplifier be utilized between the detector and the picture tube, capable of providing a relatively high gain throughout the entire band width of a conventional picture signal which is about 3.5 megacycles per second. It is, therefore, seen that it is necessary to 3,065,376 Patented Nov. 20, 1962 provide a relatively high power and expensive video amplifier because of the already mentioned high output drive voltage required, and of the large total shunt capacitance associated with a picture tube drive circuit. This shunt capacitance is the sum of various capacitances such as: the output capacitance of the video amplifier tube, the input capacitance of the picture tube and the capacitances due to wiring, sockets and other'components associated with the picture tubes drive circuit.

The insistent demand of the television industry for a reliable low drive cathode ray tube of good performance and low cost has generated a large amount of work and research to improve the transconductance of present television tubes. In my copending application entitled Electron Device, Serial No. 705,583, filed December 27, 1957, and assigned to the same assignee, I have described a high transconductance wide band television gun which has been referred to as the screen grid amplifier gun. This electron gun is a combination of-a conventional screen grid gun with an amplifier constituted in large measure by the existing electrodes in the electron gun; It is a very simple and economical structure which may include only one more electrode than the conventional low transconductance type gun. In the copending application, the gun is provided with three grids which participate in the beam modulation process. The structure consists of an auxiliary input grid and the two conventional control and screen grids of the immersion lens. These two conventional grids are coupled together and are operated out of phase with respect to the auxiliary input grid.

In this invention, there is provided an improved structure which operates from What is referred to in the television work as a black negative signal. In most current design television sets the video signal supplied by the picture detector to the final video amplifier tube, which drives the picture tube, is of the black negative type. This means that the electron beam decreases when the video signal goes more negative. Thus, black is achieved when the video component of the signal reaches its peak negative value. The negative polarity video signal supplied by the picture detector not only drives the final video amplifier tube but is often used to control at the same time the automatic gain control (or A.G.C.) circuit of the receiver. The high transconductance gun described in my copending patent application 705,583 demands a black positive signal instead of a black negative signal. In certain cases this may be an inconvenience because if, in order to achieve the proper signal polarity required by the picture tube the second detectors polarity is reversed, such signal may not be suitablefor the A.G.C. circuit unless this circuit is redesigned. The gun described in this invention eliminates this difliculty' by operating with a black negative signal.

Another important characteristic'of the new gun resides in its extremely low total shunt capacitance. *It is well known that the shunt capacitance should be kept as low as'possible to achieve beam modulation with the widest possible bandwidth by means of the smallest possible amount of power.

To illustrate this point, a few specific examples are given here. in a typical television set, the total shunt capacitance associated with the cathode ray beam modulation system of a current television picturetube is somewhere around 20-38 micromicrofarads, of which only about 6 micromicrofarads are contributed by the electron gun these various components. The high transconductance gun of my copending patent application 705,583 represents a decisive improvement in this respect because it eliminates many of these external components just mentioned including the final video amplifier tube and socket. For this reason, the shunt capacitance contributed by the circuitry in the case of high G gun of my copending application 705,583 may be only 3-5 micromicrofarads and the total gun and circuit capacitance may be kept to about 8-12 micromicrofarads. With the new gun described in this invention, a further reduction of total shunt capacitance is made possible mainly on account of the additional simplification of the external circuitry. In the gun embodiment illustrated herein, the beam modulating electrode herein is associated with only one external resistor of very small physical size, in one embodiment. In the other embodiment illustrated, the beam modulating electrode has no external circuit component at all associated with it. This latter embodiment therefore is highly suitable for all those important applications involving the very wide or extremely Wide bandwidth modulation of cathode ray beams, required by the reproduction of high definition television pictures, where low shunt capacitance is essential to maintain the power demand down to a minimum.

Another interesting characteristic of this gun is that it preserves in the modulated cathode ray beam the direct current component contained in the input signal.

The new gun allows, furthermore, a greater freedom in gun design to meet particular gun specifications because it offers the possibility of applying a set of voltages to the electron optical section of the gun quite different from the set of voltages applied to the transconductance section or amplifier unit of the gun.

Accordingly, an object of this invention is a high transconductance gun capable of the very high resolution performance, which requires a black negative type of drive signal.

Another object is to provide a gun as above, particularly suitable for extremely wideband operation.

Still another object is to provide a gun as above, wherein the direct current component of the input drive signal is preserved in its entirety in the cathode ray beam modulated in accordance with said signal.

