US2717322A - Cathode ray tube guns - Google Patents

Description

Sept. 6, 1955 D. c. BALLARD 2,717,322

CATHODE RAY TUBE GUNS Filed Nov. 1, 1952 A AA A AA A j? fi ENTOR d z! i 4/ 49 QMMQ ATTOR NE 1 United States Patent CATHODE RAY TUBE GUNS David C. Ballard, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware 7 Application November 1, 1952, Serial No. 318,191

10 Claims. (Cl. 313-76) This invention relates to cathode ray tubes and particularly to electron guns suitable for use in such tubes.

The type of cathode ray tube with which the present invention is most directly concerned is the kinescope or'picture tube as used in television sets. A conventional kinescope comprises a large funnel-shaped envelope, a fluorescent face plate or viewing screen closing the side end of the envelope and an electron gunlocated in the neck of the funnel.

The electron gun normally comprises a thermionic cathode, an electrode for controlling the flow of electrons from the surface of the cathode, and one or more accelerating electrodes which are at high potentials during operation of the tube. The electron gun shoots a narrow beam of electrons in the direction of the fluorescent screen of the kinescope. Besides the previously mentioned parts of the electron gun, means are provided, either inside or outside the tube, for focusing the electron beam at the fluorescent screen and for scanning or sweeping the electron beam over the inner surface of the viewing screen to produce a picture or other image.

During operation of television kinescopes a gradual degrading of the picture or image quality has been noticed. Two things, fuzziness of the electron beam and general loss of focus of the image have been observed. In many cases the loss of focus becomes so extreme that sharpness of the picture can not be restored by a readjustment of the focus controls of the television set. i

Examination of some kinescopes exhibiting this lack of sharpness of image has revealed that a metallic coating has been deposited on the elongated glass neck of the kinescope. The metalized coating becomes charged and thus distorts the focusing fields (which normally focus the electron beam).

The source of metal vapor which settles on the neck r of the tube has been found in many cases to be one of the accelerating electrodes of the electron gun. This accelerating electrode is substantially tubular in form and has an apertured disc at or near one end of the electrode. The electron beam ordinarily passes through the aperture, which is sometimes termed a limiting aperture because it restricts the diameter of the electron beam. Because of the intensity of the electron beam, the edges of the aperture of that electrode are heated by electron bombardment to the extent that metal is slowly evaporated from its surface. The immediate effect of the evaporation is not noticeable, but over the life of the tube a gradual degrading of picture quality, evidenced by loss of focus and fuzziness or distortion of the spot produced by impingement of the electron beam on the screen, results.

The rate of evaporation of metal from this accelerating electrode is appreciably accelerated if the electron beam is improperly centered in the aperture. In many kinescopes the centering of the electron beam through the limiting aperture is accomplished during manufacture of the tube, but in tubes which have so called ion traps centering of the electron beam during tube manufacture is not done. The ion trap, which prevents negative ions from bombarding and damaging the viewing screenof the tubes, achieves its function by taking advantage of the fact that ions are deflected much like electrons by an electrostatic field but, unlike electrons, are relatively unaffected by a magnetic field. The electron guns of ion trap tubes are so constructed that both ions and electrons are electrostatically deflected towards a part of the tube structure where the ions can do no harm, but an external ion trap magnetplaced close to the electron gun aligns only the electrons of the beam back intothe tube axis. The adjustment of the external magnet is rather critical if a minimum number of electrons are to bombard the limiting aperture of the accelerating electrode, yet it is common practice to determine the proper position of the'magnet by moving the magnet along and around the neck of the tube until good picture 'quality is observed on the kinescope screen. Critical centering of the electron beam with such an adjustment procedure is a matter of chance.

. However, even if the proper position of the magnet is determined by instruments which indicate minimum electron bombardment of the aperture by the electron beam, ordinary jarring of the kinescope, such as occurs when the receiver is moved, often proves sufficient to slightly change the position of the ion trap magnet and adversely affect the centering of the electron beam.

De-focusing of the image is not the only adverse affect of the bombardment of the electron gun electrodes by the electron beam. Gases may be releasedduring vaporization of the metal, metalvapor may settle on insulators and cause short circuits, or the electrode aperture shape may become distorted, thus changing the shape of the electron beam which passes through the aperture.

