US3156029A

US3156029A - Electron gun and fabrication thereof

Info

Publication number: US3156029A
Application number: US131867A
Authority: US
Inventors: James O Simon
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1961-08-16
Filing date: 1961-08-16
Publication date: 1964-11-10
Anticipated expiration: 1981-11-10
Also published as: BE620963A; GB1005225A; NL282118A; DE1219133B

Description

Nov. 10, 1964 J. o. SIMON ELECTRON GUN AND FABRICATION THEREOF 2 Sheets-Sheet 1 Filed Aug. 16, 1961 INVENTOR. J M-' 0. 5/4/04/ firm: y

Nov. 10, 1964 J. 0. SIMON 3,156,029

ELECTRON GUN AND FABRICATION THEREOF Filed Aug. 16, 1961 2 Sheets-Sheet 2 16 (//////////J r///////////2 s ========f 4 a INVENTOR. w JiMis' 0. 5mm

5 ygw 47' away United States Patent Ce 3,1567% 3,156,029 ELETRON GUN AND FABRICATION THEREOF .larnes 0; Simon, Gas City, Ind, assignor to Radio (301'- poration of America, a corporation oi Delaware Filed Aug. 16, 1961, Ser. No. 131,867 3 Claims. (Cl. 2925.13)

This invention relates to electron guns for cathode ray tubes and to apparatus for and a method of accurately adjusting the cathode-control grid spacing to a desired amount to obtain a desired cutolf characteristic for the gun.

In one type of electron gun a tubular cathode having an electron emissive end wall is mounted concentrically within a control grid cup with the emissive end wall spaced adjacent to an apertured end Wall of the grid cup. The cathode is supported relative to the control grid by means fixed to a portion of the cathode tubular wall spaced from the emissive end wall. The support means may, for example, comprise a ceramic washer mounted transversely within the control grid cup or studs embedded in a glass rod to which the control grid is similarly mounted. Such electron gun further include one or more accelerating and/or focusing electrodes which are spaced from the control grid on the side thereof opposite the cathode.

The electron beam cutoff characteristic of such an electron gun as described above is a critical function of the spacing between the cathode emissive end wall and the apertured plate portion of the control grid. Even a minute change in this spacing may markedly change the cutoff characteristic of the electron gun.

Mechanical spacers placed between the cathode and the control grid have been used to obtain the desired spacing, but have failed to provide a consistently high degree of accuracy in quantity production. Other spacing techniques include telescoping the cathode into the control grid while electrically sensing (e.g., by are discharge or capacitance measurement) the spacing therebetween, and then when the desired spacing is obtained, fixing the cathode and control grid relative to each other such as by welding. Although the capacitance sensing technique provides a high degree of accuracy in sensing changes of spacing, difficulty has been encountered in maintaining the accurate spacing so provided. The physical contacting and/or the thermal expansion of the gun electrodes during the subsequent welding operation causes the accurately obtained spacing to be upset. Experience has shown that the welding operation frequently results in the cathode and control grid spacing being decreased by an amount which varies from gun to gun. Because the resulting spacing error varies, it cannot easily be compensated for; because the resulting spacing is undersize, it is not correctable by any known conventional technique.

Accordingly, it is an object of my invention to provide a new and improved electron gun of the type described and apparatus for and method of accurately adjusting the cathode-control grid spacing thereof during manufacture of the gun.

It is also an object of my invention to provide a new and improved apparatus for and a method of increasing to an accurately controlled critical value the spacing of cathode-control grid assemblies of the type described which have previously been spaced too closely together and then fixed in place.

My invention may be used for increasing the cathodecontrol grid spacing, whether such spacing was intentionally made too close as a step in the practice of my improved method of obtaining a desired cathode control grid spacing, or whether such spacing was unintentionally made too close in a practice of some prior art spacing Patented Nov. 1c, 1964 technique. In the latter case, an electron gun assembly which would otherwise be'scrapped may be salvaged.

Briefly, according to my invention, a cathode-control grid assembly of the type described is provided in which the tubular Wall of the cathode is radially bulged between the emissive end Wall and the portion of the tubular wall to which the cathode support means is fixed. Such bulging serves to axially shorten the cathode and thereby move the emissive end wall back and away from the apertured plate of the control grid to increase the spacing thcrebetween.