Another object furthermore is a gun, as specified above,

which allows a greater freedom in the design of the electron-optical and transconductance sections of the gun.

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

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

FIG. 2 is an enlarged view of a portion of the gun in FIG. 1 in section along line II of FIG. 3;

FIG. 3 is a top view of a portion of the gun shown in FIG. 2, partly in section;

'FIG. 4 is a schematic view of an electron gun structure in accordance with the structure shown in FIGS. 2 and 3 to assist in the explanation of the devices;

FIG. 5 is a modified schematic view of the structure shown in FIG. 4;

FIG. 6 is another modification of the schematic structure shown in FIG. 4;

FIG. 7 is a modification of the electron gun shown in FIGS. 2 and 3 taken along line VII of FIG. 8;

FIG. 8 is a view of the gun shown in FIG. 7 taken along the line VIII of FIG. 7; and

FIG. 9 is another modification of the electron gun shown in FIGS. 2 and 3, partly in section.

Referring in detail to FIG. 1, there is shown a cathode ray tube embodying the 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 or viewing portion 18 of the envelope is provided with a suitable coating 20 deposited on the inside surface. The coating 20, which is usually of a phosphor material, is normally provided with an electron permeable electrically conductive coating 22 deposited on the exposed surface of the coating 20 which is remote from the face plate 18. The flared portion 16 of the bulb is also provided with an electrically conductive coating 24 of a suitable material such as carbon or aluminum which extends into the neck portion 14 of the envelope.

An electron gun 30 is mounted within the neck portion 14 of the envelope. As previously mentioned, the gun 30 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 a suitable voltage is applied by means of an exterior terminal 26.

The general gun structure described and shown herein is of the electrostatic focus type. There is also provided around the exterior portion of the neck near the end of the gun, a suitable deflection system such as electromagnetic and illustrated herein by the deflection yoke 28. The deflection yoke 28, by application of suitable signals, provides means of deflecting the electron beam generated by the electron 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 32, a modulator grid 34, and a first accelerator anode or screen grid 36. The beam forming and modulating system has closely associated therewith an amplifying structure consisting of an indirectly heated cathode 52, a control grid 54, and an anode or intercepting electrode member 56. In the specific embodiment shown in FIGS. 2 .and 3, the screen grid or accelerator grid 36 consists of a cup-shaped member 38 of electrically conductive material with an open end on the side facing the screen 20. The other end of the member 38 which may be either totally or partially open has the opening partially closed by a bottom portion 40 with a centrally located aperture 42 therein. The cup-shaped member 38 is positioned within the tube neck so as to be substantially coaxial with the axis of the neck portion 14. Attached to the rear surface of the bottomportion 40 of the cupshaped member 38 with respect to the screen 20 is a U-shaped member 44 having its

side legs

46 and 48 lying in planes substantially parallel to the axis of the neck portion 14 and the middle portion 47 of the member transverse to the axis of the neck portion 14. The middle portion 47 of the U-shaped member 44 is attached to the bottom 40 of the cup-shaped member 38 by any suitable means such as crimping or welding. Centrally located within the middle portion 47 of the U-sh-aped member 44 is an aperture 49 which is located substantially on the axis of the neck portion 14 and of smaller diameter than the aperture 42 in the bottom of the cup-shaped member 38. The dimple in the members serve the purpose of aligning the aperture cup 36 with the aperture 49 mentioned below. The U-shaped member 44 provides support for the modulator grid 34, the cathode 32, and the associated amplifier 50 of the beam forming and modulating system to the remainder of the gun structure. End portions or projections are provided on the ends of the U-shaped member 44 which are inserted through apertures provided in two spacer members 60. The members 60 are of a suitable insulating material such as mica or ceramic. The projections are bent in well known manner to retain the spacer members 60 in a fixed position.

Positioned to the rear of the middle portion 47 of the U-shaped member 44 with respect to the screen 20 is the modulator grid 34 which is substantially parallel to the middle portion 47 of the U-shaped member 44. The modulator grid 34 is a planar member of electrically conductive material and is retained in this position by end portions which project through apertures in the spacer members 60. An aperture 3-3 is centrally located within the modulator grid 32 substantially on the axis of the neck portion 14 of the tube and in alignment with the aperture 49 in the middle section of the U-shaped member.

