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Sept. 13, 1938. ‘ M. KNOLL 2,130,280 ‘ ELECTRON DISCHARGE ‘TUBE Original Filed April 6, 1934 F119 '1 /f 66 ‘Q A . xv’ Q @/ “V INVENTOR ’ BY 7 ‘ATTORNEY 2,130,280 Patented Sept. 13, 1938 "UNITED STATES PATENT OFFICE 2,130,280 ELECTRON DISCHARGE TUBE 'Max Knoll, Berlin, Germany, assignor to ‘Tele funken Gesellschaft fur Drahtlose Telegraphic .m. b. 11., Berlin, Germany, a corporation of Germany Application April 6, 1934, Serial‘ No.' 719,287. Re_ newed July'3, 1937. In Germany April 12, 1933 12 Claims. ‘The. present invention is concerned with dis charge tubes comprising a cathode, an anode and one or more interposed grid electrodes, and ~more particularlyawith the construction and dis "5 mposition ‘of said electrodes in such tubes. The stream or. current of electrons in discharge tubes has heretofore been conceived of as being a stream of a continuous medium (the so-called “electron gas" or cloud) proceeding from the cathode or ?lament through the control electrode to the anode or. plate. However, more recent in vestigations have demonstrated that a current of electrons which propagates throimh a broken or ‘apertured electrode such as a grid, is resolved, "15 ‘under certain circumstances, into discrete con .stituent or individual pencils ‘or brushes of rays sharply separated from one another which, in the presence of sufficiently high speeds and sum :ciently low current‘densities, exhibit a behavior satisfying the laws of geometrical electron optics. ‘If an electron having a velocity of Uh enters betweentwo equipotential surfaces of potential Uo of the non-homogeneous ?eld at the opening ofv an apertured or non-continuous electrode, it 25",wil1'be refracted or de?ected, according" to the sign of the electrode, in the direction of, or away from, the ‘perpendicular. For the space into which‘ the electron will get after having passed through the two equipotential surfaces, there lBf) will then hold the following index of refraction: , On the basis of this law of refraction of elec ‘35 tron-optics, it-is possible to trace, both graphi cally?and bywcalculation,.the different electron paths or trajectories on passing through the equipotentialsurfaces in the openings of aper tured .-or non-continuous electrodes, in analogy .withthe behavior of a luminous ray inside a me exhibiting non-homogeneous refractive ness. - _ ~--What- is important and essential in connection with-the invention hereinafter to be described is 45 a proper appreciation of this fact that the laws of' electron-optics hold good not only, as is well known, ‘for highv electron velocities and com paratively- small currents, but also for relatively lowvoltages‘and. relatively large currents such 50 as occur . in ordinary ampli?ers, in spite electrostatic repulsion of the electrons of be tween one another. ‘If the electrode systems there used are investigated on the basis of the electron-optical viewpoint rather‘than the 65 hydrodynamic one, as has ‘heretofore been done, (Cl. 250—27.5) it will be discovered that the electrons, at the openings of the various apertured electrodes, have to deal with or are subject to approximately cylindrical or calotte-shaped (hemispherical) equipotential surfaces which come to act upon them in the way of small lens systems, and the result is that, posteriorly of each opening of an apertured electrode, there arises a tiny easily de ?nable pencil or brush of rays, the cross-section and shape of which is a function of the form and 10 the size of the said opening. The object of the present invention, by adopt ing suitable dimensions for the openings of the apertured electrodes, by the disposition of the electrodes themselves, by the development of their 15 surface, and by the mutual or relative positions of the openings of the various electrodes, is to act upon or influence the characteristic of the discharge tube in certain or prearranged ways. By the formation of “electric lenses” as herein 20 before mentioned, inside the openings of aper tured electrodes-foci will be set up posteriorly thereof in which the electrons will be focused. It- is possible to cause the foci to register with the openings of the next apertured electrode or 25 electrodes so that the passage of electrons there to (or absorption of electrons by the same) will be avoided. This means a great relief for these electrodes, and this is especially advantageous whenever auxiliary electrodes maintained at a 30 high positive potential are involved, as is true, e. g., of screen-grids and space-charge grids. Moreover, owing to the fact that the electrons may be'exactly concentrated or focused in the openings of a control electrode, it is feasible to secure increased controlling power or accuracy and greater slope (mutual conductance) of the tube. By intentional shifting of the foci away from the center of the opening of the following apertured electrode it is further feasible to flatten 40 the characteristic, indeed, to impart to the latter any kind of shape or trend, for instance, to ob tain also aform following an exponential