In one embodiment of apparatus for practicing the method of my invention, a servo mechanism is provided for automatically obtaining the desired cathode-control grid spacing. The servo mechanism may, for example, include a capacitance bridge for sensing the capacity between the cathode electrode and another electrode disposed in fixed spaced relation with the cathode. The output of the capacitance bridge is then used to disable a servo motor to which an expandable cathode bulging tool is mechanically coupled when a desired cathodegrid spacing is obtained.

In the drawings:

FIG. 1 includes a partial section of one embodiment of electron gun assembly of the type described to which the invention has been applied and a schematic showing of apparatus for practicing the invention;

FIG. 2 is an enlarged detail in longitudinal section of a portion of the electron gun assembly of HG. 1;

FIG. 3 is a transverse section taken along line 3-3 of FIG. 2; and

FIG. 4 is a partial section of another embodiment of the invention in an electron. gun assembly of the type described.

Referring to FIG. 1, a portion of an electron gun is shown which includes an elongated tubular cathode 10 concentrically disposed within a control grid cup 12 which has a central aperture 13. The cathode 10 includes an electron emissive end wall 14 which is closely spaced from the centrally apertured plate portion 16 of the control grid 12. The electron emissivity of the cathode wall 14 may be provided in conventional manher by a coating of metal oxides.

The cathode It) is mounted within the control grid cup 12 by means or a ceramic support washer 18. The cathode 16B is disposed within the central aperture of the ceramic washer 18 and is fixed therein by a pair of circumferential beads 29 crimped in the tubular wall 22 of the cathode. The ceramic washer 18 is sand-- wiched between flanges of a metal rim 24 and a metal retaining ring 26 which are welded together and to the cylindrical wall of the control grid cup 12. An approximate spacing of the cathode emissive wall 14 relative to the apertured grid plate 16 is obtained by telescopically sliding the cathode-ceramic washer assembly within the grid cup 12 and then welding the two

metal members

24 and 26 to the grid cup Wall when the desired approximate spacing is obtained. This approximate spacing is made less than that ultimately desired, thus permitting a subsequent practice of this invention to obtain an accurately controlled desired spacing as hereinafter described.

The electron gun assembly of FIG. 1 further includes a cup-shaped accelerating or screen grid electrode 30 spaced from the control grid cup 12 on the side thereof opposite the cathode 10. The accelerating electrode 30 has a central aperture 32 axially aligned with the aperture 13 of the control grid cup 12. Other electrodes including an additional accelerating electrode or focus electrode 36 (only a portion of which is shown) are mounted beyond the screen grid electrode 30. The electrodes of the electron gun assembly are mounted in fixed spaced relationship on a plurality of glass insulator rods 38 (only one of which is shown) by support studs 40 welded to the electrodes and embedded in the insulator rods 38.

In accordance with the practice of this invention, the tubular wall of the cathode is provided with an outwardly radially bulged portion 28 between the emissive end wall 14 and the portion of the tubular wall to which the support washer 18 is attached. As is hereinafter described in greater detail, the bulge 28 is provided for the purpose of shortening the cathode 10 to cause the emissive end wall 14 to be backed away from the apertured grid plate 16 to increase the spacing therebetween. In this way an accurate adjustment of the spacing between the cathode emissive end wall 14 and the apertured grid plate 16 is obtained.

The bulging of the cathode 10 may, for example, be provided by a conventional 120 split expander tool 42, which includes a tubular member 44 having a rounded flat head 46 on one end thereof. The tubular member 44 is axially split along three radii spaced 120 from each other. The axial splits extend from the head 46 part Way back along the tubular member 44. The axial bore of the tubular member 44 tapers from a given diameter to a smaller diameter in a direction toward the head 46 in the region of the axial splits. A tapered center plunger 48 is provided which, when advanced axially toward the head 46, causes the three sections of the head 46 to be moved radially outward in an expanding action. 46 is used to effect an outward radial bulging of the tubular wall 22 of the cathode 10.

