Cathode ray tube with longitudinally seamed body portions
I Umted States Patent 1 [111 3,  
 de Vere Krause Apr. 1, 1975 [5 CATHODE RAY TUBE WITH 2.250.622 1/1941 Bowie 313/89 x LONGITUDINALLY SEAMED BODY 2,828,433 3/1958 Frenkel 313/82 R PORTIONS 3.047.759 7/1962 McNaney 313/82 R 3.286.118 11/1966 Vause et al........ 313/318 [76] Inventor: Anthony Victor de Vere Krause, 3 3,319,818 5/1967 Hudson 313/64 X Beechcroft Ashtead England Marshall R [22] F d J l 9 1973 3.462.629 8/1969 Bell t. 313/82 TS 1e uy Appl&#39; N07 377778 Primary E.\&#39;aminerRobert Segal Related U.S. Application Data [63] Continuation of Ser. No. 123.279, March 11, 1971.  
  b d d. a 57 ABSTRACT [30] Foreign Application Priority Data A t b I d t l t vacuum u e comprlsmg an enve ope an a eas m fi it z g one electrode within that envelope, wherein the enveg&#39; gi z 5214/7 lope includes two portions joined together along a mg Om seam and the required electrical connection is made to the electrode from outside the envelope by means of at least one conductor which passes through the seam. Preferably, the conductor serves to locate and [58] Field of Search 313/82 R, 32 TS, 317, 318 support the electrode in the tube [56] References Cited UNITED STATES PATENTS 6 Claims, 38 Drawlng Figures CATHODE RAY TUBE WITH LONGITUDINALLY SEAMED BODY PORTIONS This application is a continuation of Ser. No. 123,279; filed Mar. ll, l97l, now abandoned.  
 BACKGROUND OF THE INVENTION This invention relates to vacuum tubes, and is particularly, although not exclusively, concerned with improvements in the construction and manufacture of a cathode ray tube such as may be used in a television display apparatus.  
  The development of miniature televisions, particularly those small enough to be carried in the pocket, has been retarded by a number of factors. Among these factors has been the design and manufacture of a cathode ray display tube which is of suitably small physical dimensions and which requires a low power supply for its operation, enabling the use of small and light batteries to power a portable set.  
  Further from the commercial point of view, the market for such televisions has been limited in view of the cost of manufacturing such a television, the cathode ray display tube contributing a significant part of that cost.  
  It is therefore an object of the present invention to provide a cathode ray display tube suitable for use in a television apparatus which is small in physical size, is of low power requirement, while still being cheap to manufacture.  
  It is to be understood however, that the construction and method of manufacture of the invention is applicable to other types of vacuum tube.  
 SUMMARY OF THE INVENTION Accordingly, in one aspect, the present invention provides a vacuum tube comprising an envelope and at least one electrode within that envelope, wherein the envelope includes two portions joined together along a seam and the required electrical connection is made to the electrode from outside the envelope by means of a conductor which passes through the seam.  
  Preferably in such a tube, the conductor serves to locate and support the electrode in the tube.  
  In a further aspect the invention provides a cathode ray tube comprising an envelope and a plurality of electrodes therein forming an electron gun assembly, said envelope including two body portions joined together along a seam, the electrodes of said gun being supported and located in the envelope by means of conductors through which the required electrical connection is made to the respective electrode said conductors passing through the seam.  
  In a preferred arrangement of such a cathode ray tube, the screen is formed on a face of a cup portion of the envelope which is joined to said two body portions along a further seam.  
  According to a further aspect of the invention, there is provided a method of manufacturing a vacuum tube comprising an envelope and at least one electrode therein, with electrical connection to the electrode from outside the envelope by means of a conductor, including the steps of locating said electrode with the conductor in a jig, providing the envelope in at least two portions, locating the two portions around the electrode in the jig and joining the two portions together along a seam with the conductor passing through the seam.  
  In such a method as applied to the manufacture of a cathode ray tube comprising an envelope and a plurality of electrodes therein forming an electron gun assembly, said envelope may preferably be provided in two body portions with a screen cup to be joined to the body portions to complete the envelope after said two body portions have been assembled around the electrodes in the jig.  
  Preferably in the above methods the completion of the envelope is carried out in a vacuum thereby eliminating the need for subsequent evacuation of the envelope.  