or logarithmic law. If, furthermore, the cathode surface is given a shape which agrees or con 45 forms with that of the equipotential surfaces occasioned by an apertured electrode, this affords a chance, especially in the presence of small inter-grid distances, to make the load of the cathode more uniform, to derive or extract from 50 the latter a higher emission, at the same control voltage, and thus to secure a further increase ‘in the slope of the tube. For a better understanding of the basic idea underlying this invention, the same will be ex 2 2,130,280 plained by reference to the accompanying draw ing. In Fig. lis shown a schematic longitudinal sec tion through the electrode system of a tetrode tube which comprises a cathode K, a space charge grid Gs, a control grid Go, and an anode A. Both grid electrodes, for instance, shall be assumed to be of the mesh or gauze metal type. Inasmuch as all parts of the same electrode are 10 at one and the same potential, there arises a ?eld distribution such as indicated in the graphs, the fine solid lines indicating the form of the equipotential surfaces. It can be readily seen that the potential surfaces between two grid wires resemble the shape of a biconcave lens, and the vicinity of an individual grid wire the form of a biconvex lens. Whether these equipotential surfaces exercise one action or the other upon the electrodes, in other words, whether they focus or disperse, will depend upon the sign of the potential gradient in the potential surface in question, seeing that the electrons in the one instance are attracted, and are repelled in the other. In view of the positive potential of the space-charge grid, the electrons emanating from the cathode are concentrated in pencils of rays B which are located exactly posteriorly of the wires of the space-charge grid. According to pre-supposition, the control grid is at a negative potential, and it is for this reason that in this case, the interstitial spaces between its wires act like another focussing lens, and focus the elec trons upon the wire strips of the anode A. It can be readily understood that the constituent ray pencils will penetrate through the control grid without an incidental impediment of their trajectories if the openings of each grid are ex actly alike. rI'his stipulation is ful?llable in vari ous ways, for instance, by that holes of like shape 40 Q In This circumstance would seem important es precise mounting and assembling in conformity with the demand hereinbefore indicated, that is, correct registering of grid openings, are still more easily ful?lled. The use of an additional auxiliary grid in a 30 space-charge grid type of tube is illustrated in Fig. 3. The electrode system comprises the cath ode K, the auxiliary Ga, the space-charge grid Gs, the control grid Go, and the anode A. What are punched in a lamination or sheet, said holes being equally spaced apart. The openings could at the same or at a negative voltage in reference have also the form of slits running parallel to the cathode. Also the so-called mesh grids made of wire gauze satisfy the said condition, at least approximately so, through in that case it is ad to the cathode, whereas the space-charge grid, the ray pencil. t is for this reason that‘ grids attached to stays constructed according to con ventional methods would appear unsatisfactory, While spiral grids, fundamentally speaking, are 55 admissible, for they may be conceived as helically wound cylinder lenses, it must be borne in mind that this uniform condition should not be dis turbed by stays or similar supporting means. From the distribution of the electrons as illus trated, it can be seen that when making the anode or plate from a wire gauze or netting of de?nite dimensions, favorable thermal radiation is feasi ble, on the one hand, while, on the other hand, provided that the pencil of rays impinge exactly 65 upon the anode surface, no electrons will be able to reach the space in the rear of the anode where they are liable to give rise to all kinds of trouble. Fig. 2 shows a cross-section of a tetrode tube comprising a cathode K, a control grid Go and an anode A. In addition an auxiliary electrode Go. is mounted between the control grid and the cathode which, for the reasons hereinafter to be discussed in more detail, is so designed and 75 arranged that the openings thereof come to reg 10 pecially when the tube is operated inside the range of positive grid voltages, inasmuch as then no grid current will be able to arise, and since the control potential source is relieved of load. As a natural result, as will be noted, such distor tions as will arise normally when working with in the positive grid potential region, will here be avoided. This construction is of utmost importance par ticularly also in connection with transmitter valves seeing that in the case of these, on the one hand, owing to higher‘ operating voltages, far larger grid currents will arise, while, on the other hand, due to the large dimensions of the electrode system, the chances for insuring 25 must be borne in mind is