US1997196A - Electron discharge device - Google Patents

Description

Filed Feb. 16, 1931 ATTORNEY Patented Apr. 9, 1935 c 1,997,196

UNITED STATES PATENT OFFICE ELEc'moN DISCHARGE nEvIcE James O. McNally, Maplewood, N. J., asslgnor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 16, 1931, Serial No. 515,965

8 Claims. (Cl. Z50-27.5)

This invention relates to electron discharge shield for the heater terminals, to reduce the devices and more particularly to such devices in inuence of the heater current on the electron which the cathode isheated by alternating curflow between the electrodes. The lelectrostatic rent. shielding may be increased by extending, the

In discharge devices employing the well known shield to segregate the heater leading-in wires 5- cylindrlcal or equi-potential cathode which is from the path of electron flow between the elecbrought to its proper operating temperature by trodes. an internal heater element which may heat the Since the low frequency hum is attributed to cathode by conduction or radiation, it is more magnetic effects as well as electrostatic effects, 10 convenient to supply alternating current to the it is necessary to diminish or overcome the mag- 10 heater element in order to produce the necessary neific field introduced by the heater elementelectron emission from the cathode. When al- 'This is accomplished in accordance with another ternating current is used as the heating source, feature of this invention by inserting a metallic various ditliculties arise due to the low frequency shield between the heater element and the cathhum which may be detected in the output circuit 0de. This shield is formed of a metal or alloy 15 of the device. This hum is attributed to disturbwhich retains its magnetic properties at the opances created in the electron ow, due to the erating temperature 0f the cathcde- Nickel and varying field of the heater element. The varycopper are not suitable for this Kfunction since ing iield is primarily caused by electrostatic and they readily 10Se their magnetic Properties at c a0 magnetic eiects introduced between the heater temperature considerably below the temperature 20 element and the surface of the cathode. These of the cathode necessary for maximum electron effects produce the disturbances in the ow of emission. Cobalt or an alloy preferably of iron electrons between the other electrodes associated and cobalt may be employed to reduce the magwith the cathode, namely, the grid and the netic iield of the heater element and thereby dianode. minish the hum in the output circuit of the de- Since the electron discharge device reproduces vice. y in amplified form in the output circuit any fre- A further feature of the invention relates to quency source supplied to the input circuit, it is reducing the capacitance eiect between the readily apparent that any current or voltage of heater and the input electrode or grid. 'I'his the frequency of varying or alternating current capacitance eiect results from the close prox- 30 supplied to the heater which is introduced in imity of the leading-in wires of the heater and either the input or output circuits distorts the the grid'. In order to overcome this diiiiculty the signal current and causes a disturbing hum effect grid leading-in wire is removed from the press 0r noise in the output circuit. through which the heater leading-in wires are An Object of the invention is to eliminate or sealed and arranged to enter the vessel through 35 substantially reduce the hum eiect in electron the top thereof, This arrangement together with discharge devices in Which the cathcde iS heated the electrostatic shield enclosing the heater by varying current. leading-in wires materially decreases the capaci- In accordance Wiih che aspect 0f this inventson tance effect between the heater and the grid and 40 the electron discharge device comprises an eVeC- prevents circulation of heater current in the grid ated Vessel including a c'thode a grdrand an or input circuit of the device. An electron disande Theathode Conslsts of an B qul'pften charge device embodying these features overtial surface 1n the form of a metallic cylinder comes to a great extent any disturbances in the lupltortei on niinlsulangfmerbe. m Ylllmh 211,11 electron flow between the electrodes and mateea er e emen s oca e or ea mg e ca rially increases the eiiiciency and adaptability of ode, by conduction or radiation, to the proper electron emitting temperature. In order to overthe devl when variable Current 1s employed as come the electrostatic effect of the heater elethe heating Source' ment, the surface of the cathode cylinder is ex- These and other features of the mvention Will tended substantially the full length of the insube n mre Clearly undfrstood from ih e followmg 50 lating member and @my a section remote from deta1led description 1n connection with the acthe end of the heater leading-in wires is coated ccmpanying drawing. with thermionicauy active material to serve as Fis. 1 is a perspective view of an electron disthe electron source. In eiect, the uncoated surcharge device made in accordance with this inface of the cathode serves as an electrostatic vention, embodying the Various features of shield- V65 assembly made in accordance with this invention, with a portion broken away to illustrate the position of the magnetic shield;

Fig. 3 is a top plan view of the cathode assembly shown in Fig. 2; l

Fig. 4 illustrates in cross section a modified cathode structure with the heater formed of two concentric spirals; and

Fig. 5 is an enlarged side view of the electrostatic shield segregating the heater leading-in wires from the electrodes ofl the device.

