US3299317A

US3299317A - Electron tube having a quick heating cathode with means to apply a variable voltage to the quick heating cathode

Info

Publication number: US3299317A
Application number: US287263A
Authority: US
Inventors: Jr Jackson W Kendall
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1963-06-12
Filing date: 1963-06-12
Publication date: 1967-01-17
Anticipated expiration: 1984-01-17
Also published as: US3249791A; FR1398465A; DE1281585B; CH434491A; GB1055695A; NL6406606A

Description

Jan, 17; 1967 J' w. KENDALL, JR- 3,

, E iled June 12, 1963.

ELECTRON TUBEIIAVING A QUICK HEATING CATHODE WITH MEANS TO APPLY A VARIABLE VOLTAGE TO THE QUICK HEATING CATHODE S'SheetS Sheet l INVENTOR. JACKSON w. KENDALL JR ATTORNEY .1; w. KENDALL, JR v 3,299,317 ELECTRON TUBE HAVING A QUICK HEATING CATHODE WITH MEANS TO APPLY A VARIABLE VOLTAGE TO THE QUICK HEATING CATHODE I 3 Sheets-Sheet Filed June 12, 1963 M v w .r v. m 4 P. w T W 2 l 2 wowww ww w- I! p b psa o fs w l j INVENTOR. JACKSON W. KENDALL JR.

BY A W/ww ATTORNEY ELECTRON TUBE HAVING A QUICK HEATING CATHODE .9 J w. KENDALL, JR 3,

WITH MEANS TO APPLY A VARIABLE VOLTAGE TO THE QUICK HEATING CATHODE FiledJune 12', 1963 3 Sheets-Sheet 5 TO WHEATSTONE BRIDGE AMPLIFIER m SUPPLY----- 2.5V, an

' TUBE SUPPLY H5 Vac. I, I2 v, 8A 5 WHEATSTONE BRIDGE TO BRIDGE 5g AMPLIFIER B v INVENTOR. 0 I JACKSON w. KENDALL JR.

0.! [.OSECOND 1775 v M J/WBw AT TORNE Y United States PatentQ This invention relates to an electron tube and more pa rticularly to a quick heating cathode for an electron tube and the method of operating it.

A portable push to talk transmitter and other similar devices require tubes having low power consumption combined with quick warm up operation for the efficiency and long lifetime of the device. In this type of electronic equipment and also in other electrical systems utilizing tube components in cooperation with transistor components, it is essential for the tube to have the features of low power consumption, long on-ofl cycle lifetime, and

alos have warm up time occur within'approximately a tenth of a second. The tube of this invention has all of the above enumerated essential features and can be used in quick operating electrical systems.

cathodes have been made before using the principle of directly heating the cathode by a current source. However, most of the prior art arrangements generally were operated with the consumption of a great deal of power since the current to the cathode was left on continuously so that immediate cathode operation could be achieved.

Besides high power consumption, previous directly heated cathodes had other disadvantages, such as 'complicated assembly, vibration problems, poor voltage-current relationship, i. e. high current at low voltage, short life time due to inability to be cycled on and off repeatedly, and mechanical difliculties in the expansion and contraction of the cathode.

In other forms of directly heated cathodes, an overvoltage or high potential was applied to the cathode for the quick heating thereof and timing devices were utilized to switch on and off the high potential applied to the cathode. However, this type of arrangement was not adequate in the past since the cathode did not always reach its coldest temperature before the equipment user activated the circuit supplying the overvoltage to the cathode. Hence, the use of a timing mechanism was inadequate since the tube was easily destroyed due to overheating of the cathode.

Other directly heated cathode devices, using an initial overvoltage for quick heating, required complicated mechanical and electrical arrangements associated with the tube to switch the overvoltage on and oif. The overvoltage had to be applied directly to the cathode for only a short period of time, and the prior art was concerned with complex mechanical and electrical arrangements to cope with the problem of switching the overvoltage on and off and the associated cathode expansion and contraction problem. A need existed for a tube having low power consumption, quick warm up, long on-off cycle lifetime, and simple construction from a mechanical and electrical view.

Accordingly, the main object of this invention is to provide an electron tube which will heat up to operating temperature in a very short period of time and a method for quick heating an electron tube.

v A further object of this invention is to provide an electron tube which can be quickly brought to operation and use low amounts of power to maintain operation.