Positioned on the opposite side of the modulator grid 34 with respect to the active portion of the screen grid is the indirectly heated cathode 32 which is illustrated as comprised of a tubular member 27 of electrically conductive material positioned substantially parallel to the modulator grid 34 and retained in position by having the end portions of the tubular member 27 extending through apertures provided in the spacer members 60. An electron ernissive coating 29 is provided on the surface of the tubular member 27 adjacent the aperture 33 in the modulator grid 34. A heater (not shown) is provided within the tubular portion 27 to heat the electron emissive coating 29 to emission temperature.

Positioned on the remote side of the cathode 32 with respect to the modulator grid 34 is the amplifier portion 50 of the gun 30. The amplifier 50 consists of an indirectly heated cathode 52 having a tubular portion 51 with an electron emissive coating 53 on the exterior surface. The cathode 52 is positioned substantially parallel to the beam forming cathode 32 and is retained in position by end portions positioned within apertures in the spacer members 60. The control grid 54 of the amplifier 50 is of suitable electrically conductive material in the form of a mesh or a plurality of parallel wires and surrounds the cathode 52 and is retained in position by side rods which extend through apertures provided in the spacer members 60. Additional grid members may be used if desired. The anode or intercepting electrode 56 is of electrically conductive material and surrounds the control grid 54 in the manner shown in FIG. 2. The member 56 is also substantially parallel to the beam forming cathode '32 and is transverse to the axis of the neck portion 14. Projections are provided on the member 56 in the same manner as the other electrode which extend through the spacer members 60 so as to retain the member 56 therein. This technique of assembling the amplifier 50 to the beam forming cathode and modulator system is similar to that used in the receiving tube art.

The principal focusing electrode lens system consists of a first anode 70 comprised of a tubular or skirt member and a second anode 80, also a tubular member, spaced along the axis of the tube from the first anode 70. The end of the skirt portion of the first anode nearest the screen 20 is closed by a diaphragm transverse and substantially perpendicular to the axis of the tube and having a centrally located aperture therein. The end of the second anode 80 which faces the first anode 70 also is provided with a diaphragm having an aperture therein substantially perpendicular to the axis of the envelope. Means in the form of contact members 72 are provided on the end of the second anode 80 nearest the screen 20 for making electrical contacts to the coating 24 and also to assist in positioning the electron gun within the tube neck. The first and second anodes 70 and 80 are also connected together electrically and are supplied with voltage from the coating 24. A sleeve or focusing electrode 75 with a larger diameter than the skirt portion of the first and second anodes 70 and 80 surrounds the space between the first and second anodes 70 and 80*and is coaxial with said anodes. The principal focusing electrode system comprised of the electrodes 70, 75 and 80 is of the conventional univoltage type structure and is only an example of a suitable principal focusing lens system. The screen grid, first anode, second anode, and focusing electrode are normally spaced by providing A 6 radially projecting anchor pins 72 from the electrodes and embedding the pins within longitudinal glass support rods 74.

Referring in detail to FIG. 4, a schematic Showing the beam forming and modulating system of the electron gun illustrated in FIGS. 2 and 3 is shown to describe the operation of the device. The first anode 70 is normally operated at a potential of about 15,000 volts and a suitable source is illustrated in FIG. 4 by the battery 82 which has its positive terminal connected to the anode 70 and the negative terminal connected to ground. The screen grid 36 is connected to the positive terminal of a suitable source illustrated as a battery 84 to provide the necessary screen grid voltage which may be for this example 300 volts. The modulator grid 34 of the electron beam forming section is also connected to a terminal of the battery 84 to provide a variable positive potential of about to volts with respect to ground.

The cathode of the electron beam forming section is illustrated as the electrode 88 having the electron emissive coating 29 thereon. It should be remembered that the effective anode or collector of the electron beam forming section is the screen 20. The screen 20 is at the same potential as the first anode 70 so we can consider for purposes of explanation that the first anode 70 is the anode of the electron beam section.

The amplifier section 50 consists of the intercepting electrode 56 which is shown in FIG. 4 as a part of the member 88 and in physical and electrical contact with the electron emissive coating 29. The control grid 54 of the amplifier 50 is connected to a video source 90 and is supplied with a bias potential by means of battery 92 to provide a negative bias with respect to ground of a few volts. The cathode 52 of the amplifier 50 may be connected to ground potential as illustrated by FIG. 4.

