US1929661A - Electron tube - Google Patents

Description

Oct- 10, 1933. c J VON wEDEL 1,929,661

ELECTRON TUBE Filed Aug. 31, 1928 cttmtm w Patented Oct. 10, 1933 1,929,661 ELECTRON TUBE Carl J.. R. H. von Wdel, Berlin, signor, by mesne assignments,

Germany, as-

to Electrons,

Inc., a corporation of Delaware Application August 31, 1928'. Serial No. 303,154 10 Claims. (01. 250-275) The present invention relates generally to electron tubes, and particularly to the three electrode type of electron tube commonly used in connection with alternating current amplification and repetition.

A particular object of the invention is to avoid an undesirable result arising in the ordinary mode of fabrication of some types of tubes, and which has been particularly noticeable in the fabrication of that type of tube known as the indirectly heated cathode type.

There is an increasing tendency towards the use of the so-called coated type of cathode to supply the electron emission in electron tubes, the emissive coatings involving various compounds including the alkali earth metals, such as barium, calcium and strontium, as the basic emissive element.

In many cases of electron tube use, it is desirable that only the cathode emit electrons, but it has been found that in the production of tubes having coated cathodes, conditions arise resulting in vaporizing of the alkali earth metals, which vapors condense on the other electrodes, and later result in emission from these other electrodes when the tubes are put to use. This result is particularly annoying when the emission takes place from the grid electrode of the usual three electrode tube, this emission resulting in a flow of current in the grid circuit seriously interfering with proper grid biasing and other features which must be nicely predetermined in order to secure best operation from systems employing such tubes.

The present invention provides a satisfactory way for preventing this subsequent emission effect in a tube even though fabricated in such a Way that vaporizing of emissive substances of the cathode coating takes place.

The invention will be better understood by reference during the detailed explanation to the figure of the accompanying drawing which illustrates a form of indirectly heated cathode type of three-electrode vacuum tube.

In the figure VT is the usual glass or other envelope of such tubes having at the base a stem S which supports the electrode structure and through which the lead-in wires pass in sealed fashion. The envelope is pumped to a high vacuum, or may contain a small quantity of a selected gas, depending upon the purposes of the tube. The cathode construction is shown to comprise the usual cylindrical thimble C which is shown a pair the heating element,

inside of of twisted wires H forming it being understood that the two parts of the twisted pair of wires are suitably insulated one from the other, as for example in the manner described and claimed in my copending application Serial No. 243,042

of December 28, 1927. The cathode structure is shown surrounded by a grid electrode comprising a spiral wire. The grid is shown surrounded by the plate electrode comprising a metallic cylinder P. The cathode structure is supported by an upright W which also serves the function 01 completing the conductive connection betweenv the point 0 of the cathode structure and externally of the tube by way of the lead-in wire passing through the stem S.

In the usual practice of fabricating such a tube as that shown the cathode thimble C is superficially coated with raw chemical compounds which include the necessary elements which form the final desired emissive compounds by chemical conversion upon heating of the cathode during fabrication. The extent of heating required for the chemical conversion and the proper degasing of the electrodes and coating requires reaching such a high temperature that there inevitably results a vaporizing in part of the alkali earth metal or metals of the compounds, and it is seen that the closely associated grid structure G, which during the operation is cooler than the cathode structure, provides a readily accessible condensation surface for these vapors, with the result that there is an electron emissive deposit formed on the grid'to later come into action for electron emission when the normal operation of the tube heats the grid sufficiently high to bring about electron emission, it being seen that the grid is sufficiently close to the cathode to take up a temperature not greatly less than that of the cathode. There is also condensation of the vapors of the emissive substances on the surface of the plate electrode P. While the subsequent emission of electrons from the plate electrode is not wanted in usual operation, yet this secondary emission is not as serious as is the secondary emission from the grid because the sensitiveness of operation of systems employing vacuum tubes to flows of current in the grid circuit is quite exacting.