A still further object of this invention is to provide a cathode which will have a long on-oif cycle lifetime.

3,299,317 Patentetl dan. 17, 1967 A further object of this invention is to provide a simply constructed quick heating cathode arrangement which will permit the cathode to repeatedly expand and contract during on-off cycling. 1 p

Another object of this invention is to provide a cathode which is initially directly heated and subsequently heated directly and indirectly. i v I I Y:

A further object of, this invention is to'provide a. cathode'which will have a' cathode 'temp'eratu'revserising arrangement to control voltage applied directly tothe' cathode so as to prevent destruction of the cathode. x

I Briefly described, the invention resides in 1a cathode assembly which comprises a cathode'member and a support for the cathode member. Current carrying means are providedin the cathodej assembly and are permanently connected in series with the cathode member.- The current carrying means serve as a current pathand as a heat source for the cathode member. The current carrying means is also a flaccid element which permits the cathode member to expand and contract in response to temperature variations. j

A cathode heating arrangement is also described wherein cathode temperature sensing means'are provided to cooperate with the cathode assembly sojas to control the amount of voltage which is supplied to the cathode member. p j In addition, an electron tube having an envelope enclosing electrodes including a cathode is described. The electron tube contains the current carrying means perma: nently connected in series with the cathode' to serve as a current path and as'a heat source for the cathode. The cathode temperature sensing means to control the voltage applied to the cathode are also defined in the description of the electron tube. I

A method for quick heating a cathode assembly and an electron tube is also described. The method requires that a high potential be applied to the cathode using the current carrying meansorelement as a current path. The high potential applied to the cathode is 'switched "to" 'a low potential after the cathode has reached'a designated temperature. The cathode-isheated directly by the curre-nt passing through the current carrying element and indirectly with the current carrying means serving as'a heat source since it is'compo'sedofa material having a high electrical resistance when hot. r

These and other features of this invention will become more clearly apparent upon review of the following description when'taken in conjunction with theaccompanying figures in which: Y

FIGURE 1 is an elevational view in section of 'a'tub illustrative of this invention; FIGURE 2 is an elevational view in section of the cathode assembly of the tube of FIGURE 1; a I

FIGURE 3 is a schematic diagram of the cathode tem perature sensing circuit;

FIGURE 4 is a schematic digram of one type-of switching circuit used with the circuit of FIGURES; and

FIGURE 5 is a graph showing the temperature vs. time characteristics for cathode warm up.

Referring now to FIGURE 1 of the drawing, a tetrode is shown to illustrate the invention. Electron tube 8 has an anode 10 which is a cylindrical or cup-shaped metal element mounted on a ceramic base 12 by means of IL shaped metal flange 14, L-shaped metal flange'1-5, ceramic cylinder section 16 and curved metal flange'18. Ametal flange 17 is brazed to the outside of the anode and a cup-shaped metal contact member 19' is heliarced to the flange 17 so as to provide an anode terminal; The metal contact member 19 also provides a protective cap -for anode tubulation 21 which is pinched and sealed-at 2'3; after exhausting contaminants from the tube. Assembly of the various metallic and ceramic elements mentioned support flange thereto.

herein is done pursuant to well known ceramic metallizing and brazing techniques used in joining metal to ceramic.

Referring to FIGURES 1 and 2, a copper index pin 20 is mounted in the center of the ceramic base 12. Eight contact pins 22, only four of which are shown in the sectional views of FIGURES 1 and 2, are also mounted in the ceramic base 12 by metallizing the ceramic and brazing washer 13 thereto. The pins 22 provide leads for the electrodes of the tube 8.