that conditions should be such that the openings of the space-charge grid and the auxiliary grid will exactly come to register, wheras the wires of the control grid will coincide correctly with the middle of the said opening. The auxiliary grid again is maintained visable to roll the gauze down or ?atten it in order that it may present throughout the same thickness or gauge; for the requirement of having uniform thickness in apertured electrodes in so far as they are traversed by the stream of elec 50 trons, is essential for the uniform formation of 60 ister or coincide exactly with the openings of the control grid. The auxiliary grid is main tained at the same or at a negative potential in relation to the cathode. As a consequence, the spaces between the grid wires act as condenser lenses and they thus focus and concentrate the electrons in the interstices between the wires of the control grid. Hence, the electrons find no chance to impinge upon the control grid proper. as usual, is connected with a positive biasing potential. In this manner conditions, on the one hand, are made such that the space-charge grid will not be struck by electrons, as is true of Fig. 1. As a result the appreciable consumption of energy that has so- far been inseparable from the use of a space-charge grid and which has proved a hindrance to the wider introduction of this type of tube, is obviated. On the other hand, 50 the electrons through the space-charge grid can be focussed in the openings of the control grid, and they are there subjected to a maximum control action. Fig. 4 represents a cross-section through the 55 electrode system of a screen-grid tube containing the cathode K, the control grid Go, the screen grid Gsc, and the anode A. The two grids may each consist, for example, of a sheet-metal cyl inder in which are punched slots running paral 60 lel to the cathode. The two apertured electrodes are so disposed in reference to each other that the openings will come to register precisely. The potential of the control grid should always be negative in reference to the cathode. In this manner, the openings of the control grid act like cylindric condenser lenses, and as a result they cause the electrons to become concentrated in the openings of the screen grid with the conse— quence that the screen grid can be practically rendered “currentless”. The practical result and success of this step manifests itself not merely in a saving of current by the suppression of current in the screen grid, but also in the circumstance 75 13 that the tubes become uniform,‘ contradlstinct vfrom-‘what has heretofore'been'the case-where‘ it was ‘largely a ' question. of ' chance'whether the unwhes of'the-screenrgrid'would collect or‘ab sorb electrons or not, 'wlththe ‘result: that rather great disparities were observed inthe size'of the currents ?owing in the screen grid. Variations in this vregard became particularly annoying whenever the screen-grid voltage was tapped 10 from. a voltage divider or potentiometer. A further embodiment of the basic idea of this invention is indicated in Fig. 5. It is well known that in the presence of a very small distance be tween control grid and cathode, the electrons will not always be derived in a uniform way along the entire surface of the cathode, indeed, that some portions of the cathode become subject to marked loads and that these local areas furnish practi cally the whole electron emission, whereas neigh boring portions would give off no electrons. This circumstance proves unfavorable from the view point of life of the cathode, and it imposes a limitation so far as the slope (mutual conduc tance) is concerned in the presence of inter 25 grid distances falling below the size of grip open ings. Now, another object of this invention is a novel formation of the cathode surface such that it will match and adhere to the equipoten tial surfaces produced by virtue of the grid con struction. Fig. 5 is a longitudinal section through the electrode system of a triode tube containing the cathode K, the control grid Go and the anode A. The trend or shape of the potential surfaces is 35 indicated by the horizontal solid lines. The cathode surface has ridges or depressions the shape of which is a function of the control elec trode. If the latter, for instance, consists of a 3. *An electron discharge tube: comprising an indirectly heated'cathode having a pluralityof uniformly spaced similarly-shaped electron emit ting surfaces, a perforated electrode positioned adjacent said cathode vwith ‘its ‘perforations in registry with the said-similarlyshaped cathode emitting surfaces, and 'a'n'additional " electrode surrounding the perforated- electrode. 4. .An' electron discharge tubercomprising 8. cy lindrical equi-potential cathode having formed 10 on its surface a helically-grooved electron emit ting surface of uniform pitch, a similarly pitched helically-wound grid electrode surrounding the cathode so that the spaces between successive grid turns register with the grooved emitting sur 15 face, and an additional electrode surrounding the cathode and the grid electrode. 