US2051950A - Electron discharge tube - Google Patents

Description

Aug. 25, 1936.

JOBST El' A l.

ELECTRON DISCHARGE TUBE Filed May Patented ug. 25, 1936 Y `UNITED STATES 2,051,950,A I `ELECTRON DISCHARGE` TEUBE if Gunther Jobst'and Franz Sammer, Berlin, Germany, assignors to Telefunken Gesellschaft Afr Drahtlose Telegraphie m. b. H., Berlin,

- Germany, alrcolfporationof Germany ApplicationMay 16, 1934, Serial No. 725,8384 Y In Germany May 17, 1933A n Y Y 1 Claim.

The present inventionis concerned with'a discharge tube and more particularly to means for shielding the discharge paths' thereof by the `aid of one or more metallic envelopesor enclosures which do'not contain insulating parts and` means maintained at indenite potentials.

It is obvious that discharge paths lin electron tubes, which are known as the mostl sensitive relay system available at the present time, will respend even to the feeblest disturbances, the

sources of which may be both outside as' Well as n inside the discharge vessel, so that the-most careful shielding of the-tubes andlmore particularly of the discharge paths thereof is desirable. This consideration'is especially important in the-input:

stages of anampliiier or receiver, for it is here where small useful or working amplitudes are involved whose level might fall below the stray or disturbance level-or might `be influenced-by the latter. In line Withthese conceptions a number of. protective Ways and means have been suggestn ed in the earlier art with a View to either rendering stray 'elds coming infrom -theoutside harmless or else to exclude the influence of charges that have accumulated on tlf'letubewall.V Among these may be mentioned'themetallization of fthe glass bulb upon the inner or outersurfacesthe disposition of a `metallic cylinder'between Ythe glass wall and the electrode system,or else the encasing of the electrodesystem insidea metallic box-like structure. However, ithasV been-found that the said protective steps are not invariably adequate; in fact, they prove particularly defi-.

cient inthe attempt to-preclude the vdisturbances which are occasioned. by such electrons as stray I away from the discharge path proper and the action of stray iieldsuupon the electrons. In order that thismay be accomplished-tothe greatest possible degree it is, on the contrary, necessary according to the present invention not only to surround the electrode system all around by a metallic sheath in a way resembling a Faradaycage arrangement from which only such means as are required for the supporting of the electrodes and for the supplying or lead-in of the current are projected, but in addition, care must be taken that no insulating parts will be contained inside such a cage.

For a better understanding of the basic idea of the invention, reference may here be made to the following relations and phenomena inside a discharge tube: Every electrode system of the form of construction heretofore known offers a chance for electrons drifting or flying away at diierent points along the path of the discharge -to move elsewhere through thefglass bulb. Such points where straying away ofV electrons occurs.' are located at the open ends, vfor instancain electrode systems having electrodes open at vboth 5 ends and beinguof cylindrical orprismatic form. Electrons will also-ily ou vor drift in the Qspace surrounding theelectrodes whenever the outermost electrode, which servesV as the anode and Y which is'maintainedat'a high positive potential, l0 is apertured or formed of Wire gauze material. This'type ofv anode electrode has Vbeen adopted for thermal reasons with a view to Vpromotimr rapid heat `ccnductionor radiation from the interior of the discharge spaceand toprecludng undel5 sirable temperature rises yconduciveto-thermalemission of unheated electrodes, as vfor example, the grid. The escapeof electronsfromthe dischargepath proper isals'o favored particularly inthe case of electrode systems of the vkind ingo which the vouter Velectrode constitutes no closed surface as is 'frequently Vtruel with plate-shaped n, anodes. Thevdisturbances which uare liable to becaused by electronsV in 'Y the vacuous vspace fas stated lmay vbe of'widely diierent nature, Vand 25 they depend both upn the action of outside' iields as well as' the-'electrical state gif-insulation* parts.V

Electrons located between the electrode system and the glass wallarecausative of space charges, the existence-and distribution Vof 4which 30 is frequently a functional? uncontrollable Yfactors: They aretherefore oftenwquiteY unstable, indeed,V they evidence inclination to Aexperience suddenV changes which Vmanifest themselves, for instance, in the form of crackling noisesio'r hisses.` 35 Under certain ycircumstances there may Occur periodic oscillatory movements' of these free space chargeslin lthevvay-asfolcaserveclin the case of Barkhaus'en-Kurz oscillations. At anyrate, it can be readily understood thatthese space- 40 chargesl are considerablymore'responsivefto'outside elds than the stream of electrons passing inside the discharge space proper between the electrodes and controlled by definite potentials.

