US3276836A

US3276836A - Process for upgrading vacuum tubes

Info

Publication number: US3276836A
Application number: US329153A
Authority: US
Inventors: Neher Henry Victor
Original assignee: Victoreen Instrument Co
Current assignee: Victoreen Instrument Co
Priority date: 1963-12-09
Filing date: 1963-12-09
Publication date: 1966-10-04
Anticipated expiration: 1983-10-04

Description

Oct. 4, 1966 H. v. NEHER 3,276,836

PROcEss FOR UPGRADING VACUUM TUBES Filed Dec. 9, 1965 05; E QRQM +3 44.

United States Patent 3,276,836 PROCESS FOR UPGRADING VACUUM TUBES Henry Victor Neher, Pasadena, Calif., assignor to Victoreen Instrument Company, Cleveland, Ohio, a company of Ohio Filed Dec. 9, 1963. Ser. No. 329,153 3 Claims. (Cl. 316-2) This invention has to do with measuring and improving the quality of vacuum tubes, with particular reference to the amount of residual gas that is present in such tubes.

More particularly, the invention provides procedures for measuring the pressure of residual gas within a vacuum tube and for pumping the tube to virtually eliminate such residual gas, all without opening the tube and without requiring any special equipment within the tube.

In the commercial manufacture of vacuum tubes, it is not feasible to produce as high a vacuum within the tube envelope as is desirable for some tube applications. In normal manufacture, vacuum tubes are pumped down to a pressure of about 10* mm. Hg and then sealed off. Even if the pumping technique could be improved on a production basis, the act of heating the glass tubulation to seal off the tube releases a small amount of gas into the tube, sharply limiting the resulting vacuum. Some of the residual gas can be cleaned up by providing in each tube a quantity of getter, such as BaAl, that is I electrically heated and vaporized after the tube has been sealed ofi. However, the vacuum actually produced by such known and customary techniques, typically l0 to 10- mm. Hg, is not fully satisfactory for all purposes. Moreover, during normal use of the tube gases tend to escape from the metal, glass and other materials inside the tube, further lowering the vacuum.

Treatment of finished tubes to improve their vacuum is particularly desirable in the special vacuum tubes of electrometer type which are used primarily to measure extremely small currents, say of the order of 1() amperes. Any residual gas inside the tube tends to become ionized by the electric fields between tube electrodes, and the flow of such ions produces spurious currents that afiect such measurements. Even a very low pressure of residual gas in an electrometer tube can cause a variable grid current of the same order of magnitude as the current being measured. Although such spurious grid current can be partially controlled by limiting all tube voltages to values less than the ionization potential of the residual gas, the performance of the tube is thereby severely limited. Moreover, the residual gas tends to cause progressive deterioration of the cathode surface, so that an instrument in which the tube is used requires frequent recalibration.

A primary object of the present invention is to provide a process for treating complete vacuum tubes to effectively eliminate the residual gas that they normally contain. That is accomplished without opening the tube envelope, and without requiring any special chemical or physical apparatus within the tube.

In a known procedure for obtaining a very high vacuum, known as ion pumping, special electrodes are provided in the chamber to be evacuated. After preliminary evacuation, an intense electric field is produced between those electrodes, and a strong magnetic field is superimposed, parallel to the' electric field. The action of those fields, in combination with the specially provided electrode surfaces, produces a slow but progressive disappearance of the gas from the chamber. Apparently stray electrons present in the chamber are accelerated by the imposed fields, ionizing the gas by collision; and the gas ions are then driven into the electrode surfaces with sufiicient energy to become bound.

In accordance with the present invention, it has been discovered that ion pumping can be carried out effectively in tetrode or pentode vacuum tubes of conventional design and construction without provision of any special electrodes. An electric field of suitable form and intensity for that purpose can be produced by direct connection of selected tube electrodes to the opposite terminals of a suitable source of direct current voltage. In accordance with the invention, the tube is then placed in a magnetic field in such orientation that the lines of magnetic force are essentially perpendicular to the tube plate.

In preferred form of the invention the entire vacuum tube is heated during the ion pumping procedure to an elevated temperature, typically of the order to to 300 C.

A further aspect of the present invention provides a process for measuring the actual pressure of residual gas existing within a tetrode or pentode vacuum tube of conventional construction. That is accomplished, like the ion pumping procedure, without opening the tube envelope or otherwise affecting the tube, and without requiring provision of any special structure within the tube.

For that purpose, the tube filament is heated, typically by application of the normal filament voltage; a positive voltage, such as 100 volts, is applied to the screen grid to draw electrons from the filament, and a negative bias is applied to the control grid of sufiicient value to limit the screen grid current to a suitable value. The tube plate is maintained at a moderate negative potential with respect to the filament. A sensitive current measuring device is connected in series with the tube plate. With that arrangement, the observed current flowing from the plate is due to positive ions reaching the plate, and is essentially a linear measure of the residual gas pressure within the tube. That method of measuring the gas pressure is particularly useful for monitoring the progressive improvement in the tube vacuum produced by the ion pumping procedure described above.