In the copending application of Meyer et a1, Serial No. 196,996, filed June 6, 1927 and the copending application of Spanner et a1, Serial No. 251,736, filed February 3, 1928, it is disclosed that alkali earth metals are used to make up double metal compounds for electron emission purposes, that the resulting compounds are effectively or non-effectively electron emissive depending upon the acid character of the metal included in the radical of the double metal compound; and it is further disclosed therein that in the case of such compounds in which the oxy-acid of the metal used in the radical is more acid than titanic acid that the emissive character of the compound is of no practical value. It is also disclosed in the aforementioned applications that it is a.practical matter to form such emissive compounds by using the surface metal of an electrode as one element of the compound, as by bringing raw chemical compounds containing the desired alkaline earth metals in contact with the metal surface of the electrode after oxidization of its surface, as by heating in the presence of oxygen, and forming the final compound from this arrangement.

The present invention takes advantage of the non-emissive character of a double metal compound including a metal in its radical the oxyacid of which is more acid than titanic acid by making the grid and plate electrodes, or other electrodes of an electron tubeof a metal the oxyacid of which is more acid than titanic acid, or at least superficially coating the desired electrodes with such a metal as by electro-plating, and oxidizing the surface of these electrodes before commencement of the usual heating, pumping and de-gasing process in the making of tubes, so that when evaporation of the alkali earth metal of the cathode compound takes place, the condensation on another electrode will result in the formation on its oxidized surface of a nonemissive compound of the character described. Platinum and tungsten are good examples of metals usually employed in electron tube electrode construction the oxy-acids of which are more acid than titanic acid, and thus can be used for the electrodes desired not to be emissive, and if oxidized to the desired degree before the commencement of the final steps in the making of the tube, will effectively form the desired nonemissive compounds to prevent the dificulties with subsequent secondary electron emission herein outlined.

Having fully described my invention, I claim:

1. An electron discharge device including a cathode having an electron emissive coating including an alkaline earth metal compound, a nonemissive electrode having thereon a compound of a metal the oxy-acid of which is at least as acid as the corresponding acid of platinum, and a gaseous medium surrounding the electrodes.

2. A discharge tube comprising an electron emissive cathode including an alkaline earth metal, a gaseous medium, and a non-emissive electrode having a surface of tungsten oxide.

3. A discharge tube comprising an electron emissive cathode including an alkaline earth metal, a gaseous medium, and a non-emissive electrode having a surface of platinum oxide.

4. A three electrode discharge tube comprising an oxide coated cathode, a plate electrode, an interposed grid electrode and a gaseous medium, the grid electrode having a surface of tungsten oxide.

5. A three electrode discharge tube comprisingan oxide coated cathode, a plate electrode, an interposed grid electrode and a gaseous medium, the grid electrode having a surface of platinum oxide.

6. A discharge tube comprisingan electron emissive cathode including an alkaline earth metal, a gaseous medium, and a non-emissive electrode having on its surface an oxide of one of the metals tungsten and platinum.

7. A three electrode discharge tube comprising an electron emissive cathode including an alkaline earth metal, an anode, an interposed grid electrode and a gaseous medium, the said grid electrode having on its surface an oxide'of one of the metals tungsten and platinum.

8. A three-electrode discharge tube comprising a cathode having an electron emissive surface formed in part of an alkline earth metal, a plate electrode, a grid electrode, and a gaseous medium,

the grid electrode having a surface of a compound of a metal the oxy-acid of which is at least as acid as the corresponding acid of platinum, the said compound being adapted to combine with the alkaline earth metal vapor under ionic bombardment at operating temperatures.

9. A discharge tube comprising an' electron emissive cathode including an alkaline earth metal, a gaseous medium and a non-emissive elec trode having a body of tungsten and a surface of tungsten oxide.

10. A discharge tube comprising an electron emissive cathode including an alkaline earth metal, a gaseous medium, and a non-emissive electrode having a surface containing an oxide of a metal the oxy-acid of which is at least as acid as the corresponding acid of platinum.

CARL J. R. H. VON WE'DEL.

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