US2773212A

US2773212A - Electron gun

Info

Publication number: US2773212A
Application number: US374240A
Authority: US
Inventors: James A Hall
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1953-08-14
Filing date: 1953-08-14
Publication date: 1956-12-04
Anticipated expiration: 1973-12-04
Also published as: NL96222C; BE530971A; GB761008A; DE1015948B; FR1109867A; NL189231B

Description

J. A. HALL ELECTRON GUN Filed Aug. 14, 1955 Fig.2.

MIJ@ i Fig.3.

INVENTOIR James A.II0|II.

BY l

ATTORNEYy 9 I I l a I r'lld'l'lrll.'

BILII WITNESSESI United Safes Patent@ ELECTRGN GUN Application August 14,1953, Serial'No., 374,240 5 Claims.` (Cl. 313-82) My invention relates to cathode-ray tubes and infpar ticular relates to an improvement in the` cathode-ray guns which project electron beams onto the fluorescent screens of such tubes. n

The electron beams in cathode-ray tubes are'proj'ected by an assemblage ot electrodes known as`an electron gun such as is diagrammed in Fig. l which comprises` a `heated cathode l having a control electrode 2 enclosing its electron-emissive end-portion except forl an aperture4 3. Facing the control electrode 2 is ascreen-electrode 4 comprising a disc 5 with an aperture 6 aligned with aperture 3,. and a cylindrical wall which may have a rolled Aedge 7. Facing the wall 7 is the circular edge of another cylindrical member 9 which may be .joined tocylindrical member 13 by flanges 11 and 12. The member 13 is -closed at `its remote end by aV disc with an aperture`17 `located on the axis of member 13. To provide'for an .ion trap `of a type now widely used in television picture itubes, the rim of the screen-grid electrodewally'? and :adjacent edge of member 9 may be slanted at an angle of abouti() to l3 degrees relative to theV plane perpen- (dicular to the axis of the screen electrode 4. Theelectrostatic field produced between these electrodes at'ythe normaloperating voltages bends the unwanted beamiof negative ions away from the axis of electrode 9 sothat .it cannot'pass through the aperture "17, andl'th'ere'fore Jeannot reach and'damage the fluorescent `screen of tthe picture tube. The electron beam is caused'to follow .a curved path through the aperture 17 by thefaction of :a magnetic field inthe directionperpendicular tothe 4plane' of the diagram. This magnetic eld'iscustomarly supplied by a magnet whose pole pieces 'are located approximately over the screen-grid electrode 4.'-f`The path yof the heavier negative ions is little affected by this -magnetic` eld. i

Coaxial withV cylinder 13`but spaced therefrom'by a *gap is a cylinder 14, closed at its nearer end-by a disc fhav'ing an aperture` 13 locatedA on itszaxis. Thefdiscs in `:parts 13 and 1d may have'various shapes,` but areusually -of the dished shape indicated in Figure 2; The cylinders yof -

parts

13 and 14 may be parallel sided `or1 may have 'asection of smaller diameter near the :closed ends of .these parts.

Surrounding the gap between .cylinders 13 and11'4 is Va larger `cylindrical sleeve 16 called the focussingl electrode. Sleeve 16 is insulatingly supported coaxiallyyand :approximately symmetricallyrelative to the gap; between *cylinders 13 and 1e. In some designs,` af-similarly lo- Ycatedring or discwith centrally located-aperture is lprovidedinstead of the sleeve 16 toproduce thefocussing lteld.

`type tubes in the receiver. Thus the range of 'adjustment ice The outer end of'cylinder 14 customarily has a wide flange which'is provided with springs 15 of flexible metal which engage the walls of thecathode-ray tube and assist in` positioning 'the cathode-ray gunjstructure` centrally, therein. Wh'ilecylinders 9.and 13 may becoaxial, they are' customarily joinedso that their axes intersect at an anglefof 3 to`5.5 degrees, crossing `in the plane of the flanges or other joining means, so that theelectron beam after completing its curved `path yin the region of the magnetic and axially unsymmetrical electrostatic teolds,`wil1 follow aM path along theaxes (of parts 13; 14 and16. "The cylinders 27 4, 9, 13 and V14 are usually all of the saine inside diameter, the cathode 1 is insulatingly supported within the control electrode 2 andall electrodes are providedwith leads by which electrical. potentials may be' impressed on them from voltage sources .outside the cathode-ray tube. The above-described assemblage of cylinders constitutes an electron-lens systemby 'which the electrons emanating from' cathode 1 may be focussed into a spot or" smalldiameteron theluorescent screen 19 at the remote endof the cathode-ray tube f2() which kforms a vacuum-tightenclosurefor the rabove-,described structure as widely used today in television pictureptubes. The electron `beam is projected along .the axes of electrodes 13,414 and 16 and is then deected `by two setsof mutually perpendicular magnetic dellecting coilsi21and 22 to scan the picture screenrll in a mannervnow `well known.