A principal object of the present invention is to pro-1 vide a cathode ray tube having consistently good focus quality over a long useful life.

Another object of the present invention is to provide a cathode ray tube which will maintain-uniform beam spot shape over a long useful life.

Yet another object of the present invention is to eliminate aperture damage by impinging electrons.

Still another object of the present invention is to prevent evaporation .of the metal from surfaces bombarded by the electron beam.

A further object of the present invention is to reduce the possibility of short circuits between electrodes of an electron gun structure.

'Referring to the accompanying drawing, in which corresponding parts have the same reference numerals:

Fig. 1 illustrates a cathode ray tube having an electrostatic focus type of electron gun which incorporates the present invention, and

Fig. 2 illustrates a magnetic focus type of electron gun which incorporates the present invention.

Figure 1 discloses a cathode ray tube having an electron gun in which an electron beam is focused by an electrostatic lens. The gun structure also includes a negative ion trapping means. The cathode ray tube is formed of an envelope 10, which may be entirely of glass or partially glass and partially metal. The envelope 10 consists of a large bulb portion 12 and a tubular neck portion 14 mounted on a common axis 16. The large structure 22 for forming and focusing an electron beam on the fluorescent screen 20. The electron gun 22 includes a beam forming portion having a source of electron emission, which is a cathode electrode ,24 formed of a short metal tube closed at .the .end 'facing the screen 20. The closed end of cathode 24 which forms an electron emitting portion is coated in a well-known manner with thermionic emitting material such as the mixed oxides .of barium and strontium. A filament 26 Within the tubular cathode 24 heats .the cathode to provide thermionic emission during tube operation. Enclosing the cathode 24 is a control gridelectrode 28 having .a closed end .portion with a small aperture positioned over the electron emitting surface of cathode 24. Spaced from control electrode 28 is an accelerating grid structure 36 formed as a cup, or thimble-shaped electrode, for drawing the electron emission from the cathode 24 through the aperture of the control grid. A second accelerating grid 32 is mounted closely spaced from grid 36 and is formed as a tubular cylinder. and 32 are at an angle to their common axis to provide an asymmetric lens field to deflect the electron beam ofi the common axis to trap the negative ions .of the beam, and as described below.

The electron gun 22 is provided with a beam focusing portion including the end of the accelerating grid 32 adjacent to the screen which is closed by a plate or disc 33 having an aperture 34 at its center. Spaced from the plate is a second cup shaped accelerating electrode 36 having an aperture at its center. The electrode 32 and the electrode 36 are spaced apart by insulating spacer elements 33. The lead 37 conductively connects the electrode 36 to the accelerating electrode 32. Positioned intermediate the disc 33 and electrode 36 is a focusing grid or electrode 44. As shown, the focusing electrode 44 consists essentially of a ring or washer having an aperture comparable in diameter to electrode 32.

The beam forming electrodes 28, and 32 are mounted together as a unit by metallic support members 41 projecting out from each respective electrode and secure ly fixed or clamped about ceramic support rods 50. This mounting means is similar to that shown in United States Patent 2,335,818 to Trumbull et al., for example. The focusing electrode 44 is mounted on the glass spacer elements 38 which separate electrode 32 and accelerating electrode 36. Apertured disc 33 and electrode 36, as Well as the focusing electrode 44, are each mounted coaxially with the tube axis 16 whereas the apertures of electrodes 28 and 39 are on an axis which is offset from the tube axis 16. In a successfully operated tube of the type described, the apertures of electrodes '28 and '30 are on an axis which is at an angle of substantially five and one-half degrees to the tube axis 16. In this manner, the beam forming portion of the electron gun, including the electron emitting area of cathode 24, as well as electrodes 28, 30 and 32, is mounted off-set from the beam focusing gun portion, formed by electrodes 34, 44 and 36.