As shown in FIGURE 1, cathode cylinder 24, control grid cylinder 26, and screen grid cylinder 28 are mounted in concentric alignment in the electrontube 8. An electron emitting oxide coating is applied to the exterior surface of the cathode cylinder 24. The cathode cylinder 24 is composed of a thin foil, preferably having small holes 25 aligned diagonally. The holes 25 are formed by etching. The electrical resistance of the cathode cylinder 24 is raised by making the cylinder thin and also by providing theholes 25 which create a longer electron path. The cathode cylinder 24 is composed of high electrical resistance metal material, such as a one mil thick Hastelloy B metal which is a trade name of a nickel based alloy comp-rising small amounts of molybdenum, iron and carbon. Metal caps 30, '32and 34 connect the cathode cylinder 24, the control grid cylinder 26 and the screen grid cylinder 28, respectively, to an insulated support pin 36. The screen grid cylinder 28 is brazed to a curved metal flange 38 at its lowest end. A support flange 40 is bolted by bolt 41 to the curved metal flange 38 so as to support the screen grid assembly comprising the screen grid cylinder 2'8 and the curved metal flange 38. The support flange 40 is connected at its lowest end to the ceramic base 12 by metallizing the ceramic and then brazing the The support flange 40 is also joined to the U-shaped flange 14 by brazing.

Referring to FIGURES 1 and 2, a four-pronged cathode cylinder metalsupport member 42 is mounted on four of the pins 22. Current can be supplied directly to the cathode cylinder 24 through electrical connection with the four pins 22 on which the cathode cylinder metal support member 42 is mounted. The cathode cylinder 24 is connected to the cathode cylinder metal support member 42 by spot welding. Leads for the control grid 26 are made through connections 44 to two of the pins 22. A molybdenum wire braid 46 comprising forty to fifty .003 inch diameter molybdenum wires is formed in a hollow, substantially cylindrical configuration. The braid 46 is mountedon metal pin 48 the bottom portion of which is located within and brazed to the copper index pin 20. The molybdenum wire braid 46 has one end of each of the .003 inch diameter wires spot welded to the metal pin 48 and the other ends of the wires are spot welded between the cap 30 and the cathode cylinder 24. Thus, the braid 46 is permanently connected in series with the cathode cylinder 24 and provides a current path for the cathode cylinder 24. The end of the braid 46 which is located between cathode cylinder 24 and the end cap 30 has a larger diameter thanthe end of the braid 46 which is spot welded to the pin 48. This is achieved by cutting and stretching the end of the molybdenum braid 46 to achieve the wide configuration necessary for connection between the cathode cylinder 24 and the end cap 30. The molybdenum braid 46 is a flaccid member which yields readily, and thus permits the cathode cylinder 24 to expand and contract in its longitudinal direction without stress. The molybdenum braid 46 has a high electrical resistance when hot and a low electrical resistance when cold, thus permitting large amounts of current to initially surge through the braid 46 to heat the cathode directly. After, the initial high current surge through the braid 46, the braid 46 becomes hot and its electrical resistance becomes high. The braid 46, when hot, has a lower amount of current passing through it and heats the cathode cylinder 24 directly with this current. Thebraid 46 also heats the cathode cylinder 24 indirectly due to its high electrical resistance when hot.

A molybdenum control lead wire 59 is connected to the end cap between the cathode cylinder 24 and the end cap 30. The lowest end of the control lead 50 is connected to one of the pins 22 for use in the circuit of FIGURE 3.

Referring to FIGURE 3, a Wheatstone bridge control circuit is shown comprising variable heater return lead resistance 52 which is the wire braid 46. A constant lead resistance 54 which is the cathode cylinder 24, a constant resistance 56, a' variable resistance 58 for matching variations in resistance of tube components thus allowing the bridge to be balanced, and the control lead 50 comprise the remainder of the circuit of the Wheatstone bridge. The cathode cylinder 24 of Hastelloy B and the heater return lead 52 of molybdenum have different rates of change in resistance during warm up. Due to the relatively higher electrical resistance of the molybdenum wire braid 46 when hot, the balance of the Wheatstone bridge circuit is upset. The resulting voltage across the bridge, after a small amount of amplification by the Wheatstone bridge amplifier 59, shown in block form in the circuit of FIGURE 4, operates the relay 60.

The circuit of FIGURE 4 comprises a conventional switching relay 60 which, when energized, removes the overvoltage supplied to the cathode cylinder 24 and only the low heater voltage is supplied to the series circuit comprising the cathode cylinder 24 and the wire braid 46.

Curve A of FIGURE 5 depicts the warmup time of a cathode in relation to temperature. T on the ordinate indicates the temperature for electron emission by the cathode. T indicates the cathode burn out temperature or the temperature at which the cathode is destroyed. In order to bring the temperature of the cathode up to where electrons are emitting in one tenth of a second, it is necessary to use an overvoltage.