The current flowing in the amplifier section 50, which consists of the cathode 52, control grid 54 and intercepting electrode 88, is controlled by the potential applied to the control grid 54. The electrons constituting the anode current are collected by the electrode 88 which is common with the beam electron emissive coating 29. The intercepting electrode 56 and the beam electron emissive coating 29 are at the same potential or are electrically connected if they are not a common electrode. The bias applied to the modulator grid 34 of the immersion lens system is adjusted to obtain black correspondence of the black level of the input signal. This means that the beam current in the beam forming section is at its cutoif point when the input signal reaches its peak negative value. To accomplish this, the bias on the modulator grid 34 must be less positive than the potential of the beam forming electron emissive coating 29 by a certain amount equal to the so-called beam cutoff voltage. As the video signal voltage from the video source 90 becomes less negative, the anode current increases and the potential of the beam forming electron emissive coating 29 decreases from the peak level existing when the beam is cutoif to a certain value which depends upon video signal. Because of this decrease, the potential difference between coating 29 and the modulator grid 34, electrons may now leave the cathode 29 and flow through the modulator grid aperture 33, thus providing the beam current. The intensity of the beam current depends upon the drop in potential undergone by the cathode 29.

In other words, a negative variation of the input signal amplitude causes a negative variation of the anode current which in turn produces a positive variation in the potential of the emissive coating 29. This, in turn, causes a negative variation or decrease of the beam current provided by the coating 29.

It is therefore seen that the potential of the beam forming electron emissive coating 29 varies in opposite phase to the input video signal, while the beam current in the beam forming system varies in phase with both the anode current in the amplifier 50 and the video signal applied to 7 the control grid 54. Thus the current in the beam forming section reaches its maximum level when the potentials of the control grid 54 and of the beam forming cathode 29 attain their maximum and minimum levels, respectively.

In practice, the modulator grid 34 is usually never allowed to attain a potential positive with respect to that of the cathode 29. Therefore the cathode 29 potential will not descend below the biasing potential applied to the modulator grid 34. The maximum variation of potential required by the cathode 29 in order to force the beam to go from the condition of beam cutofi to the condition of peak beam current level can, therefore, be equal at most to the beam cutoif value mentioned previously.

In the embodiment shown in FIG. 4, the beam forming cathode 29' is provided with an electron emissive coating of such an area to provide the necessary electrons for the beam forming section and also in addition to provide electrons for another section, entirely distinct from the beam forming section.

This section basically constitutes an auxiliary load impedance of the amplifier section 50. It offers the means to design guns capable of meeting specific operational requirements concerning such characteristics as: drive, gamma, bandwidth, etc.

As a result of the presence of this section, the amplifier may, for example, be operated along a steep slope region of its transfer characteristic thus minimizing the drive requirement of the gun. The amplifier may be operated instead in an arbitrarily chosen non-linear region of said transfer characteristic thus varying the gamma of the gun, namely the exponent of the power law relationship existing between the beam current and the drive signal applied to the gun. In conventional design guns, said exponent has a value of about three. The guns gamma is determined by the geometry of the guns immersion lens. The bandwidth of the section may be designed to obtain the desired bandwidth simply by proper choice of the auxiliary load impedance level. It is obvious that the current in the amplifier 50 is equal to the summation of the auxiliary load current and the beam forming current. The amplifier current, the beam forming current and the auxiliary load current assume their minimum values when the potential of the electrode 88 has its peak negative value with respect to the cathode 52 of the amplifier 50. The auxiliary load current will generally flow even when the beam current is cutofi.

The brightness of the picture may be varied by varying the bias on the modulator grid 34. The contrast may be varied by varying the amplitude of the input signal or by varying the input signal bias or means of a variable resistor placed in series with the cathode 52 as illustrated in FIG. 6. The amplifier section 50 has been shown as a triode, but it should be appreciated that any number of grids or electrodes may be used in the amplifier to obtain the desired characteristics.

In FIG. 6, there is shown a modification in which the electron emissive coating 29 on the electrode 88 is of only sufficient area to supply the electrons for the beam forming system. In this case, the auxiliary load impedance is constituted by an external resistor 93 connected between the electrode 88 and the screen grid 36.

In FIG. 5, an apertured portion 93 of the modulator grid 34 is provided between the coating 29 and the screen grid 36.

The auxiliary load impedance is constituted in this particular instance by one or more beams or sheets of current flowing through apertures 95 in the modulator grid electrode other than the aperture 33 of said modulator grid electrode which forms the beam current.

The current flowing through these auxiliary apertures 95 is intercepted by the screen grid electrode. As previously mentioned, this current generally flows even when the beam current is zero.