Referring to the drawing, the electron discharge device, in accordance with this invention,V

comprises an enclosing vessel Ill having an inwardly projecting stem or press II in which the leading-in wires for the electrodes are sealed. An insulating base I2 is attached to the vessel and the leading-in wires projecting through the stem are soldered to the prongs I3 projecting from the closed end of the base I2. A pair of rigid upright metallic members or rods I4 and I5 extend from the press within the vessel and constitute the main supporting members for the electrodes. A cylindrical or equi-potential cathode surface I6 is centrally positioned between the upright members I4 and I 5 and is supported in the axis of the vessel at one end by a wire connection I1 which is attached to the free end of the supporting member I4. A pair of leading-in wires I8 and I9 embedded in the press II are attached to the heater element extending within the cathode I6. A cylindrical metallic plate electrode or anode 20 is coaxially spaced about the cylindrical cathode I6 and rigidly attached to the upright member I5 which serves as a current conductor for-the anode 2U. An insulating member 2|, preferably in the form of an elongated glass bead, is arranged in'an offset position transverse to the longitudinal members I4 and I5 and connected to these members by bent end wires 22 and 23. A similar insulating member or bead 24 is arranged parallel to the insulating bead v2| approximately at the center of the vessel and connected to the upright members I4 and I5 respectively by bent end wires 25 and 26. Near the center of each insulating member, 2| and 24, is astraight wire or support 21 extending in the same direction as the end wires in the respective insulating members. A helical wire grid or control electrode 28 is interposed between the cathode I6 and the anode 20 and coaxially positioned with respect thereto and welded to an upright metallic member 29 having both its ends bent at right angles and connected to the supporting wire 21 embedded in each of the insulating beads 2| and 24. A leading-in wire 30 is sealed in the top of the vessel I0 and connected to one end of the grid supporting member 29. A terminal cap 3| is aixed to the sealed portion at the top of the vessel and the leading-in wire 30 is attached thereto.

The cylindrical or equi-potential cathode I6 is brought to its proper electron emitting temperature by an internal heater element in the form of a hairpin heater 32 threaded through parallel bores in an insulating rod 33, such as quartz, on which the cylindrical cathode I6 is supported. The heater element 32 is usually supplied with current from an alternating current source to maintain the cathode at the proper operating temperature. However, when alternating current is used as the heating source, various difiiculties arise, due to the presence of low frequency hum or noise which creates disturbances in the electron flow between the cathode and the other electrodes in the device. These disturbances are amplified in the output circuit of the device along with the signal current and results in the reproduction of undesirable hum noise which interferes with high quality reproduction of speech and music. 'I'he hum or amount of low frequencies perceptible in the output circuit and produced when the equi-potential cathode is heated by alternating current may be attributed to either or both of two sources. 'I'he first may be considered to be the disturbance created in the electron flow by the varying field of the heater. The contributing causes of this varying Aleid are the electrostatic and magnetic fields of the heater with respect to the cathode. Another cause of the disturbances is the capacitance between the heater or the heater leading-in wires and the other electrodes of the device, principally between the cathode and the grid or control electrode.

All of the above mentioned causes contribute to the total degree of hum produced in the device and merely eliminating one of the causes does not substantially reduce the total volume of hum present in the operation of the device. I t is the main purpose of this invention to sub'- stantially eliminate or reduce all of the causes whereby the hum level is hardly perceptible, if at all. The electrostatic field, in accordance with this invention, is substantially reduced by extending the length of the cathode I6 to substantially the full length of the quartz insulator 33 so that one end of the cathode cylinder is closely adjacent the heater leading-in wires I8 and I9. It will be seen fromFig. 1 that 4the upper portion of the cathode is coated with thermionic material, whereas the lower portion is uncoated. The extension of the cathode surface .along the full length of the insulator substantiallyeliminates the electrostatic field effect between the heater and the cathode because the lower end of the uncoated cathode serves as an electrostatic shield between the heater and the cathode coated surface.'

