US2130280A - Electron discharge tube - Google Patents

Description

Sept. 13, 1938. M. KNOLL 2,130,280

ELECTRON DISCHARGE TUBE Original Filed April 6, 1934 66 Q Q INVENTOR A x V BY 7 ATTORNE Patented Sept. 13, 1938 "UNITED STATES PATENT OFFICE ELECTRON DISCHARGE TUBE Germany Application April 6, 1934, Serial No.' 719,287.

newed July'3, 1937. 1933 12 Claims.

The. present invention is concerned with discharge tubes comprising a cathode, an anode and one or more interposed grid electrodes, and -more particularly with the construction and disposition 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 filament through the control electrode to the anode or. plate. However, more recent investigations have demonstrated that a current of electrons which propagates throimh a broken or apertured electrode such as a grid, is resolved, 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 currentdensities, 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 field at the opening of an apertured or non-continuous electrode, it ",wil1'be refracted or deflected, 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 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 graphically and bywcalculation,.the different electron paths or trajectories on passing through the equipotentialsurfaces in the openings of apertured .-or non-continuous electrodes, in analogy withthe behavior of a luminous ray inside a meexhibiting non-homogeneous refractiveness.

---Whatis important and essential in connection with-the invention hereinafter to be described is a proper appreciation of this fact that the laws of' electron-optics hold good not only, as is well known, for high electron velocities and comparativelysmall currents, but also for relatively lowvoltagesand. relatively large currents such as occur in ordinary amplifiers, in spite of electrostatic repulsion of the electrons between one another. If the electrode systems there used are investigated on the basis of the electron-optical viewpoint ratherthan the hydrodynamic one, as has heretofore been done,

In Germany April 12,

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 definable pencil or brush of rays, the cross-section and shape of which is a function of the form and the size of the said opening.

The object of the present invention, by adopting suitable dimensions for the openings of the apertured electrodes, by the disposition of the electrodes themselves, by the development of their 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 hereinbefore mentioned, inside the openings of apertured electrodes-foci will be set up posteriorly thereof in which the electrons will be focused. Itis possible to cause the foci to register with the openings of the next apertured electrode or electrodes so that the passage of electrons thereto (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 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 the characteristic, indeed, to impart to the latter any kind of shape or trend, for instance, to obtain also aform following an exponential or logarithmic law. If, furthermore, the cathode surface is given a shape which agrees or conforms 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 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 explained by reference to the accompanying drawing.

In Fig. lis shown a schematic longitudinal section through the electrode system of a tetrode tube which comprises a cathode K, a spacecharge 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 at one and the same potential, there arises a field 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 electrons 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 exactly alike. I'his stipulation is fulfillable in various ways, for instance, by that holes of like shape are punched in a lamination or sheet, said holes being equally spaced apart. The openings could 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 advisable to roll the gauze down or flatten it in order that it may present throughout the same thickness or gauge; for the requirement of having uniform thickness in apertured electrodes in sofar as they are traversed by the stream of electrons, is essential for the uniform formation of the ray pencil. t is for this reason that grids attached to stays constructed according to conventional methods would appear unsatisfactory, While spiral grids, fundamentally speaking, are admissible, for they may be conceived as helically wound cylinder lenses, it must be borne in mind that this uniform condition should not be disturbed by stays or similar supporting means. From the distribution of the electrons as illustrated, it can be seen that when making the anode or plate from a wire gauze or netting of definite dimensions, favorable thermal radiation is feasible, on the one hand, while, on the other hand, provided that the pencil of rays impinge exactly 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 arranged that the openings thereof come to register or coincide exactly with the openings of the control grid. The auxiliary grid is maintained 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. This circumstance would seem important especially 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 distortions as will arise normally when working within the positive grid potential region, will here be avoided.

This construction is of utmost importance particularly 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 precise mounting and assembling in conformity with the demand hereinbefore indicated, that is, correct registering of grid openings, are still more easily fulfilled.

The use of an additional auxiliary grid in a space-charge grid type of tube is illustrated in Fig. 3. The electrode system comprises the cathode K, the auxiliary Ga, the space-charge grid Gs, the control grid Go, and the anode A. What 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 at the same or at a negative voltage in reference to the cathode, whereas the space-charge grid, 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 sofar 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, 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 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 cylinder in which are punched slots running parallel 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 that the tubes become uniform, contradlstinct from-what has heretofore'been'the case-where it was largely a question of chance'whether the unwhes of'the-screen grid'would collect orabsorb electrons or not, 'wlththe result: that rather great disparities were observed inthe size'of the currents flowing in the screen grid. Variations in this regard became particularly annoying whenever the screen-grid voltage was tapped 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 between 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 practically the whole electron emission, whereas neighboring portions would give off no electrons. This circumstance proves unfavorable from the viewpoint of life of the cathode, and it imposes a limitation so far as the slope (mutual conductance) is concerned in the presence of intergrid distances falling below the size of grip openings. Now, another object of this invention is a novel formation of the cathode surface such that it will match and adhere to the equipotential surfaces produced by virtue of the grid construction.