Theelectron optical characteristics of this assemblage dependsupon the relative `dimensions andspacings "of the component cylinders, sleeves and diaphragm` apertures,` and onl the magnitudes of thelelectrical potentials limpressed on them. lt may be noted that.` dimensions and magnitudes` in this electron-optical system are relative, so that an assemblage inwhich all dimensions are the same as those of another assemblage .multiplied by the same factorwill produce vsimilaroptical patterns in `the latter provided it is supplied with electricalpotentials fwhich are the same as those` impressed on theformer.

The dimensions in the Fig. l assemblage..may therefore *be expressed as ratios to the inside diameter D. of cylinder 2. While this or a similar system is/widely used in ymost recent electrostatically focussed picture tubezguns,

a number of undesirable features `have been found `which l `will nowfpoint out.

A. The bias voltage on control electrode 2.` isdmade `manually variable to enable control of picture brightness by varying the current in the electron beam., `The potential needed for cylinder 16 to maintain the best4 spot focus, hereinaliter called they focus voltage, becomes rapidly more negative as the picture brightness, Vand `hence beam current, is increased. This'tocussing voltage is "customarily derived from the supply ywhich furnishes 250 to `350volts D. @for anode power for `the receiving available may be insulrcient to `maintain the best focus with variation or picture brightness.

' B. Thevoltage applied to control electrode 2 is modu- -lated by the signal from the'video amplifier ofthe television receiver to vary the brightness of the fluorescent spot on the picture `tube screen by` varying tliebeam current asi-each picture element is scanned; In tubes .using electrostatically focussed` guns of the prior: art-` the ,size of the uorescent spot becomes` rapidly: larger: .and

the detail in the reproduced picture less distinct as the electronbeam current is increased. This effect, known colloquially as blooming, causes loss of detail in picture highlights. I television reception in weak signal areas, stray signals originating in the receiver circuits cause a random pattern of white dots to be superimposed on the reproduced picture. This type of interference is known to workers in the field as snow. The increase of fluorescent spot size with increasing brightness then becomes especially undersirable since the white dots will be large and much more objectionable to the viewer.

C. The potentialapplied to

electrodes

9, 13, and 14, hereinafter called the anode potential, which is usually between l kv. and 18 kv. positive with respect to the picture tube cathode, is subject to wide variation during receiver operation. This variation is due both to fluctuations in the voltage supplied by the domestic powersupply lines and to variations in the load conditions within the television receiver since the anode potential supply has very poor regulation. A drop of 1000 or more volts for a 200 microampere change in picture tube anode current is typical of present practice. The anode supply voltage may also vary during the life of the receiver due to the gradual deterioration with life of other tubes and components in the receiver.

These variations tend to degrade picture definition since the focus voltage required to maintain best picture definition becomes more negative as the anode potential is decreased, in the order of 50 volts for every kilovolt change in the anode Voltage.

This effect becomes much more objectionable in light of A and B above, since these effects all tend to make the focussing voltage required more negative. Further, the causes listed tend to add, since lowering the anode voltage tends to make the picture dim, while attempts to increase picture brightness by changing the bias voltage applied to the control electrode to increase the tube current will further decrease the anode voltage because of the poor power supply regulation.