A single operation of the 120 split expander tool 42 produces three equally spaced outward radial bulges 28. A more circumferentially continuous radial bulging of the cathode can be provided by a multiple operation of the expander tool 42, with the tool being slightly rotated relative to the cathode after each bulging operation thereof. The bulging of the cathode need not be circumferentially continuous. However, it is preferred that the bulging be circumferentially symmetrical such as provided by a circumferentially continuous bulge or by a number of equal size bulges equally circumferentially spaced from each other. Circumferentially symmetrical bulging avoids undesirable tilting of the cathode emissive end wall 14 relative to the control grid apertured plate 16.

The expander tool 42 is illustrated only by way of example. Other known devices for bulging the cathode may alternatively be used, for example, one which compresses a rubber O-ring to effect a circumferentially continuous outward radial bulging thereof. Such an alternative bulging tool is described, for example, with reference to FIG. 3 of U.S. Patent 2,507,979, issued to J. Kelar on May 16, 1950, and entitled Anchoring Cathode.

FIGS. 2 and 3 illustrate the effect of cathode-to-control grid spacing that is obtained by a bulging of the tubular wall of the cathode sleeve. The cathode 10, provided with three bulges 28 equally circumferentially spaced from each other, is illustrated in its bulged condition in solid lines and in its condition before bulging in dotted lines.

The spacing of the emissive end wall 14 from the apertured control grid plate 16 of the cathode before bulging is designated with the letter S. The spacing of the emissive end wall 14 from the apertured control grid plate 16 of the cathode after bulging is designated with the letter S. The radius of the tubular wall of the cathode before bulging is designated with the letter R, and the radius of the bulged cathode at the bulge is designated by the letter R.

As illustrated in FIG. 2, a bulging of the tubular Wall 22 of the cathode 10 to locally change its radius from R to R serves to shorten the portion of the cathode between the support means 18 and the emissive end wall 14 by an amount equal to SS and thus move the Such expanding action of the head (i emissive end wall 14 back away from the apertured grid plate 16 by this same amount.

As an example of the practice of this invention, an electron gun of the type illustrated in FIGS. 1-3 and described below has been incorporated in a cathode ray picture tube type 19XP4. The cathode had a radius before bulging of about 62 mils, and the ceramic support washer 18 was spaced about 100 mils from the emissive end wall 14. The cathode was rough-spaced to a distance of about 4 mils from the control grid apertured plate 16 by capacitance sensing techniques and then fixed relative. thereto by welding. The cathode was then provided with three 120-spaced outward radial bulges, each of which locally increased the cathode radius from about 62 mils to about 70 mils. Such bulging served to increase the cathode-control grid spacing by about one mil to an accurately controlled desired distance of about 5 mils.

FIG. 4 illustrates a form of electron gun which is different from that illustrated in FIGS. 1-3, and to which the invention has been applied. The electron gun of FIG. 4 comprises an elongated tubular cathode 50, which has an emissive end wall 52, and which is supported coaxially within a tubular cathode shield 54 by a cathode support ring 5-5 between the cathode 50 and the shield 54. A centrally apertured control grid cup 58 is coaxially telescoped over the shield 54 and the emissive end wall 52 of the cathode 50. An accelerating or screen grid electrode 60 is spaced from the control grid cup 58 on the side thereof opposite the cathode 50. The cathode-

cathode shield assembly

50, 54, the control grid cup 58, and the screen grid cup 60 are all mounted in fixed spaced relationship on a plurality of glass insulator support rods 62 (only one of which is shown) by studs 64 welded to the electrodes and embedded in the glass rods 62.

The tubular wall 66 of the cathode St) is provided with one or more radial bulges 68 between the emissive end wall 52 and the portion of the tubular wall 66 at which the support ring 56 is attached. As in the assembly of FIG. 1, bulges serve the purpose of adjusting the spacing between the emissive end wall 52 and the apertured end wall of the control grid cup 58.

While the bulging of the

cathode sleeve

10 or 50 with the expander tool 42 or other suitable apparatus may be manually performed, it is preferred that the bulging operation be provided automatically, utilizing a serve mechanism system such as that schematically illustrated in FIG. 1.