The disturbing hum due to the magnetic field in an equi-potential device heated by alternating current is overcome in accordance with this invention by shielding the cathode surface from the magnetic effect of the heater. The surface of the cathode in an equi-potential device operates at approximately 750 C. so that any magnetic shield placed between the cathode surface and the heater must necessarily operate at a somewhat higher temperature. The cathode sheath or core which is coated with the thermionically active material toproduce the emission of electrons is usually formed of nickel and since its magnetic properties are lost at about a temperature of 350 C., it cannot be considered as an effective magnetic shield.

In accordance with one aspect of this invention a magnetic shield 34 is interposed between the cathode surface I6 and the insulator 33 as shown in Fig. 2. In order for this shield to be ,effective it must be made of material which does not lose its magnetic properties at the operating temperature of the cathode. 'I'his is accomplished by forming the shield 34 of cobalt or iron or an alloy containing cobalt, such as an iron-cobalt alloy. An alloy containing approximately 10% cobalt retains its magnetic properties to a ternadjoining edges 'of the strip being welded together. The cathode surface I6 is placed on the insulator to enclose\the magnetic shield 34. It is, of course, understood that the magnetic shield may consist of a cylinder' which slides over the insulator 33 and is interposed between the insulator and the cathode surface.

In accordance with another aspect of this invention, the interposed magnetic shield may be omitted and the cathode sheath or core made of the magnetic shield material and coated in the usual. manner to serve as an electron source.

This results in the cathode I6 serving both functions of electrostatic and magnetic shielding since the cathode cylinder will then be formed of a material which does not lose its magnetic properties at the operating temperature of the cathode and'at the same time serves as a carrier for the emission material.

In accordance with another aspect of this invention, the magnetic eld between the .heater and the cathode may be overcome by an arrangement as shown in Fig. 4. The cathode assembly in this figure consists of a heater element having a central helical portion 35 of small diameter and closely spaced turns enclosed in an insulating tube 36. A larger diameter helical spiral 31 is arranged coaxially with respect to the helix 35 with adjacent turns widely spaced. The two helices are attached together at one end as shown at 38. An insulating tube 39 surrounds the helical portion 31 and a metallic sheath or sleeve 40 is supported on the outer insulator 39 to serve as the cathode surface. It is possible to arrange the two spirals, both being wound right-hand or left-hand, and select the number of turns per iich in each spiral to obtain approximately zero field at any particular point outside of the cathode. In eiect, any disturbing field caused by the small diameter helix 35 is neutralized by the large di.

ameter helix 31 and therefore the resultant iield outside of the cathode will be approximately zero.

In addition to the electrostatic and magnetic fields produced in the flow of electrons from the cathode, there is also an additional diiliculty experienced in equi-potential type devices due to the alternating current supply to the heater. This diillculty is caused by the capacitance effect produced between the heater or the heater leading-in wires and the -grid or control electrode. In accordance with this invention the .capacitance effect between the heater leading-in wires and the control electrode is diminished or substantially eliminated by an arrangement as shown in Figs. 1 and 5 in which a metallic shield is placed between the heater leading-in wires and the electrodes of the device. This shield consists of an inverted box-like member 4I having a central aperture through which the cylindrical cathode I6 extends and encloses the wires vI8 and I9 to segregate the heater leading-in wires from the electrodes in the device. The shield 4I is supported by links 42 embedded in the stem I I. The shield 4I is provided with extending apron portions 4Ia to overcome any effect of theA heater leadingin wires sealed in the glass stem. This arrangement serves as a shield between `the heater leading-in wires I8 and I9 and the control grid 28 and eliminates capacitance effects therebetween. In order to increase the freedom from capacity of the grid-the leading-in wire for the grid 30 is brought through the top of the tube as explained heretofore. Another feature of the invention which aids in securing low capacitance between the heater leads and the grid is that the grid is wholly supported from the parallel insulating beads 2I and 24 and is entirely removed from the stem II in which the remainder of the leading-in wires are sealed.