D. The focus voltage required becomes rapidly more negative as the voltage applied to the screen grid electrode 4 is increased. A 100 volt change in this voltage will cause a 200 volt change in the focus voltage. The percentage of the electrons leaving cathode 1 which finally arrive at the fluorescent screen is a function of the operating voltages. At high brightnesses, the electron beam diameter in the gun may exceed the diameter of aperture 17, and further attempts to increase the brightness will increase the percentage of electrons which are intercepted by the end of electrode 13. For a given current of electrons from the cathode, the beam diameter at this point is increased if the voltage on screen-grid electrode 4 is increased, or if the anode voltage on

electrodes

9 and 13 is decreased. Since high picture brightness is desired and since only the electrons which reach the fluorescent screen'contribute to picture brightness, the electron gun must be designed so that little if any of the electron beam is intercepted by electrode 13 under the least favorable conditions of operation expected in the field.

E. To find'a good market, cathode-ray tubes must fit a wide variety of different television receivers, and the magnitudes of the screen-grid (electrode 4) and anode (

electrodes

9, 13, 14) supply voltages varies widely between television sets of different manufacturers. Typical ranges for these voltages are 275 to 500 volts for the screen-grid voltage and l0 to 18 kilovolts for the'anode voltage. Furthermore, the picture brightness requirements considered necessary by various receiver manufacturers range from 12 to 70 foot lamberts. Because variations in picture brightness, screen-grid voltage and anode voltage all change the focus voltage required by the picture tube, and because the range of focus voltage supplied in a television receiver is small (or zero in some cases) it was impossible todesign one electrostatically focussed cathode-ray tube which would perform satisfactorily in all receivers designed for this type oftube. As a result, cathode-ray tube manufacturers find it necessary to build two or three tubes actually differing from each other for each nominal tube type.

F. The magnetic field impressed near the screen-grid 4 as part of the ion-trapping device is of non-uniform intensity across the region traversed by the electron beam; in electron guns of prior design, the cross section of the beam is distorted from a true circle, the shape of the light spot produced on the picture screen is distorted from a circle and picture resolution is degraded. Both the uniformity and strength of this magnetic field vary between sets of different manufacture, so that a compensating non-uniform electric field cannot be used to correct this beam distortion.

G. In the electron guns described, slight non-uniformities in the electric field between electrodes 4 and 9 distorted the cross section of the beam from a circle, with loss of picture definition. These field non-uniformities were both those caused by slight distortions of elec trodes 4 and 9, and the general field asymmetry in this@ region required for the ion-trapping action.

One object of my invention is accordingly to provide l a new and improved structure for cathode-ray tubes.

Another object is to provide a new and improved type? of electron gun for cathode-ray tubes.

Another object is to provide a new and improved type* of electrostatically focussed electron gii for cathode# ray tubes.

Another object is to provide a new and improved type" of electron gun of the socalled low voltage? electro-A statically focussed type for cathode-ray tubes.

Another object is to provide a cathode-ray gun of this@ type in which the focussing of the beam shall less sensitive to variations of anode potential relative tofl cathode than cathode-ray guns of the prior art.

Another object is to provide a cathode-ray gun of this type in which variation of the focus voltage required to compensate for variations in the screen-grid voltage shall be greatly reduced.

Another object is to provide a cathode-ray gun of this type in which variation of the focus voltage with bias voltage of the control electrode shall be smaller than that of the prior art.

Another object is to produce a cathode-ray gun in which variation of the focus voltage required to compensate the effect on beam focus of variations in picture brightness shall be less than in conventional cathode-raf.I tubes operating in conventional receivers.

Another object is to provide a cathode-ray gun in which the size of the light spot on the picture-screen is smaller for a given gun length than in cathode-ray guns of conventional type.

. Another object is to produce an electrostatically focused cathode-ray gun in which substantially all focussing occurs'- in a single electron lens.

Still another object is to produce a cathode-ray gun in which focussing in the electron lens comprising the screen' grid and the first anode is reduced virtually to zero.

Still another object is to produce a cathode-ray gun in which the percentage of the electrons emitted by the 'cathode which arrive at the picture screen is little affected by variations in the anode voltage or the screen-grid voltage.

Yet another object is to provide a cathode-ray gun which is less sensitive to variations in the magnetic field comprised in the ion trap than are prior art cathode-ray guns.

Yet another object is to provide a cathode-ray gun in which larger tolerances in the symmetry of the electron lens comprising the screen-grid first anode may be permitted than in cathode-ray guns of the prior art.