 BRIEF DESCRIPTION OF THE DRAWING In order to promote a fuller understanding of the above, and other, aspects of the present invention, embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:-  
 FIG. 1 is a side elevation of a cathode ray tube,  
 FIG. 2 is a plan view of the tube of FIG. I,  
 FIG. 3 is a face view of the tube of FIG. 1,  
  FIG. 4 is a schematic cross-sectional view taken on the line IV-IV of FIG. 2,  
  FIG. 5 is a schematic cross-sectional view taken on the line V-V of FIG. 1,  
  FIG. 6 is a schematic cross-sectional view taken on the line VI-VI of FIG. 4,  
  FIG. 7 is a schematic cross-sectional view taken on the line VII-VII of FIG. 4,  
  FIG. 8 is a schematic cross-sectional view taken on the line VIII-VIII of FIG. 5,  
  FIGS. 9 and 10 show the screen potential control electrode of the tube of FIG. 1,  
  FIGS. 11 and 12 show one of the Y deflection plates of the tube of FIG. 1,  
  FIGS. 13 and 14 show the interplate screen of the tube lfFIG. l,  
  FIGS. [5 and 16 show the X deflection plates of the tube of FIG. 1,  
  FIGS. 17 and 18 show the No. 3 anode of the tube of FIG. 1,  
  FIGS. I9 and 20 show the No. 2 anode of the tube of FIG. 1,  
  FIGS. 21 and 22 show the No. l anode of the tube of FIG. 1,  
 FIGS. 23 and 24 show the grid of the tube of FIG. 1,  
  FIG. 25 shows the assembly of the grid and filament of the tube of FIG. 1,  
  FIG. 26 shows a typical portion of the seam of the envelope of FIG. 1,  
  FIG. 27 shows a schematic cross-section on the line XXVII-XXVII of FIG. 26,  
  FIG. 28 shows a similar view to that of FIG. 25 of a further assembly of grid and filament,  
  FIG. 29 shows an enlarged view of the filament of FIG. 28,  
  FIG. 30 shows a part schematic view on the arrow A of FIG. 28 with anode No. l omitted.  
  FIG. 31 shows a mandrel assembly for use in the assembly of the tube of FIG. 1,  
  FIG. 31A is an enlarged fragmentary plan and side view of the upper end portion of FIG. 31,  
 FIG. 32 shows a side view of the assembly of FIG. 31,  
  FIG. 32A is an enlarged fragmentary view of the upper end portion of FIG. 32,  
 FIG. 33 shows a plan view of the assembly of FIG. 31,  
  FIG. 34 shows the mandrel assembly of FIG. 31 together with retaining fingers in use in the assembly of a tube of FIG. 1,  
  FIG. 35 shows a side view of the apparatus of FIG. 34 and,  
  FIG. 36 shows a typical oven arrangement for sealing a tube in the apparatus of FIG. 34.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. I, 2 and 3 show the external arrangement of the cathode ray tube of this embodiment.  
  The glass envelope of cathode ray tube is formed in three parts, as best seen in the side view of FIG. I, the parts comprising two body halves I and 11, and a screen cup 12 having a screen end face 13. The three body parts 10, II and 12 are joined together to form a complete envelope by soldering with a low melting point glass frit in a manner to be described in more detail below.  
  As best seen in FIGS. 4 and 5 which are crosssectional views of the cathode ray tube, the various electrodes of the electron gun of the tube which is of the electrostatic deflection type, are supported individually on a series of supporting wires, the supporting wires passing through the envelope along the seam where the two body halves I0 and 11 are joined together. The supporting wires also serve for electrical connection to the respective electrodes.  
  The electrodes of the gun will now be described in brief detail. The first electrode immediately behind the screen of the tube is a screen potential control electrode which is shown on an enlarged scale in FIGS. 9 and I0. It is provided with supporting wires 21 and 22 and is positioned towards the front of the tube immediately behind the joint between the screen cup 12 and the two body halves l0 and 11. The next electrodes are the two Y deflection plates 23 and 24, the plate 23 being shown in more detail in FIGS. II and 12, each is provided with supporting wires 25 and 26. The next electrode is the interplate screen 27 shown in more detail in FIGS. 13 and 14 and provided with supporting wires 28. The screen 27 is provided with a central slit 29 through which the electron beam may be directed from the X plates. The next electrodes are the X plates 30 and 3] shown in more detail in FIGS. 15 and 16, and provided with supporting wires 32 and 33. The next electrode is an anode, referred to as No. 3 anode, indicated at 34 and shown in more detail in FIGS. 17 and 18. The No. 3 anode is provided with a central aperture 35 through which the electron beam may pass, and supporting wires 36.  