A full understanding of the invention, and of its further objects and advantages, will be had from the following description of certain detailed illustrative procedures by which it may be carried out. The particulars of that description and of the accompanying drawings which form a part of it, are intended only as illustration, and not as a limitation upon the scope of the invention, which is defined in the appended claims.

In the drawings:

' FIG. 1 is a schematic diagram representing illustrative circuit arrangements for treating a completed vacuum tube to improve its vacuum in accordance with the present invention;

FIG. 2 is a schematic elevation representing a vacuum tube undergoing such treatment; and

FIG. 3 is a schematic diagram representing illustrative circuit arrangements for measuring the residual gas pressure in a completed vacuum tube in accordance with the present lnvention.

In each of FIGS. 1 and 3 a conventional vacuum tube is represented schematically at 10, with cathode 12, control grid 14, screen grid 16, suppressor grid 18 and plate 20. The cathode is shown as a filament, but may, alternatively, be of heater type. The suppressor grid is shown 'illustratively connected internally of the tube envelope extend between the pole faces of the magnet, as indicated at 30, the sense of the field being unimportant for the present purpose. Tube is supported in the magnetic field 30 in such orientation that the lines of magnetic force pass through the tube transversely of the tube plate and preferably substantially perpendicularly. The intensity of magnetic field 30 is preferably as high as can be applied economically without danger of deflecting the tube elements permanenntly or causing contact of adjacent elements. A field of the order of 2500 gauss is ordinarily satisfactory.

A source of direct current voltage is indicated schematically at 34, typically comprising a conventional transformer and rectifier for transforming alternating current power from a source 35 into direct current power of the desired voltage at the negative and positive output terminals 36 and 37, respectively. The output voltage from source 34 is preferably of the order of 1000 volts. Voltages much below about 500 volts are found to be relatively ineffective for the present purpose, while voltages much above about 2000 volts involve danger of arcing within the vacuum tube.

In the illustrative circuit of FIG. 1, negative terminal 36 of direct current voltage source 34 is connected via the line 38 to plate 20 and also to control grid 14 of the vacuum tube; and positive terminal 37 of the voltage source is connected via the line 39 to screen grid 16 of the tube.

Protective resistances

41 and 42, typically of the order of l megohm, are preferably inserted in

lines

38 and 39. Suppressor grid 18 and filament 12 may be allowed to float, as shown, or one or both of them may be connected to a positive potential, such, for example, as the line 39.

In carrying out the present invention, the tube is treated by maintaining the described connections, or their equivalent, and a magnetic field such as 30 for an extended time period of the order of 10 to 50 hours. During that treatment any electrons appearing between the control grid and plate of the tube, from whatever source, are strongly attracted toward the positive screen grid. Most such electrons pass through the screen grid without striking it, and are again attracted toward it, leading to an oscillatory motion back and forth through the apertures of the screen grid. The presence of magnetic field 30 causes such electrons to follow a path that is a helix or spiral about the lines of magnetic force and that is therefore far longer than the relatively straight path that would otherwise be followed. Any gas molecules that are present in the tube tend to be bombarded by such fast moving electrons and ionized. The resulting positive ions are attracted to the strongly negative plate and control grid, which they strike with suflicient velocity to have a good probability of becoming bound. The number of free gas molecules is thus progressively reduced .by the .described tube treatment.

It has been discovered, further, that the eflectiveness of the described treatment can be considerably increased by maintaining the vacuum tube during the treatment at an elevated temperature, such as from about 100 to about 300 C. That can be accomplished by surrounding the tube envelope by a furnace enclosure, indicated at 50 in FIG. 2. That enclosure may be made of asbestos in which is embedded an electrical winding 52 of high resistance material, such as Nichrome, for example. The winding is connected to electric power source 35 via a control apparatus, indicated schematically at 54, which is adjustable to vary the current in winding 52 and thereby maintain the vacuum tube at the desired temperature. Adjustment of control 54 is facilitated by providing in furnace 50 a temperature indicating device, such as the thermometer 58. A particular advantage of maintaining an elevated temperature during the described tube treatment is that the resulting improvement in the tube vacuum tends to be more stable during'subsequent use of the tube than when the treatment'is carried out at normal temperature. i

In FIG. 3, tube 10 is connected in an illustrative circuit for measuring the residual gas pressure in accordance with the present invention. The filament 12 or the cathode heater is supplied with essentially the normal heating voltage from a source indicated schematically as the battery 60. The relative potentials to be described for supply to the various tube electrodes may be obtained in any suitable manner. The multiple-brush potentiometer structure indicated at 62, with direct current voltage source 64 connected ,to its terminals, is illustrative. The tube cathode is connected to an intermediate point of potentiometer 62,?which point will be considered as zero potential, and may be grounded as. indicated at 65.