Still another object is to produce a cathode-ray gun which is less sensitive to non-uniformity within field of the iontrap magnet than are cathode-ray guns of the prior art.

, Yet another objectis toproducea cathod'efraygun `in which the focussing effect at `the rst electronglens, adjafl cent1 the screen-grid electrode, is reduced and the focus` sing effect of the second electron lens, between theanode cylinders and focussing electrode, is normal; j

Other objects of my invention will become apparent upon reading the following description taken in connection with. the drawings in which:

Figure 1 isa schematic view showing the relation ofthe cathode-ray gun embodying the principles of my invention to the otherstructural elements'A of a cathode-ray tube.

Fig. 2is a schematic view partly raygun `of .the prior art; and

Fig; 3 Yis asimilar view` of a cathode-ray gun embodyingthe principles of my invention;

Fig; 1 is believed to have already been described sul-A ciently so that further description of it isA superfluous.

Referring toFig". 2 the cathode 1"c`ornpr`ise`s a" metal cylinder 30 having a cupped end 33fsurfaced `with thefr'r'n ionically-emissive material which is heated byian'electri'cal heater-winding 23. The cathode is supported by a` ceramic collar from" the control electrode 2, which has its end'facing cuppedend 22 closed exceptfor` central'"aperture3.` The distance from the emissive surface on cathode '1"t`o the aperture 3 isabout 0.01 D; the other important dirnen-A sions Vof-cathode 1 and control electrode 2 are tabulatedA below.

Facing the aperture 3 lin control electrode 2 is `the centralraperture 6 in the lat end of screen-grid 4,: and ,facing 30 the otherV end ofV the latter is the open end ofjcylindiicalf rst-anode-section 9; control electrode 2, 'screen-gridf4 and rst-anode-section 9 are rigidly intercnnectedby tivov or `more rods of insulating

material

25, 26 to which 'their'` support-arms 27 are attached. The adjacent edgesvbf elec-i trodes 4 and 9`are in parallel planes inclined to 'a plane` perpendicular to the axis of screen-grid 4 by an angleof about, 10 to 13 degrees, and the edge of screen-grid '4' inlay"l bei rolled over as shown or may be meelycut and pol'- ished to remove sharp edges and burrs which mightlcaus'ej local distortionsof the electrostatieeld. "y t" The right-hand side of rst-anode-section 9 isY joined by flanges 11,12 or sorne other suitable joining means tfolfsec-` tion 13 which has its remoteend closed except' for af central aperture 17.` which is aligned withvthe" central ap j ture 1S in the closed end ofanode-.section 14. "The corn mon" axis'of electrodes 4 and 9 may be inclined byabout;q 3 to`5.5 degrees to the common axis of electrodes` 13,"147` and'16, intersecting the latter at a point in `the planeofthe ariges or other joining means 11 12. T he anode-sections 143"a`nd:14 Yare electrically interconnected, and are` lgidly v` interconnected mechanically by a set of insulatingy i 28, 29 to which their support arms 31 are attached.. A sleeve or focussing electrode 16atxed-tosupport rods 28,` 29 surrounds thespagebetweenapertures 1] and 18. A wide ange 32 to which are attached flexible spring-plates isprovided at the right-hand. end of anode-section lat.V The apertures 3, 6, 17 and 18.are positioned longthe' curved path which` electrons` will. travel through the Vl'elec'- trostatic fields of these electrodes and the magnetic' field of an ion-trap magnet (not shown) mounted outside the tube near electrode 4.

In-leads through the wall of tube 2,0 make it possible to impress the desired potentials onthe electrodes 111,12, 4,'9, 13,14, 16`and 23. Thus cathode 1 may he grounded; control electrode 2 may be impressed with signal voltages and with a variable negative bias to control brightness of the picture on Screen 19; screen-grid electrode 4 in section of a cathodewith a Xed position voltage usually between 250 and 500 volts;

electrodes

9, 13 and 14 with the anode supply voltage of 10 to 18 kilovolts; and sleeve 16 with a focus voitage usually variable over a portion of the range from l00 to +450 volts.