Fig. 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 indicated by the horizontal solid lines. The cathode surface has ridges or depressions the shape of which is a function of the control electrode. If the latter, for instance, consists of a mesh or gauze or of a punched perforated sheet or lamination, the said depressions are made hemispheric (calotte-shaped) so that to each grid mesh there corresponds a depression which comes to be positioned exactly in the rear of the grid opening. In the case of rodlet-type grids, corrugations will, in analogy, be arranged parallel to the axis of the cathode, as shown in Fig. 6, while with spirally-wound grids there should be provided a helical groove having the same pitch as the grid.

Instead of providing the cathode surface with depressions, roughly the identical effect is attainable by making the cathode bl-partite, in other words, a cylindrical electron-emission surface of the kind heretofore customary, and further, at 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 cathode, an anode, a grid electrode interposed between cathode and anode, and means for controlling the electrostatic field in the vicinity of the grid electrode whereby the electron flow therethrough will not be impeded, said means comprising arcuate electron emitting surfaces formed on the surface of the cathode, which arcuate surfaces are arranged in registry with the apertures of the grid electrode.

2. An electron discharge tube comprising an equipotential cathode having a plurality of depressions 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 additional electrode surrounding the grid electrode.

3. *An electron discharge tube: comprising an indirectly heated'cathode having a pluralityof uniformly spaced similarly-shaped electron emitting surfaces, a perforated electrode positioned adjacent said cathode with its perforations in registry with the said-similarlyshaped cathode emitting surfaces, and 'a'n'additional electrode surrounding the perforatedelectrode.

4. An' electron discharge tubercomprising 8. cylindrical equi-potential cathode having formed on its surface a helically-grooved electron emitting 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 surface, and an additional electrode surrounding the cathode and the grid electrode.

5. An electron discharge tube comprising an equipotential cathode having a plurality of depressions distributed along its length, a grid elec trode surrounding the cathode and having its spaces aligned with the cathode depressions, and an additional electrode surrounding the grid electrode.

6. An electron discharge tube comprising a cathode, an anode, a grid electrode interposed betwen cathode and anode, and means for controlling the electrostatic field in the vicinity of the grid electrode whereby the electron flow therethrough will not be impeded, said means comprising arcuate electron emitting surfaces formed on the surface of the cathode and extending 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 grid rods are in registry with the arcuate cathode surfaces.

8. An electron discharge tube comprising an equipotential cathode having a plurality of longitudinally extending depressions uniformly distributed around its surface, a grid electrode provided with longitudinally extending conductors surrounding the cathode and having the spaces between successive grid conductors aligned with the cathode depressions, and an additional electrode surrounding the grid electrode.

9. An electron discharge tube comprising a cathode, an anode, a grid electrode interposed between cathode and anode, and means for controlling the electrostatic field in the vicinity of the grid electrode whereby the electron flow there through will not be impeded, said means comprising spaced electron emitting surfaces in the form 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 parallel relation to the cathode axis.

10. An electron discharge tube comprising an equipotential cathode having a plurality of longitudinally extending electron emitting strips uniformly distributed around its surface, a grid electrode provided with longitudinally extending conductors surrounding the cathode and having the spaces between successive grid conductors substantially aligned with the electron emitting strips, and an additional electrode surrounding the grid electrode.

11. An electron discharge tube comprising an equipotential cathode having a plurality of longitudinally extending depressions which are oxide coated and uniformly distributed around its surface, a grid electrode provided with longitudinally extending conductors surrounding the cathode and having the spaces between successive grid conductors substantially aligned with the oxide coated cathode depressions, and an additional electrode surrounding the grid electrode.

12. An electron discharge tube comprising an equipotential cathode having a plurality of longitudinally extending oxide coated strips uniformly distributed around its surface, a grid electrode provided with longitudinally extending conductors surrounding the cathode and having the spaces between successive grid conductors substantially aligned with the cathode strips, and an additional electrode surrounding the grid electrode.

MAX KNOLL.

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