  The next electrode is an anode referred to as No. 2 anode indicated at 37 and shown in more detail in FIGS. I9 and 20. It is to this anode that a beam focusing potential is applied to the tube when in use, and it is provided with an aperture 38 for the beam and with supporting wires 39. The next electrode is an anode referred to as No. 1 anode indicated at 40 which is shown in more detail in FIGS. 21 and 22 and which is provided with an aperture 41 for the beam and supporting wires 42. To complete the electron gun assembly is the grid indicated at 43 and shown in more detail in FIGS. 23 and 24, which forms an assembly with a directly heated cathode filament 44, the assembly being shown in more detail in FIG. 25. The grid electrode 43 is provided with a central aperture 45 through which electrons emitted from the directly heated cathode 44 may pass.  
  The assembly of grid 43 and filament 44 forms a rigid structure, the supporting wires 46 being rigidly attached to the grid electrode 43 to support it and the assembly in the envelope of the tube. Attached to the supporting wires 46 are two further supporting wires 47 to which the ends of the filament 44 are rigidly attached. The respective wires 46 and 47 are rigidly attached to one another but electrically insulated from one another, by means of glass beads 48 into which the respective supporting wires are fused. The grid electrode 43 is provided with apertures 49 to afford clearance for these glass beads, and may be provided with depressions 50 for attachment to the support wires 46 in an alternative assembly to be discussed below. The grid electrode 23 is also provided with stiffening grooves 51.  
  Thus it can be seen that all the electrodes of the electron gun of the cathode ray tube are located and supported in the envelope of the tube by means of the supporting wires. This transfers the location of the various electrodes in the gun assembly to the actual envelope of the tube as opposed to the location by separate internal spacers and supports as in tubes of previous construction where the gun was a separate assembly independent of the envelope. This feature enables a considerable reduction in the overall size of the envelope to accommodate the particular assembly of electrodes resulting in a smaller size of tube envelope for a given gun and screen size.  
  In the embodiment shown particularly in FIG. 25, the filament 44 is of a coiled coil tungsten construction. In an alternative arrangement, shown particularly in FIGS. 28, 29 and 30, the filament may be of a foil strip type which is more particularly disclosed in our copending British Patent Application No. 5214/71. In this arrangement, the filament indicated at 52 is in the form of a strip of thin foil material, such as nickel which is arranged in a zig-zag pattern to cover an area behind the aperture 45 in the grid 43 as particularly shown in FIG. 30. The foil pattern of the filament 52 extends between conductor members 53 and 54 which are in turn attached rigidly to the supporting wires 47 by the intermediary of members 55 and 56. Again, in this arrangement the grid 43 is attached to the supporting wires 45 by means of the indentations 50 mentioned above, and the supporting wires 46 and 47 are connected together by means of the glass beads 48 to complete a rigid assembly as before. The conductor members 53 and 54 are temporarily joined by bars 57 during the formation of the assembly, and the bars 57 are severed either mechanically or electrically only when the assembly has been completed, thus eliminating the possibility of strain to the filament pattern 52 during the formation of the sub-assembly. In this way it can be seen that the filament pattern 52 is again correctly located beneath the grid aperture 45 and located axially at the same time. The foil construction of the filament arrangement is particularly advantageous in providing a filament of sufficient electrical resistance to achieve a low power consumption while still enabling an adequate surface area of filament material to be disposed within the are covered by the grid aperture 45.  
  The portions of the envelope of the tube are preferably joined together along their respective seams by means of a low melting point glass frit. As particularly seen in FIG. 26, the edges of the body halves and 11 may be provided with grooves 60 at the appropriate points where the supporting wires for the electrodes pass through the joint between them. The glass frit indicated generally at 61 apart from serving to join the two portions together, also serves to form a seal around the supporting wire where it passes through the seam.  