A suitable servo mechanism may comprise a capacitance bridge 70, a phase discriminator 72, a chopper amplifier 74, and a servo motor 76. The capacitance bridge 70, which may be of a type capable of sensing capacitance differences of 0.0001 [.LiLf. In the bulging operation it is connected between the cathode 10 and the accelerating electrode 30 to sense the capacitance C therebetween.

In the bridge 70 two sections C and C of a ditferential capacitor are serially connected between the ends of a center tapped secondary winding 7 8 of a quadrawound toroidal transformer. The center tap of the winding 78 is grounded. A factor capacitor C whose size deter mines the measurement range of the bridge, is connected between the junction of the two sections C and C of the differential capacitor and a first one of a pair of input terminals AA of the bridge. The other output terminal is connected to the junction of the section C of the differential capacitor and an end of the winding 78. An output E of the bridge 70 is taken between ground and the first one of the terminals AA.

A separate standard capacitor C having terminals A'A' is provided which has a value equal to the capacitance C between the cathode 10 and the screen grid 30 when the cathode is at the desired spacing from the control grid 12.

The phase discriminator 72 includes a center tapped transformer secondary winding 80, between the ends of 8 J which are serially connected a first diode D a pair of resistors R and R and a second diode D;,. A first capacitor C is connected in parallel with resistor R and a second capacitor C is connected in parallel with resistor R The output E of the bridge 70 is fed through a transformer 82 whose secondary is connected between the center tap of the winding 80 and the common junction of resistors R and R and capacitors C and C The output E of the discriminator 72 is taken across the serially connected resistors R and R The bridge 70 is prepared for measuring the electrode spacing capacitance C by first connecting the terminals A'A of the standard capacitor C to the bridge input terminals AA. An A.C. reference signal E is fed to the bridge through the quadrawound toroidal transformer, and the bridge is balanced by adjusting the differential capacitor C -C to increase the capacitance in one .eg of the bridge and decrease the capacitance in the other leg. The reference signal may, for example, comprise a 465 kc. output from a crystal oscillator.

Cathode-grid spacing adjustment is made with the standard capacitor C disconnected from the bridge terminals AA and the terminals A"A" from the cathode and screen grid connected to the bridge terminals AA. The control grid 12. is grounded.

The reference signal E is fed both to the bridge 70 and to the discriminator 72 through the transformer windings 78 and 88, respectively. If the cathode-control grid spacing is such that C =C the bridge 70 is balanced and the output E, from the bridge is zero. In this event capacitors C and C become equally and oppositely charged, and the output E from the phase discriminator 72 is zero. If the cathode-control grid spacing is less than that which is desired, then C is greater than that required to balance the bridge and an output E from the bridge 70 of a given phase is produced; it the cathode-control grid spacing is greater than that which is desired, then C is less than that required to balance the bridge and the bridge output E is of the opposite phase. In either case, capacitors C and C become charged to unequal voltages of opposite polarity. Depending upon the phase of E the charge on one or the other of capacitors C and C Will be the greater and will thus produce a DO. output E of one polarity or the other.

The output E of the discriminator 72 is fed to a chopper amplifier 74 whose output is in turn coupled to the servo motor 76 through a control unit 84 to energize the motor and thereby actuate the plunger 48 of the expander tool 42. It B is of one polarity, the plunger 48 will be advanced; if E is of the other polarity the plunger 48 will be Withdrawn. The servo system is so arranged that if C is greater than that required to balance the bridgeindicating too close a cathode-control grid spacingthe plunger 48 is advanced to bulge the cathode 10 and thereby increase the cathode-control grid spacing.

In operation, with the bridge having been balanced against C the expander tool 42 is inserted into the cathode sleeve 10 and the bridge 7 0 and discriminator 72 are energized with the reference signal E, to feed a signal E to the amplifier 74 indicative of the cathode-control grid spacing. A switch (not shown) in the control unit 84 is then closed to energize the motor and advance the plunger 48 of the expander tool 42 upwardly as shown in FIG. 1 to cause a bulging of the tubular Wall 22 of the cathode 10. This action serves to shorten the cathode sleeve 10 and move the emissive wall 14 away from the apertured grid plate 16. Such movement serves in turn to decrease the capacitance C between the cathode and accelerating electrode. Such decrease of capacitance tends to balance the bridge 70. When the desired cathode-control grid spacing is obtained and the bridge 70 thus balanced, the

output E of the discriminator 72 is zero and the servo motor is disabled.