5. An electron discharge tube comprising an equipotential cathode having a plurality of de pressions distributed along its length, a grid elec— 20 trode surrounding the cathode and having its spaces aligned with the cathode depressions, and an additional electrode surrounding the grid elec trode. 6. An electron discharge tube comprising a 25 cathode, an anode, a grid electrode interposed be twen cathode and anode, and means for con trolling the electrostatic ?eld in the vicinity of the grid electrode whereby the electron flow therethrough will not be impeded, said means 30 comprising arcuate electron emitting surfaces formed on the surface of the cathode and extend ing in directions parallel to the cathode axis, the grid electrode being in the form of rods which are also arranged in parallel relation to the cathode axis. 7. An electron discharge tube according to the preceding claim wherein the spacings between the mesh or gauze or of a punched perforated sheet 40 or lamination, the said depressions are made grid rods are in registry with the arcuate cathode surfaces. 40 8. An electron discharge tube comprising an equipotential cathode having a plurality of longi grid opening. In the case of rodlet-type grids, 45 corrugations will, in analogy, be arranged paral lel to the axis of the cathode, as shown in Fig. 6, while with spirally-wound grids there should be tributed around its surface, a grid electrode pro vided with longitudinally extending conductors surrounding the cathode and having the spaces between successive grid conductors aligned with the cathode depressions, and an additional elec trode surrounding the grid electrode. 9. An electron discharge tube comprising a cathode, an anode, a grid electrode interposed 50 between cathode and anode, and means for con trolling the electrostatic ?eld in the vicinity of the grid electrode whereby the electron ?ow there— through will not be impeded, said means compris hemispheric (calotte-shaped) so that to each grid mesh there corresponds a depression which comes to be positioned exactly in the rear of the provided a helical groove having the same pitch as the grid. 50 Instead of providing the cathode surface with depressions, roughly the identical effect is attain able by making the cathode bl-partite, in other words, a cylindrical electron-emission surface of the kind heretofore customary, and further, at 55 close proximity in front thereof and conductively connected therewith, a grid whose openings come to register with those of the control electrode. What I claim is: 1. An electron discharge tube comprising a 60 cathode, an anode, a grid electrode interposed be tween cathode and anode, and means for con trolling the electrostatic ?eld in the vicinity of the grid electrode whereby the electron ?ow therethrough will not be impeded, said means 65 comprising arcuate electron emitting surfaces formed on the surface of the cathode, which ar cuate surfaces are arranged in registry with the apertures of the grid electrode. 2. An electron discharge tube comprising an 70 equipotential cathode having a plurality of de pressions uniformly distributed along its length, a helical grid electrode surrounding the cathode and having its spaces between successive turns aligned with the cathode depressions, and an ad 75 ditional electrode surrounding the grid electrode. tudinally extending depressions uniformly dis ing spaced electron emitting surfaces in the form 55 of narrow elongated strips formed on the surface of the cathode and extending in directions paral— lel to the cathode axis, the grid electrode being in the form of rods which are also arranged in 60 parallel relation to the cathode axis. 10. An electron discharge tube comprising an equipotential cathode having a plurality of longi tudinally extending electron emitting strips uni formly distributed around its surface, a grid elec trode provided with longitudinally extending con 65 ductors surrounding the cathode and having the spaces between successive grid conductors sub stantially aligned with the electron emitting strips, and an additional electrode surrounding 70 the grid electrode. 11. An electron discharge tube comprising an equipotential cathode having a plurality of longi tudinally extending depressions which are oxide coated and uniformly distributed around its sur 75 4 2,130,280 face, a grid electrode provided with longitudinally extending conductors surrounding the cathode and having the spaces between successive grid conductors substantially aligned with the oxide Cl coated cathode depressions, and an additional electrode surrounding the grid electrode. 12. An electron discharge tube comprising an equipotential cathode having a plurality of longi tudinally extending oxide coated strips uniformly distributed around its surface, a grid electrode provided with longitudinally extending conduc tors surrounding the cathode and having the spaces between successive grid conductors sub stantially aligned with the cathode strips, and an additional electrode surrounding the grid elec trode. MAX KNOLL.