  In order to assemble the tube envelope with the electrodes inside, the electrodes of the electron gun are first assembled in a jig. The jig arrangement is particularly shown in FIGS. 31, 32, 33, 34, and 35. The jig comprises a central mandrel assembly (FIGS. 31, 32 and 33 which serves to locate the electrodes along the axis of the electron gun. The mandrel comprises a base indicated at 70, a shoulder 71 extending upwardly from the base, a wide strip mandrel 72 extending upwardly from the shoulder 71, and a central circular crosssection mandrel 73 extending upwards from the strip mandrel 72. The mandrel 73 actually extends as a rod through the whole height of the mandrel assembly and is replacable in case of damage.  
  The mandrel is arranged in a jig assembly as particularly shown in FIGS. 34 and 35, as a central support for the electrodes of the electron gun, and means are provided to individually locate and retain the various electrode components in their correct disposition along the mandrel.  
  The top of the cylindrical mandrel portion 73 is provided with a shoulder 74 (FIG. 31A) and a spiggot 75. The grid filament assembly discussed above is located on the spigot 75 by means of the grid aperture 45. In order to prevent the grid assembly tilting on the spiggot 75 it is supported from below by a pair of spring fingers 76 on each side as shown in FIG. 34 which abut below its edge and angularly located by a pair of spring fingers 77 on each side, the fingers 77 being arranged to pass and engage on either side of the supporting wires 46. The tops of the fingers 76 and 77 are lipped to hold the grid down, but allow subsequent withdrawal of the jig after assembly, as indicated at 131.  
  The No. 1 anode 40 is located on the cylindrical mandrel 73 by its aperture 41 and supported in its correct vertical position by means of spring fingers 78 on either side as seen in FIG. 35, the fingers 78 being provided with a notch at the top to receive a spigot 79, appropriately provided on the anode 40 as shown in FIGS. 21 and 22. The support wires 42 also pass between the fingers 77 and 76 on each side as seen in FIG. 35. The fingers 78 are also lipped to hold the anode down with the spigot 79 in the notch.  
  The No. 2 anode 37 is supported vertically by spring fingers 80 which are positioned behind the finger 78 as seen in FIG. 34 and is located sideways as seen in FIG. 34 by two spring fingers 81 which engage on either side of the shoulders 82 formed on the No. 2 anode as seen in FIG. 19, and by the support wires 39 passing between the fingers 76 and 77. The tops of the fingers 81 are lipped as above to hold the anode down.  
  The No. 3 anode 34 is again located by its aperture 35 on the cylindrical mandrel 73, angularly located by its support wires 36 passing between the pairs of fingers 77 and 76 and is supported vertically by fingers 83 abutting beneath it, the fingers 83 being positioned behind (as seen in FIG. 34) the fingers 81 and 80, the fingers being provided with projections 130 to hold the anode 34 down.  
  The X deflection plates 30 and 31 are supported 0 the mandrel with a notch 84 on the upper edge engaging the cylindrical mandrel 73, and a notch 85 in the lower edge engaging on a shoulder 86 formed at the top of the strip mandrel 72. The strip mandrel 72 is chamfered at a point 87 at its upper end to correspond with the angle of the adjacent part of the X deflection plates. The support wires 32 and 33 of the X deflection plates are provided with shoulders 87 and 88 respectively, these shoulders being formed by tubes fixed over the support wires, and spring fingers 89 and 90 are provided on each side (as seen in FIG. 34) to engage on those shoulders and retain the Y plates in the position as discussed above. The fingers 89 and 90 lie behind (as seen in FIG. 35) the fingers 77 and 76. The tops of the fingers 89 and 90 are also lipped to hold the plates down in position.  
  The inter-plate screen 27 is located with its slit 29 over the strip mandrel portion 72, and it is supported vertically by spring fingers 91 which lie behind (as seen in FIG. 34) the fingers 83 and abut below the screen. The supporting wires 28 pass, on each side, through the slot between the fingers 77 and 76, 89 and 90 and an additional locating finger 92 is provided to abut each side of the screen as seen in FIG. 35, again lipped to hold the screen in place.  