A collector electrode, consisting of a conductive wall coating 52, extends from a point adjacent the screen .20 along the inner surface of the bulb portion 12 into the neck portion 14 to a point adjacent the electrode 36. Bulb spacers 54 serve to accurately center the end of .gun 22 Within the tube neck 12, as well as to electrically tie accelerating electrodes 32 and 36 to the collector wall coating 52. Mounted on the end of the gun structure 22 are getter tabs 56 to provide a source .of gettering material after the tube 10 has been exhausted. The electrode structure is fixed to a press or stem portion 45 by .leads 47 welded to respective electrodes and sealed through the press 45. A base member 48 is cemented to the end of the envelope portion 14. Leads 47 pass through base 48 and are soldered to base pins 49lfixed to base 47.

The operation of the gun structure, shown in Figure l is such that the electron emission from the cathode 24 is formed into an electron beam by control grid .28 :and

The adjacent ends of electrodes 30 s accelerating grid 39. During tube operation, the control grid 28 is given a bias, negative to the potential of the cathode 24, in order to cut off the electron emission from the cathode through the aperture of grid 28. Signal pulses drive the control grid 28 in a positive direction to permit electrons to pass through grid 28 in quantities proportional to the signal voltages. is operated at around 300 volts positive relative to cathode potential, which may be considered as ground. The operating potential of grids 28 and 30 bunch the electron trajectories in a manner that they tend to converge to a point at which the beam has a minimum cross-sectional area. This point is known as the first cross-over and is adjacent to the aperture in the end wall of accelerating electrode 30. The electrons, as they enter into the second accelerating grid 32, which is operated at around 12,000 volts, are diverging from each other and from the axis of the apertures of electrodes 28 and 30. The electrostatic field between accelerating grids 30 and 32 provides a small amount of focusing of the electrons, but, as the electrons pass down the tubular portion of electrode 32, they still remain divergent. Electrode 44 is operated at around 350 volts positive relative to cathode potential. The electrostatic fields set up between electrode 44 .and the more positive electrodes 32 and 36 constitute an electrostatic lens field, which is of a nature to cause the electron paths to converge to a small point of focus on the fluorescent screen 20.

The electron beam is scanned over the surface of the fluorescent screen 28 by two pairs of coils positioned in a neck yoke 64. The coil of each pair are connected in series and are positioned on opposite sides of the tube neck 14, and are coaxial to each other. The coils are connected to sources of sawtooth voltages (not shown) to provide line and frame scansion of the beam over fluorescent screen 20. Such a deflection system is well known in the prior art and does not constitute part of this invention.

In order to center the electron beam on the center Operating potentials for the various voltages are provided by a voltage source 68.

A more detailed description of the operation of electrostatic focus electron .guns may be found in Saunders and Swedlunds co-pending application Serial No. 225,126, filed May 8, 1951.

As previously mentioned, the adjacent ends of electrodes 30 and 32, being at an angle to their common axis, deflect the electron beam and the negative ions of that beam off their normal path and towards the side of electrode 32. However, an ion trap magnet 60, which is placed alongthe neck 14 of the tube, is positioned so that its magnetic field deflects the electron beam back onto the axis 16 of :the tube. The negative ions, which are relatively unaffected by the field of magnet 60, continue on their deflected path and impinge on electrode 32.

As mentioned above, the electron beam is still-divergent as it progresses through the tubular portion of electrode 32. Consequently, even though the electron beam is properly centered on the limiting aperture 34 of disc .33 on the end of electrode 32, some electrons impinge on the disc surface adjacent the aperture .34. Because of the high velocity of the electron beam (due to the high accelerating potential of electrode 32), the impinging electrons cause intense localized heating of disc 33. The heating in turn causes metal to be evaporated from the surface of the disc 33 and collect on cooler surfaces within the tube, such as the glass tube neck 14. During operation of the tube-the metallic deposits 58 on tube neck 14 be: come charged by stray electrons :and distort the focusing field between ring 44, disc 33,, and electrode 36. This The accelerating grid 30' distortion of the focusing field often becomes so severe that it is not possible to bring the beam back into focus with the limited focus voltage available in the receiver in which the tube is used. Furthermore, it would be uneconomical to provide a separate higher voltage focus potential source.

Evaporation of metal from the edges of the aperture 34 in disc 33 also gradually enlarges the limiting aperture so that eventually the shape of beam spot becomes distorted and fuzzy rather than sharp and well defined.