While the invention has been disclosed in a number of specic structures, it is of course understood that various modifications may be made 10 in the detail construction of the equi-potential cathode structure. Furthermore, theinvention may be applied to other electron discharge devices in which two or more grids are employed or in gaseous dischargedevices without departing from the scope of the invention as defined in \the appended claims.

What is claimed is:

1. An electron discharge device comprising a plurality of electrodes including a cylindrical equipotential cathode, a heater element having a small diameter central spiral section and a larger diameter coaxial section surrounded by said cathode, leading-in wires connected to said sections, a-helical grid surrounding said cathode, a cylindrical anode exterior` to said grid and cathode, and means interposed between said leading-in wires and electrodes to substantially reduce electrostatic effects within said device.

2. An electron discharge device comprising an enclosing vessel having a stem, a pair of metallic members projecting from said stem, an equipotential cathode supported by one of said members, an anode surrounding said cathode and supported by the other of said members, a heater element within said cathode, leading-in wires for said heater element projecting from said stem, a grid between said cathode and anode, means outside the electron path and shielded by said anode insulatingly supporting said grid wholly from said metallic members, and a leadingin wire for said grid sealed in said vessel remote from said s em.

3. An electron discharge device comprising an enclosing vessel having a stem, a pair of metallic members projecting from said stem, an equipotential cathode supported by one of said members, an anode surrounding said cathode and supported by the other of said members, a heater element within said cathode, leading-in wiresfor said heater element projecting from said stem. a pair of parallel insulating members supported by said metallic members, outside the surface of said anode and located between said anode and the side wall of said vessel, a grid supported Wholly by said insulating members and located between said cathode and anode, and a leadingin wire sealed in one end of said vessel remote from said stem and attached to said grid.

4. An electron discharge device comprising an enclosing vessel having a press, an equipotential cathode and an anode supported from said press, a grid interposed between said cathode and anode, a pair of insulating beads located beyond the exterior surface of said anode, elbow-shaped 65 supports connecting the ends of said beads to the anode and cathode supports, stub wires projecting vfrom the center of said beads, a support for said grid having bent terminations connected to said stub wires, a heater element within said 7( cathode having current conducting terminations sealed in said press, and a leading-in wire for said grid extending through one end of said vessel remote from said heater terminations.

5. In a discharge device having a stem, sup- 7 porting members extending from said stem, an equipotential cathode supported by one of said members, a heater element for said cathode, leading-in wires for said heater element in said stem, an anode surrounding said cathode and supported from said other member, a pair of parallel insulating members extending transversely adjacent the opposite ends and exterior to the surface of said anode and connected to said supporting members, a control grid between said cathode and anode, a leading-in Wire for said grid extending through the top of said. device and a support for said grid connected to said insulating members.

6. In a discharge device having a stem, a tubular cathode, a heater element Within said cathode, leading-in conductors for said heater element in said stem, a grid and an anode surrounding said cathode, a box-like shield enclosing said leadingin conductors, said shield having a portion embracing said cathode and flanged portions, and supports in said stem engaging said iianged portions.

7. In a discharge device having a stem, a tubular cathode, a heater element within said cathode, leading-in conductors for said heater element in said stem, a grid and an anode surrounding said cathode, a box-like shield enclosing said leadingin conductors, said shield having apron portions extending downwardly on opposite sides of said stem, and supports in said stem engaging said shield.

8. An electron discharge device comprising a vessel having a stem, a pair of metallic upright supports in said stem, an anode attached to one of said supports, a cathode unit including a heater element, an insulating member and a cathode sleeve carried by the other support, a cobalt alloy magnetic shield between said sleeve and insulating member, a metallic shield supporting said cathode unit from said stem and enclosing the heater element terminations, a grid intermediate said anode and said cathode. unit, said grid being insulatingly supported solely from said upright 20 supports, and a connection for said grid extending through the end of said vessel remote from said stem.

JAMES O. MCNALLY.

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