In FIGS. 7 and 8, there is shown a modified structure of that shown in FIGS. 2 and 3. In this device, the beam forming cathode and the amplifier cathode 102 are heated by the same heater element 104, the control grid 106 is in the form of a planar type structure and the electrons, emitted by 102, are collected by an intercepting electrode 108 positioned on the opposite side of the control grid 106 with respect to the

cathodes

100 and 102.

In FIG. 9, there is shown a modification of the structures shown in FIGS. 2 and 3 in which the screen grid 110 is separated from the remaining portion of the beam forming and modulating structure and in which the modulator grid 112 is formed in the shape of a cup member with the open portion facing to the rear with respect to the screen 20 and provided with a spacer member 114 mounted along the edge of the cup-shaped member by which the beam forming cathode and the amplifier section are held in position. The beam forming cathode 116 is positioned within the modulator grid 112 and consists of a tubular member 118 having a closed end portion adjacent a centrally located aperture 123 in the modulator grid 112 with an electron emissive coating provided thereon. The spacer 114 which is positioned across the top portion of the cup-shaped member is provided with an aperture to which a support member 124 is attached to support the beam forming cathode 116 within the modulator grid 112.

The amplifier section of the device comprises a tubular member of similar diameter as the beam forming cathode member 118 and coaxially aligned. The member 130 is supported on one end by the spacer 114 attached to the modulator grid 112 and is positioned on its opposite end by a second spacer 114. An electron emissive coating 132 is provided on the tubular portion 136 of the amplifier cathode between two spacers. A control grid 134 is positioned around the amplifier coating 132 and is also supported by the spacers 114 and an anode or interceptor electrode 136 is positioned so as to surround the control grid 134 and is fastened to the spacers in a well known manner in order to provide a compact electrode structure. The anode or intercepting member 136 of the amplifier is electrically connected to the beam forming cathode 116 and a heater element 140 is inserted through the amplifier cathode 132 and extends into the beam forming cathode 116 so that a comgron heater is provided for both of the cathode memers.

While I have shown my invention in several forms, it will be obvious to those skilled in the art that it is not so limited but is susceptible of 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 electrode system comprising a first and a second electron discharge path, said first path comprising a first cathode operating at a first potential, an electrode operating at a second potential positive with respect to said first potential and positioned to intercept the electrons emitted from said first cathode, a first control grid positioned between said first cathode and said intercepting electrode for controlling a predetermined area of the electron flow in said first discharge path, said second path comprising a second cathode operating at the potential of said intercepting electrode, a control grid having an aperture therein positioned adjacent to said second cathode and a screen electrode also having an aperture therein and aligned with the aperture in said control grid, said control grid and said screen electrode operating at potentials to form an electron beam of smaller area than the electron flow in said first path of the electrons emitted from said second cathode.

2. An electron beam forming and modulating electrode system comprising a first, second and third electron discharge path, said first path comprising a first cathode operating at a first potential, an electrode operating at a second potential positive with respect to said first potential and positioned to intercept the electrons emitted from said first cathode, a control grid positioned between said first cathode and said intercepting electrode for controlling the electron fiow in said first discharge path, said second path comprising a second cathode operating at the potential of said intercepting electrode, a modulating grid having an aperture therein positioned adjacent to said second cathode and a screen electrode also having an aperture therein and aligned with the aperture in said modulating grid, said modulating grid and said screen electrode operating at potentials to form an electron beam of the electrons emitted from said second cathode, said third path comprising said second cathode and said screen electrode, said screen electrode intercepting a portion of the electrons from said second cathode.

3. A cathode ray tube comprising an electron sensitive screen, an electron beam forming and modulating electrode system to generate a modulated electron beam for scanning a raster on said screen, said beam forming system comprising a first cathode, a modulating grid having an aperture therein and positioned adjacent said first cathode to control the intensity of said electron beam, said modulating system comprising a second cathode, an electrode for collecting the electrons emitted from said second cathode, a control grid positioned between said second cathode and said collecting electrode to control the electron flow between said cathode and said collecting electrode, said electron flow being of greater area than said electron beam, means for electrically connecting said first cathode and said collecting electrode and means for applying a video signal to said control grid to modulate the intensity of said electron beam.