Plate 20 is connected to a point of. potentiometer 62 that is negative with respect to the cathode by a potential of the order of 10 volts, a specific illustrative value being 9 volts. Screen grid 16 is connected to a point of potentiometer ,62 that is strongly positive with respect to the cathode by a potential of the order of 100 volts, a typical specific value being volts. The screen grid then functions as a conventional plate, attracting electrons released from the cathode. Control grid 14 is connected to a point of potentiometer 62 that is sufficiently negative with respect to the cathode to limit he curren flawing to the screen grid to a value suitable to the tube in question. For a miniature tube of electrometer type, for example, a suitable value is to 200 microamperes, which require a negative bias of the order of -50 volts on the control grid, a specific illustrative value being -60 volts.

With the described connections, electrons are eflectively prevented from reaching tube plate 20 by its negative potential, so that the only plate current is due to positive ions produced within the tube, primarily by electrons accelerated toward the positive screen grid. The density of such ions,'for a given valuev of the electron flow to the screen grid, is essentially proportional to the density of gas molecules present to be ionized. Hence the magnitude of the current flowing from the plate provides an accurate measure of the residual gas pressure within the tube. That plate current may be measured by connecting a sensitive current measuring device, such as a conventional galvanometer, in series with the plate lead, as indicated at 70. The residual gas pressure in millimeters of mercury is typically given approximately by 10- times the plate current expressed in amperes times 10- However, since the primary interest is in performance of the tube, it is. usually sufficient to calibrate the current measuring device in terms of observed performance of the tube, rather than absolute gas pressure.

The invention is fully applicable to a wide variety of vacuum tubes, so long as the tube includes at least one grid in addition to the plate, cathode and control grid. If additional grid or grids are present, they may be allowed to float, or tied to any suitable voltage that does not interfere with the described tube operation. When there is a suppressor grid of suitable form as well as a screen grid and control grid any two of those three grids may be connected in the manner that has been described for the screen grid and control grid. In the circuit for ion pumping, for example, it is suflicient that the positive grid be intermediate the negative grid and plate. Many further modifications may be made in the particulars of the described procedures without departing from the proper scope of the invention, which is defined in the appended claims.

I claim:

1. The method of treating a vacuum tube to improve the vacuum therein, said vacuum tube comprising an evacuated sealed envelope containing cathode, plate, control grid, and at least one auxiliary grid between the control grid and the plate; said method comprising connecting the plate and one of said grids that is not adjacent the plate to the negative terminal of a direct current voltage source of the order of 1000 volts,

connecting a second one of said grids that is between said one grid and the plate to the positive terminal of such voltage source,

producing a magnetic field of the order of 2500 gauss with magnetic lines of force passing through the tube transversely of the grids and plate,

and maintaining said field and said connections for a time period of the order of to 50 hours. 2. The method of treating a vacuum tube to improve the vacuum therein, said vacuum tube comprising an evacuated sealed envelope containing cathode, plate, control grid, and at least one auxiliary grid between control grid and the plate; said method comprising connecting the plate and one of said grids that is not adjacent the plate to the negative terminal of a direct current voltage source of the order of 1000 volts,

heating the tube to a temperature of the order of 100 to 300 degrees centigrade,

and maintaining said temperature, said magnetic field 6 and said connections for a time period of the order of 10 to hours.

3. The method of measuring the pressure of residual gas in a vacuum tube that comprises an evacuated, sealed envelope containing heatable cathode, plate, control grid, and at least one auxiliary grid between the control grid and the plate, said method comprising supplying to the cathode a heating current of approxi mately normal magnitude, connecting the cathode and the auxiliary grid in a circuit that includes a source of direct current voltage to maintain the auxiliary grid of the order of 100 volts positive with respect to the cathode,

connecting the control grid to a source of negative bias of the order of volts with respect to the cathode,

connecting the plate in a circuit that includes a source of direct current voltage to maintain the plate of the order of 10 volts negative with respect to the cathode,

and measuring the current that flows from the plate in the last said circuit by virtue of positively charged gas ions reaching the plate.

No references cited.

FRANK E. BAILEY, Primary Examiner.

Claims

1. THE METHOD OF TREATING A VACUUM TUBE TO IMPROVE THE VACUUM THEREIN, SAID VACUUM TUBE COMPRISING AN EVACUATED SEALED ENVELOPE CONTAINING CATHODE, PLATE, CONTROL GRID, AND AT LEAST ONE AUXILIARY GRID BETWEEN THE CONTROL GRID AND THE PLATE; SAID METHOD COMPRISING CONNECTING THE PLATE AND ONE OF SAID GRIDS THAT IS NOT ADJACENT THE PLATE TO THE NEGATIVE TERMINAL OF A DIRECT CURRENT VOLTAGE SOURCE OF THE ORDER OF 1000 VOLTS, CONNECTING A SECOND ONE OF SAID GRIDS THAT IS BETWEEN SAID ONE GRID AND THE PLATE TO THE POSITIVE TERMINAL OF SUCH VOLTAGE SOURCE, PRODUCING A MAGNETIC FIELD OF THE ORDER OF 2500 GAUSS WITH MAGNETIC LINES OF FORCE PASSING THROUGH THE TUBE TRANSVERSELY OF THE GRIDS AND PLATE, AND MAINTAINING SAID FIELD AND SAID CONNECTIONS FOR A TIME PERIOD OF THE ORDER OF 10 TO 50 HOURS.