The degradation of picture quality due to distortion of the focusing field by deposits of evaporated metal on the tube neck takes place over a prolonged perod of time if the electron beam is properly centered and if the tube is operated at considerably less than maximum brightness. The other extreme of picture quality degradation occurs when a substantial portion of the electron beam impinges on disc 33 and the tube is operated at maximum brightness (maximum beam current) in order to produce a bright picture. Under these circumstances, one or more silvery rings 58 appear on the neck 14 of the tube within a matter of minutes, and loss of focus and loss of focus control usually occurs. Centering of the electron beam in the limiting aperture is primarily accomplished by proper adjustment of the ion trap magnet 60 which deflects the electron beam onto the tube axis 16. As mentioned earlier, the proper adjustment of the ion trap magnet is rather critical and is out of the control of the tube manufacturer. In addition, the adjustment of the magnet may change due to mechanical jarring or other causes not controlled by the one who installs the tube. It is therefore evident that evaporation of metal from the disc 33 must be substantially eliminated if the image quality of the kinescope is to remain good over a long useful life.

However, disc 33 is usually made of thin stainless steel which may be readily stamped to the proper shape, and thus has poor heat conduction characteristics for rapidly dissipating by conduction the localized spot heating of the type caused by impingement of high velocity electrons. Other metals which have better heat conducting properties are not used in making disc 33 because of difliculties in parts shaping, poisoning of the screen phosphors by traces of the metal, or prohibitive cost. For the same reasons, it is not practical to solve the evaporation problem by increasing the cross-sectional thickness of disc 33.

In accordance with the present invention, evaporation of metal from the disc 33 is prevented by increasing the ability of the disc 33 to dissipate heat by radiation and thereby hold the disc below the temperature at which metal will evaporate. One or both surfaces of the disc 33 are roughened, as by sandblasting, or blackened by carbonizing, or by oxidizing in sodium dichromate solution, for example, or both roughening and blackening in order to increase its heat radiating properties.

Tubes incorporating stainless steel discs 33 which have had their surfaces roughened or blackened in accordance with the present invention have shown no visible signs of evaporation of metal (either by loss of focus or by silvery deposits on the bulb neck 14) over a considerable period of time. On the other hand, prior art tubes tested under the same conditions showed marked defocusing after a short time.

Referring now to Fig. 2, there is illustrated a section of a cathode ray tube having a magnetic focus type of electron gun. In the tube of Fig. 2, the electron gun functions in a manner similar to electron gun 22, except that after the electrons emerge from the limiting aperture 34 they are focused by the focusing field of coil 62. However, since the bulb coating 52 extends toward the base of the tube past disc 33, metal evaporated from disc 33 collects on the already conductive bulb coating, and no defocusing effect is produced as is the case with electrostatic focus tubes where the coating 52 does not extend so far down the tube neck 14. However, the aperture 34 is subject to gradual enlargement (or actual puncturing should the electron beam be sufiiciently off center) due to evaporation of metal from the disc 33. This produces a fuzzy or distorted spot shape which, as mentioned before, degrades the quality of the image on the screen.

From the foregoing it is apparent that the present invention provides an improved electron gun inwhich the metal evaporation caused by, electron impingement on electrode surfaces is substantially eliminated.

What is claimed is:

l. A cathode ray tube comprising, an enclosing envelope, a target within said envelope, an electron emitting cathode, means for forming an electron beamfrom electrons emanating from said cathode and for directing said beam along a path toward said target, said means including a hollow electrode spaced along said path from said cathode, said hollow electrode having an apertured transverse wall of thin metal extending across said path of said electron beam, one surface of said apertured wall being roughened to prevent evaporation of metal therefrom.

2. A cathode ray tube comprising, an enclosing envelope, a target within said envelope, an electron emitting cathode, means for forming an electron beam from electrons emanating from said cathode and for directing said beam along a path toward said target, said means including a hollow electrode spaced along said path from said cathode, said hollow electrode having an apertured transverse wall of thin metal extending across said path of said'electron beam, one surface of said apertured wall being blackened to prevent evaporation of metal therefrom.