For guns of the prior art, the relative dimensions of theseelectrodes are expressedfas ratios to-thediameter D of lcylinderZ.

Cylinder 'Diameter Length i Voltage outsraayl.. 0.72. 16.

i Inside, 1t--- Y 1-30.

Inside, 1, `0.71 (maximum ng +275 to 500V.. `-Ins1de,i1 f- 41.8rmarirnumningun. u +1 0to 181m` Cylinder Inside Length Voltage diameter 1` .71 1otsrsknovons.`

1 1,02 10 to llkilovolts. 1. 3 j 1 4 .-*100` A.rofl-450.

Aperture` Inside u l tf i diameter between j e" 'length" 2- 4 0.045 j 4- 9. .2 i ra-ifr .75,

In a typical-case the distance between aperture 18 and the" center 'of a" 21 incli"diame`t`er fluorescent picture; screen' is about 18I Voltages listed are potential abovey the cathode for' an assemblage in'whi'ch D is 0.50 inch.

The electron-'optical system `in`Fi`g. 2 produces focusing at two points subsequent` to' the cathode lens; the

the'focal lengthof the electron lens formed'in andbe' tween electrodes 4 and 9 by reducing the curvature of the equipotential surfaces of 'the "electric "'eld within the; screen-grid 4 wherernost of rthe focussing effect of this lens resides. Thisfchangeof dimensions` also reduces the objectdistance for 'thislens so that it becomes small compared to the focalV length and makes they focussing etect of the lens negligible.

The above-mentioned increases'of focal length of the lens' resl'tsffro'r reducingV theft curvature of "the equipot'e'nt'ial surfaces-on the low potential side' o f the lens.

VAny' 'other methods 'of-reducing"thisl curvature and/or increasingv the focal'length are withinmthe broad scope of rny invention.' l l 'The region 'of the electron gun betweenjaperturesxl'l and 18 which is intended 'to yfocus the bearnlmust then be`s1ightly changed ina manner known to workersl in,"

the" fieldtoincr'ease' thestre'n'gtli of this lens.

'The following advantageous results are attained by this modificatio'noftheelectrnguii; "l f 1; The change of thelfocussingfvoltage required when picture brightn'ess changes is gre'atlyfreducedi i. e. the

chang of2'50 to 300 volts per' 100 microarnperes change of beam. current in present cathde-r'a'ybes" is' dropped 2. The adjustment of focussing voltage required in present tubes with changes in screen-grid voltage is reduced to approximately one-sixth,

3. The adjustment of focussing now required whenv i anode voltage changes, amounting to about 5 percent ofY the latter, drops to around 3 percent.

4. The defocussing of the electron beam consequent upon the inevitable momentary variations in intensity from point to point in the picture is greatly reduced.

5. The cathode-ray spot size at the picture screen is decreased compared with conventional cathode-ray guns of the same length thus increasing picture resolution in the ratio of from 800 to 1000 lines. This enables receiver designers to attain desired resolution without the necessity for providing supply voltages outside usual values.

6. The percentage of electron beam current which reaches the picture screen will be little affected over a wide range, by the magnitude of the screen-grid and anode voltages.

7. Because the focussing action'of the lens between the screen-grid and the first anode has been greatly reduced, ellipticity of cathode-ray spots on the screen is nearly eliminated and asymmetry in the electron lens formed by the screen-grid and first anode produces less aberration and astigmatism in the cathode-ray spot on the picture screen.

8. Non-uniformity and astigmatism in the cathode-ray beam arising from the departures from uniformity in the magnetic field of the ion-trap magnet are greatly reduced when the focussing effect of the screen-grid-tofirst-anode lens is reduced by my construction.

9. As a consequence of these advantages the same tube may be marketed for use in makes of receiver having anode voltages from 10 to 18 kv. and screen-grid voltages from 250 to 500 volts, Vwhich of our present knowledge includes all the receiver designs which are now sold or have been sold for useY with low voltage electrostatically focussed cathode-ray tubes.