The switch in the control unit 84 is then opened and a suitable voltage (source not shown) is applied to the servo motor through the control unit 84 to withdraw the plunger 48 so that the expander tool 42 may be removed from Within the cathode 10.

I claim:

1. In the method of assembling an apertured control electrode and an elongated tubular cathode having a coated electron emissive end wall closely spaced from said control electrode, the steps of: fixing said coated end wall in such close proximity to said control electrode that it can no longer be suitably contacted and pushed away from said control electrode, and then setting the spacing between said coated end wall and said control electrode by outwardly bulging the tubular sidewall of said cathode to thereby shorten the initial length of said cathode and thereby increase the distance between said coated end wall and said control grid to a desired amount.

2. In the assemblying of an electron gun including a control electrode and an elongated tubular cathode having an end wall with an electron emissive coating thereon which is closely spaced from said control electrode, the method comprising the steps of supporting said cathode from a portion of its tubular wall so as to fix the coated end wall thereof relative to and in close proximity to said control electrode, adjusting the spacing between said coated end wall and said control electrode by outwardly bulging the tubular sidewall of said cathode at a location between said coated end wall and said portion from which the cathode is supported to shorten the initial length of said cathode and thereby separate the said coated end wall and said control electrode further apart.

3. In the fabrication of an electron gun including a tubular cathode, a control electrode, and an accelerating electrode disposed in the order named, said tubular cathode having an end wall coated with electron emissive material closely spaced from said control electrode and being supported relative to said control electrode from a portion of its tubular wall longitudinally spaced from said end wall, the method of spacing said coated wall from said control electrode comprising the steps of: fixing said cathode coated wall in close proximity to said control electrode at a spacing therefrom closer than that ultimately desired, and then increasing the spacing of said cathode coated wall from said control electrode to an ultimately desired distance by outwardly bulging the cathode tubular wall between the support portion thereof and said coated end wall in a plurality of circumferentially spaced individual bulges to shorten the length of said tubular cathode and thereby separate said cathode coated wall and said control electrode further apart.

References Cited in the file of this patent UNITED STATES PATENTS 2,106,451 Brown Jan. 28, 1936 2,411,184 Beggs Nov. 19, 1946 2,443,916 Kelar June 22, 1948 2,748,463 Mueller June 5, 1956 2,814,090 Cheatle Nov. 26, 1957 2,831,238 Chase et a1 Apr. 22, 1958 2,884,684 Wolke et a1. May 5, 1959 2,951,279 Mozek Sept. 6, 1960 3,026,438 Warne Mar. 20, 1962 3,030,578 Lessman Apr. 17, 1962 3,059,466 Urmengi Oct. 23, 1962 3,073,160 Shawhan Jan. 19, 1963

Claims

1. IN THE METHOD OF ASSEMBLING AN APERTURED CONTROL ELECTRODE AND AN ELONGATED TUBULAR CATHODE HAVING A COATED ELECTRON EMISSIVE END WALL CLOSELY SPACED FROM SAID CONTROL ELECTRODE, THE STEPS OF: FIXING SAID COATED END WALL IN SUCH CLOSE PROXIMITY TO SAID CONTROL ELECTRODE THAT IT CAN NO LONGER BE SUITABLY CONTACTED AND PUSHED AWAY FROM SAID CONTROL ELECTRODE, AND THEN SETTING THE SPACING BETWEEN SAID COATED END WALL AND SAID CONTROL ELECTRODE BY OUTWARDLY BULGING THE TUBULAR SIDEWALL OF SAID CATHODE TO THEREBY SHORTEN THE INITIAL LENGTH OF SAID CATHODE THEREBY INCREASE THE DISTANCE BETWEEN SAID COATED END WALL AND SAID CONTROL GRID TO A DESIRED AMOUNT.