  The Y deflection plates are supported with their lower edge 100, that is to say the radiused edge nearest the screen of the tube, resting on a surface 101 provided on the base 70 of the mandrel assembly, and with their upper edges 102 resting on the respective sides of the strip mandrel 72. The support wires 26 of the Y deflector plates pass out on either side through the slot between the fingers 76 and 77, 89 and 90 respectively, and the arms 103 by which the support wires 26 are attached to the actual deflector plates, are received on each side in a slot 104 formed at the top of a rigid finger 105 extending out from the mandrel assembly base 70. The arms 106 by which the support wires 25 are attached to the plates extend outside the fingers extending from the base 70 of the mandrel assembly and are not supported. In order to locate the bottom of the X plate sideways (as seen in FIG. 34) the bottom corners engage on abutment plates 107 provided at each side of the mandrel base 70.  
  The screen potential control electrode 20 rests on the base 70 of the mandrel assembly, being located sideways (as seen in FIG. 34) by the plates 107, and supported vertically by shoulders 108 formed on those plates.  
  The various spring fingers used to locate the various electrodes as discussed above are removably attached to the base 70 of the mandrel with screws and accurately located relative to the mandrel proper by means of dowels 109 so as to be replacable when damaged.  
  Thus it can be seen that the electrodes can be assembled progressively on the mandrel, and once so assembled will be accurately located relative to one another in the correct position which they should assume in the finished cathode ray tube.  
  The mandrel assembly base 70 is provided on either side (as seen in FIG. 35) with a removable outer base portion 110 which are each provided with a recess 111 to receive the then lower edges of the body portions 10 and 11 of the tube. Thus the body portions 10 and 11 can then be positioned around an assembly of electrodes on the mandrel, with the various supporting leads extending between the joint of the two body portions, and spring clips 112 are provided on the outer base portions 110 to retain the body portions in position, the clips having an aperture 113 to engage a pip 114 formed on the side of the body portions 10 and 11. The pips 114 also serve to support the finished tube so that it will lie on a flat surface with the screen face 13 vertical.  
  Two basic procedures may be adopted for joining the body portions 10 and II together. The first is to coat the abutting edges of the body portions with a water suspension of a low melting point glass frit before placing them together around the electrodes on the assembly jig; and then to place the whole assembly of electrodes and body portions on the jig in a suitable oven as typically shown in FIG. 36, to heat the whole and thus fuse the glass frit and join the body portions together, at the same time sealing in the various supporting wires passing through the joint. The assembled pair of body portions together with the various electrodes are then removed vertically from the jig, the various spring fingers deflecting to allow this, thus to leave a completed assembly of the two body portions 10 and 11 with the electrodes of the gun located correctly in position therein. As a next step the end face of the thus formed assembly is coated with glass frit, and the screen cup 12 held in position while the whole is again heated to fuse the glass frit to join the screen cup to the remainder of the assembly. In this instance, the frit used for the second joint is preferably of a lower melting point than that used to join the body portions 10 and 1] together.  
  In an alternative procedure, the body portions l and 11 are coated with a suitable adhesive material, along the inner part of their edges as indicated at 120 in FIG. 27, the body portions are then positioned on the jig as discussed above, and the adhesive is allowed to set to hold the two body portions together while at the same time holding the various supporting wires of the electrodes in a similar manner to that described above. The adhesive assembled body portions together with the electrodes are again removed from the assembly jig, and the remainder of the joining faces of the body portions and I] coated with a low melting point glass frit suspension as indicated at 121 in FIG. 27, and the end faces to receive the screen cup 12 are also coated with frit. The screen cup 12 is then held in position with the assembled body portions and the whole again placed in an oven to fuse the glass frit, thus achieving a completion of the joint between the body portions 10 and 11, the sealing of that joint, the assembly of the screen cup 12 to the body portions and the sealing of that joint in a single operation.  
  Once the complete envelope has been sealed in either of the above methods, it is subsequently evacuated by means of a spew indicated at 122 in FIGS. 1 and 2, which is formed for the purpose in known manner per se and sealed off after evacuation.  
  in yet a further precedure which is applicable to either of the above basic methods, after the body portions 10 and 11 have been sealed together with glass frit or adhered together with adhesive, in each case with the electrodes assembled between them and removed from the assembly jig, the screen cup 12 may be attached to the body portions with a suitable glass frit between the surfaces and the whole fused for final assembly and sealing in an evacuated oven so that the envelope is sealed in an evacuated condition. This eliminates the need for subsequent evacuation.  