3. A cathode ray tube comprising, an enclosing envelope having a viewing screen, an electron emitting cathode, means for forming an electron beam from electrons emanating from said cathode and for directing said beam along a path towards said viewing screen, said means including a hollow electrode spaced along said path from said cathode, said hollow electrode having a thin metal transverse portion extending across said path ofsaid electron beam, said transverse portion having an aperture offset from said beam path, the surfaces of said transverse portion being roughened to increase their heat radiating capabilities, and means for directing said electron beam towards said aperture of said transverse portion.

4. A cathode ray tube comprising, an enclosing envelope having a viewing screen, an electron emitting cathode, means for forming an electron beam from electrons emanating from said cathode and for directing said beam along a path towards said viewing screen, said means including a hollow electrode spaced along said path from said cathode, said hollow electrode having a thin metal transverse portion extending across said path of said electron beam, said transverse portion having an aperture, offset from said beam path, the surfaces of said transverse portion being blackened to increase their heat radiating capabilities, and means for directing said electron beam towards said aperture of said transverse portion.

5. A cathode ray tube comprising, an enclosing envelope having a viewing screen, an electron emitting cathode, means for forming an electron beam from electrons emanating from said cathode and for directing said beam along a path towards said viewing screen, said means including a hollow electrode spaced along said path from said cathode, said hollow electrode having a thin metal transverse portion extending across said path of said electron beam, said transverse portion having an aperture, the surfaces of said transverse portion being roughened and blackened to increase their heat radiating capabilities, and means for directing said electron beam towards said aperture of said transverse portion.

6. A cathode ray tube comprising, an enclosing envelope having a viewing screen, an electron emitting cathode, means for forming an electron beam from electrons emanating from said cathode and for directing said beam. along a path towards said viewing screen, said means including a hollow electrode spaced. along said path, said hollow electrode'having a thin metal transverse portion extending across said beam path, the surfaces of said transverse portion being roughened to prevent evaporation of. metaltherefrom, an aperturein said transverseportion and ofiset from said beam. path, focusing means spaced along. said beampath in axial. alignment. with said aperture, and means for directing. said beam towards said aperture.

7. A cathode ray tube comprising, an enclosing envelope having a viewing screen,.an electron emitting cathode, means for forming. an electron beam from electrons ema: nating from said cathode and for directing. said beam along a path towards said viewing screen, saidmeans including; a: hollow electrode spaced alongsaidpath, said hollow electrode having a. thin metal transverse portion extending across said beam path; the-surfaces of said transverse portion being: roughened and blackened to prevent evaporation of metal therefrom, an aperture in said transverse portion, and. offset from said beam path, electrostatic focusing means spaced further along said beampath, said focusing means being. in axial alignment with: saidl aperture, and. means for directing said beanr towards said aperture.

8. A cathode ray tube comprising, an envelope, a fluo-- rescent screenwithin said. envelop,. electron gun means including a thermionic cathode electrode, gun means for forming electrons from said cathode into a. beam: and for directing said electron beam along a path towards said said means comprising a roughened surface of said wall ing a beam limiting electrode having: a thin-metal wall portion transverse to the path of said beam, said wall portion having abeam limiting aperture, and means for preventing evaporation of metal from said wall portion,

portion.

10. A cathode ray tube comprising an enclosing envelope, a target within said envelope, an electron emitting cathode, means for forming an electron beam from electrons emanating from said cathode and for directing said beam alonga path toward said target, said means including a beam limiting electrode having a thin-metal wall portion transverse to the path of saidbeam, said wall portion having. a beam limiting. aperture, and means for preventing.

evaporation of metal from said wall portion, said means comprising a blackened surface of said wall portion.

References Cited in the file of this patent UNITED STATES' PATENTS 1,981,652 Long Nov. 20, 1934 2,227,087 Hinsch Dec. 31, 1940 2,232,083 Strohfeldt Feb. 18, 1941 2,562,243 Phole July 31, 1951 2,608,666 De Gier Aug. 26, 1952 2,617,061 De Gier Nov. 4, 1952 2,637,828 Hoagland May 5, 1953 "gna

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