, Fig. 3 shows an electron gun in accordance with my invention with the length of the screen-grid reduced as just described. The remaining dimensions of the gun structure may be those specified in describing Fig. 2 and guns operating satisfactorily in substantially all picture receivers employing low voltage electrostatically focussed,

cathode-ray tubes will. result. However, it may be found desirable to depart from the dimensions specified in the foregoing tabulation, and I will therefore state in exemplification that the gap between screen-grid 4 and anode 9 may vary between 0.20 D and 0.24 D; the angle of the plane of the inclined rim of screen electrode 4 relative to a plane normal to the axis of screen-grid 4 may lie Vbetween ten and thirteen degrees; that the angle between the axes of cylinder-

sections

9 and 13 may vary from 0 to 5.5 degrees; that the rolled rim on screen-grid 4 may be omitted; and that a ring or disc of size readily calculated by those skilledv in the art may be used in place of cylinder 16.

I claim asmy invention: v A

1. An electron gun comprising Va control electrode and a screen-grid electrode, said electrodes comprising coaxi-al cylinders of inside diameter D having diaphragms .normal to their common axis and with apertures lying on said axis, an electron source on said axis on the side of said control electrode aperture remote from said screen-grid electrode, a rim of said screen-grid remote from said f control grid lying in a first plane oblique to said axis,

the major length of said screen-grid Ibeing not over .55 D, a first anode cylinder of diameter D and .having a rim adjacent to said screen-grid lying in a seco-nd plane parallel to said first plane and separated therefrom by a gap substantially 0.20 D Wide, and an electr-on lens at the other end of said first anode cylinder such that'substantially all focussing effect occurs in it.

CIL

2. The arrangement specified in claim 1 in which the distance separating said diaphragms is about 0.045 D andthe diameter of said apertures therein is about .06 D.

3. An electron gun comprising a control electrode and a screen-grid electrode, said electrodes comprising coaxialcylinders of inside diameter D having diaphragms normal to their common axis and with apertures lying on said axis, an electron source on Isaid axis on the side of said control electrode aperture remote from said screen-grid electrode, a rim of said screen-grid `remote from said con- -trol electrode lying in a first plane oblique to said axis, the major length of said screen-grid being not over .55 D, a first anode cylinder of diameter D and having a rim adjacent to said screen-grid lying in a secon-d plane parallel 4to said first plane and separated therefrom by a gap substantially `0.20 D t-o 0.24 D wide, and an electron lens of substantial focussing po'wer at `the other end of said first anode cylinder.

4. An electron gun compri-sing a control electrode and a screen-grid electrode, said electrodes comprising coaxial cylinders of inside diameter D having diaphragms normal to their common axis `and `with apertures lying -onsaid yaxis, an electron source on said axis on the side of said control electrode aperture remote from said screengrid electrode, a rim of said screen-grid remote from said control electrode lying in a first plane, the length of said screen-grid being not over .44 D, a first anode cylinder of diameter D and having a rim adjacent to s-aid screengrid lying in .a second plane pa-rallel to said first plane and separated therefrom -by a gap of about .20 D wide and an electron lens of substantial focusing power at the other end of said first Ianode cylinder.

5. An electron gun comprising a cathode, a control grid, a screen-grid, a first anode portion, a focusing electrode .and a second anode, -in the `order named, said screen-grid and said first anode comprising lcoaxial cylinders of inside diameter D having diaphragms normal to their common axis and with apertures lying on said axis, said screen-grid and said first anode portion forming a first electron lens of given focal length on the side of said first lens facing ysaid cathode, said cathode, said control grid land said screen-grid cooperating to form an electron beam object for s-aid first lens within the region of said screen-grid, said `screen-grid being of a length of less than .44 D, -a rim of said screen-grid remote from said control electrode lying in a first plane .and a rim of said first anode portion adjacent said screen-grid lying in a second plane and parallel -to said first plane and separated therefrom by a gap of about .20 D, the length of said screen-grid Ibeing such that the mid plane between said screen-grid and said first anode portion is at a predetermined distance from said cathode and the principal focus of said first lens is beyond said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,484,721- Moss Oct. 11, 1949 2,496,127 Kelar Jan. 31, 1950 2,515,305 Kelar July 18, 1950 2,638,559 Giacchett May l2, 1953 2,680,204 Swedlund June 31, 1954 OTHER REFERENCES .Fundamentals of Electrical Design (A. D. Moore), published by McGraw-Hill, 1927 (chapters V to IX).