  In either instance suitable getters may be provided within the envelope in known manner per se to trap any residual gases which may be left within the envelope after it has been sealed.  
  In either instance, a suitable jig and retaining device (not shown) is provided to hold the screen cup on the end faces of the assembled body portions during the heating cycle in an oven to fuse the glass frit.  
  In an alternative method of assembling the electron gun electrodes in the body portions 10 and 11, they may be located in a similar manner by a jig (not shown) which holds them by means of their respective supporting wires at a point outside the wall of the envelope. In this alternative method, the joints between the body portions 10 and 11 and between them and the screen cup 12 may be completed simultaneously either at atmospheric pressure with the envelope subsequently being evacuated as discussed above, or in a vacuum as discussed above. With such a method of assembly it is desirable to use more substantial supporting wires in order to achieve rigidity and accuracy of location of the electrodes, than is the case with the method of assembly discussed above where the electrodes themselves are located by the jig.  
  It will be appreciated that a number of materials may be used for the construction of the electrodes of the cathode ray tube, stainless steel, titanium or nickel copper alloys being suitable for the grid, anodes, screen, deflection plates, and screencontrol electrodes while a nickel iron alloy is preferred for the support wires for the electrodes. The coefficient of thermal expansion of the support wires is preferably of the same order to that of the glass frit used for sealing the envelope portions in order to make a good strain free seal between the supporting wires and the frit. The support wires are copper plated and borated in known manner per se to improve their compatability with the glass of the fit.  
  For the envelope portions the preferred material is a barium lithium glass of medium melting point and a preferred glass frit for sealing the portions is a devitrifiable frit such as Pyroceram&#34; frit as made by Corning Glass Company. The preferred adhesive for use in holding the body portions 10 and 11 together in accordance with one of the methods discussed above, prior to sealing with glass frit, is the adhesive lnsalute&#34; manufactured by Sauerisen Company, Pittsburg, Pa., U.S.A.  
  The inside of the face 13 of the screen cup is coated in known manner per se with a phosphor suitable to the purpose for which the cathode ray tube is intended such as in television display apparatus.  
  While the above discussion has been related particularly to embodiments of the invention in the form of cathode ray tubes, it is to be understood that the invention is applicable to other types of vacuum tubes, and finds particular application in any vacuum tube where a number of electrodes or other items are to be located relative to one another with accuracy within the envelope thethe tube.  
  Thus it can be seen that the invention provides a construction and method of manufacture of vacuum tubes, particularly cathode ray tubes which enables a compact unit to be manufactured. The supporting wires from the electrodes which also serve to make electrical connec tion to them may be connected into a conventional type of base plug, but are preferably left as flying leads by which the tube may be connected directly into a circuit, particularly a printed circuit.  
 What we claim is:  
  1. A cathode ray tube comprising a glass envelope and a plurality of electrodes therein forming an electron gun assembly; said envelope including two body portions joined together along a seam, conductors supporting and positioning the electrodes of said gun assembly within said envelope, said conductors forming electrical connections to the electrodes from exteriorly of the envelope, said conductors passing through the seam, said envelope including a screen cup having a phosphor-coated face of the tube formed therein, and a second seam joining said screen cup to said body portions; said tube including a directly heated cathode filament and a grid electrode including a metal plate with a grid aperture therein, at least one grid conductor wire supporting said grid plate in the envelope and passing through the seam between the body portions, and two filament conductor wires supporting said filament in the envelope and passing through the seam between the body portions.  
  2. A cathode ray tube as claimed in claim 1, comprising an insulating member for fastening said grid conductor wire to respectively one of the filament wires so as to form a rigid filament and grid sub-assembly.  
  3. A cathode ray tube as claimed in claim 1, comprising a glass frit joining and sealing said envelope body portions together, said glass frit having a lower melting point than said body portions.  
  4. A cathode ray tube as claimed in claim 3, said glass frit being constituted of de-vitrifiable glass material.  
  5. A cathode ray tube as claimed in claim 1, comprising an adhesive for joining together said envelope body portions, and a glass frit for sealing said body portions, said glass frit having a lower melting point than said body portions.  
  6. A cathode ray tube as claimed in claim 1, said body portions and said screen cup each being formed of barium lithium glass.  
 * i i alt