Other disturbances may originate from insulators or metal parts mounted inside the glass bulb, which parts are maintained at definite potentials and are charged up by electrons. 'I'he size and the existence or duration of these charges depends often upon accidental factors, and this introduces a further unsafe element in the function of the tube. n

Finally, secondary-emission effects oi the insulator parts may be emphasized. The'inner Wall of the glass bulb as well as the insulating v bridge means used for the supporting of the electrode system are liable vto be charged by creepage currents emanating from the plate leadin which is at a positive potential. This will frequently arise whenever the plate potential is connected before the filament or cathode has attained its full state of emission, a condition occurring generally in the standard type of house current operated receiving sets. If these areas or surfaces are struck by the electrons straying away from the discharge space or. penetrating through the apertured plate or anode at a rate of velocity which corresponds to the potential thereof, they are caused to give off secondary emission. These electron-emitting surfaces will then act like electrodes, and inasmuch as they are in. capacitive coupling relationship with the control grid they will occasion increases in damping and capacity, or decreases of these quantities, according to the phase of the voltage. Now, all oi these phenomena are not only disturbing, fundamentally speaking, but their presence becomes troublesome especially because the factors that are concerned are uncontrollable to alarge degree and are a function of accidental causes or conditions.

To preclude these diiiiculties, it is suggested according to this invention to surround the electrode system with a shielding member which is completely closed all around like a Faraday cage, care being taken to have no insulating parts, such as electrode bridges or spacers, inside the shielded or protected space and also no conductors, such as getter supports sealed-directly in the squash, that are not kept at a definite potential. Excepted from this limitation, of course, are nonmetallized parts of the ceramic cathode tube in the case of indirectly heated cathodes; for since the same, on the one hand, are connected with the cathode potential, and are, on the other hand, located outside the trajectory of the electrons, no disturbance by such areas need to be feared. But in a case of necessity it is easy to see to it that the whole part of the ceramic cathode support located within the shielding enclosure is metallized or covered by distinct shielding means or surfaces. This affords an insurance that neither uncontrollable charges will be built up nor that secondary electron emission from insulator parts will occur. The shielding enclosure or envelope -could be made also apertured so -that the thermal properties of the electrode system so Vfar as thermal radiation from the interior of the discharge space is concerned, will notbe impaired. In order that in this case the passage oi electrons in the space outside the cage may be prevented, the latter maybe connected to a fixed low potential, preferably negative, or to the cathode potential.

Inthe cage normally enclosing Vthe electrode system on all sides certain portions could be omitted at such places where no electrons are able to drift away or where no action upon the discharge process is possible. This is true, e. g., when the plate consists of an unbroken, solid sheet cylinder; in this case the shielding may be insured by two caps tted on at the end surfaces.

In the accompanying drawing which forms part of this application, Figs. 1 and 2 disclose two modications according to the invention.

Fig. l shows an electrode system having a shielding cage or enclosure surrounding the system on all sides. I is the glass bulb with the squash 2 on which the electrode system is mounted. 3 denotes the cathode, 4 a grid electrode, and 5 an anode consisting, for instance, of a metallic network or gauze. The electrode system or assembly is surrounded by a cage or enclosure which consists of a cylindrical metallic network 6 and two end surfaces 1 and 8, which may be either a solid metal disc or an apertured disc. The cage is provided with a lead 9 in order that it may thus be connected, if desired, to any convenient potential. The enclosure, however, could ,also be united inside the glass bulb with an electrode having a suitable voltage, say, for instance, the cathode. The electrodes are supported and spaced apart by the aid of insulation bridges l0 and H which are placed outside the said enclosure or shield. Also the getter support n l2 is disposed outside the cage.

Fig. 2 illustrates a tube whose electrode assembly comprises an anode or plate consisting of a solid-sheet cylinder 20. As a consequence it is suicient to provide shielding electrodes only at the two end surfaces of the anode cylinder. These consist of two caps 2| and 22 made from metal gauze which are tted on so as to engage so far over the ends of the electrode system that the escape of electrons into the space outside the enclosing shield is practically precluded. With this arrangement there is little possibility for the insulating bead 23 serving as a support for the electrode holding means being struck by electrons.

What is claimed is:

An electron discharge device comprising cathode, grid and anode electrodes, a shield member surrounding the anode electrode and having portions extending beyond the opposite ends of said anode electrode, a shield disc at each end of the shield member and joined thereto to completely enclose the electrodes contained therein, parallel support rods for said grid and anode electrodes and said shield member extending beyond the edges thereof, and insulating spacing members engaging the opposite ends of said support rods beyond the shield discs, said shield member and said shield discs being formed of wire mesh material.

GNTHER JOBST. FRANZ SAMMER.

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