4. An electron beam forming and modulating electrode system comprising a first and a second electron discharge path, said first path comprising a first cathode operating at a first potential, an electrode operating at a second potential positive with respect to said first potential and positioned to intercept the electrons emitted (from said first cathode, a control grid positioned between said first cathode and said intercepting electrode for controlling the electron flow in said first discharge path, said second path comprising a second cathode electrically connected to said intercepting electrode, a modulator grid having an aperture therein positioned adjacent to said second cathode and a screen electrode also having an aperture therein and aligned with the aperture in said control grid, said control grid and said screen electrode operating at potentials to form an electron beam of the electrons emitted from said second cathode.

5. An electron beam forming and modulating electrode system comprising a first and a second electron discharge path, said first path comprising a first cathodeoperating at a first potential, an electrode operating at a second potential positive with respect to said first potential and positioned to intercept the electrons emitted from said first cathode, a control grid positioned between said first cathode and said intercepting electrode for controlling the electron flow in said first discharge path, said second path comprising a second cathode consisting of an electron emissive coating on said intercepting electrode, a modulator grid having an aperture therein positioned adjacent to said second cathode and a screen electrode also having an aperture therein and aligned with the aperture in said control grid, said control grid and said screen electrode operating at potentials to form an electron beam of the electrons emitted from said second cathode.

6. An electron beam forming and modulating electrode system comprising a first, second and third electron discharge path, said first path comprising a first cathode operating at a first potential, an electrode operating at a second potential positive with respect to said first potential and positioned to intercept the electrons emitted from said first cathode, a control grid positioned between said first cathode and said intercepting electrode for controlling the electron flow in said first discharge path, said second path comprising a second cathode operating at the potential of said intercepting electrode, a modulating grid having an aperture therein positioned adjacent to said second cathode and a screen electrode also having an aperture therein and aligned with the aperture in said modulating grid, said modulating grid and said screen electrode operating at potentials to form an electron beam of the electron emission from a small area of said second cathode, said third path comprising said second cathode and said screen electrode, said screen electrode intercepting the remaining portion of the electrons emitted from said second cathode.

7. An electron beam forming and modulating electrode system comprising a first, second and third electron r discharge path, said first path comprising a first cathode operating at a first potential, an electrode operating at a second potential positive with respect to said first potential and positioned to intercept the electrons emitted from said first cathode, a control grid positioned between said first cathode and said intercepting electrode for controlling the electron fiow in said first discharge path, said second path comprising a second cathode operating at the potential of said intercepting electrode, a modulating grid having an aperture therein positioned adjacent to said second cathode and a screen electrode also having an aperture therein and aligned with the aperture in said modulating grid, said modulating grid and said screen electrode operating at potentials to form an electron beam of the electrons emitted from said second cathode, said third path comprising said second cathode and said screen electrode, said screen electrode intercepting the remainder of the electrons emitted from said second cathode, said modulating grid having a portion positioned in said third path and having a plurality of apertures therein through which said electrons pass.

8. An electron beam forming modulating and electrode system comprising a first and a second electron discharge path, said first path comprising a first cathode operating at a first potential, an electrode of similar area operating at a second potential positive with respect to said first potential and positioned to intercept the electrons emitted from said first cathode, a control grid positioned between said first cathode and said intercepting electrode for controlling the electron flow in said first discharge path, said second path comprising a second cathode of smaller area than said first cathode and operating at the potential of said intercepting electrode, a modulating grid having an aperture therein positioned adjacent to the electron emissive coating on said second cathode and a screen grid also having an aperture therein and aligned with the aperture in said modulating grid, said modulating grid and said screen electrode operating at potentials to form an electron beam of the electrons emitted from said second cathode and circuit means including an auxiliary load impedance connected between said intercepting electrode and said screen electrode.

9. A cathode ray tube comprising an electron sensitive screen, an electron beam forming and modulating electrode system to generate a modulating electron beam for scanning a raster on said screen, said beam forming system comprising a first cathode, a modulating grid having an aperture therein and positioned adjacent said first cathode to control the intensity of said electron beam, said modulating system comprising a second cathode, an electrode for collecting the electrons emitted from said second cathode, a control grid positioned between said second cathode and said collecting electrode to control the electron flow between said second cathode and said collecting electrode, said first cathode and said collecting electrode operating at the same electrical potential by means of electrical connection internal of said tube.

10. 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 an amplifier including a cathode, control grid 2,055,174 Kwartin Sept. 22, 1936 12 Wiene cke June 20, Schlesinger Sept. 19, Hollrnann Nov. 21, Sziklai Sept. 14, Raymond Nov. 16,

FOREIGN PATENTS France Mar. 16,