Curve B of FIGURE 5 is illustrative of the'warm up characteristic of the quick heat cathode of this invention.

There is only a small temperature difference between the temperature for the cathode to emit electrons (T and its burn out temperature (T Therefore, control of the temperature of the cathode is extremely important and this cathode temperature control is achieved by the mechanical and'electrical arrangements of this invention.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained, and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. An electron tube comprising an evacuated envelope and an anode, two terminals rigidly secured in said envelope adjacent its lower end, a thin wall cathode cylinder of high electrical resistance mounted on one of said terminals, said cathode cylinder having an electron emissive outer surface, a heater inside said cylinder and extending along the length thereof, said heater having its upper end connected to the upper end of said cathode cylinder and having its lower end connected to the other of said terminals, said heater comprising a tubular braid of thin wires forming a flaccid structure which will permit said cylinder to expand and contract free from stress by the heater, said heater wires being made of a conductor which has a high electrical resistance when hot and a low electrical resistance when cold, said cathode cylinder being made of a conductor which has a more constant resistance with change in temperature relative to the change in resistanoe of said heater with change in temperature, a third terminal in said envelope, and an electrically conductive lead interconnecting said third terminal and the upper end of said heater.

-2. An electron tube as claimed in claim 1 in which the terminal connected to said heater is a pin coaxial with said cathode cylinder, the lower end of said heater braid has a relatively small diameter portion surrounding and attached to said pin, and the upper end of said braid which is connected to the cathode cylinder has a larger diameter.

3. Electron tube apparatus comprising an electron tube having a cathode connected in series with a heater for said cathode, and means for controlling voltage to said series connected cathode and heater, said voltage controlling means comprising a variable voltage source connected to said series connected cathode and heater, said variable voltage source being adjustable to supply either a high voltage which will bring said cathode quickly to emissive temperature or a low voltage which will maintain said cathode at em-issive temperature, a Wheatstone bridge in which said cathode forms one leg of the bridge and said heater forms another leg of the bridge, and relay means responsive to unbalance of said bridge for reducing the voltage of said variable voltage source from said high voltage to said low voltage, said heater being made of a conductor which has a high electrical resistance when hot and a low electrical resistance when cold, and said cathode being made of a conductor which has -a more constant resistance with change in temperature relative to the change in resistance of said heater with change in temperature.

References Cited by the Examiner OTHER REFERENCES Woldman, N. E., Engineering Alloys, N.Y., Reinhold P ubl. Corp., 1962, Scientific Library, call No. TA490, p. 286.

JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, R. JUDD, Assistant Examiners.

Claims

3. ELECTRON TUBE APPARATUS COMPRISING AN ELECTRON TUBE HAVING A CATHODE CONNECTED IN SERIES WITH A HEATER FOR SAID CATHODE, AND MEANS FOR CONTROLLING VOLTAGE TO SAID SERIES CONNECTED CATHODE AND HEATER, SAID VOLTAGE CONTROLLING MEANS COMPRISING A VARIABLE VOLTAGE SOURCE CONNECTED TO SAID SERIES CONNECTED CATHODE AND HEATER, SAID VARIABLE VOLTAGE SOURCE BEING ADJUATABLE TO SUPPLY EITHER A HIGH VOLTAGE WHICH WILL BRING SAID CATHODE QUICKLY TO EMISSIVE TEMPERATURE OR A LOW VOLTAGE WHICH WILL MAINTAIN SAID CATHODE AT EMISSIVE TEMPERATURE, A WHEATSTONE BRIDGE IN WHICH SAID CATHODE FORMS ONE LEG OF THE BRIDGE AND SAID HEATER FORMS ANOTHER LEG OF THE BRIDGE, AND RELAY MEANS RESPONSIVE TO UNBALANCE OF SAID BRIDGE FOR REDUCING THE VOLTAGE OF SAID VARIABLE VOLTAGE SOURCE FROM SAID HIGH VOLTAGE TO SAID LOW VOLTAGE, SAID HEATER BEING MADE OF A CONDUCTOR WHICH HAS A HIGH ELECTRICAL RESISTANCE WHEN HOT AND A LOW ELECTRICAL RESISTANCE WHEN COLD, AND SAID CATHODE BEING MADE OF A CONDUCTOR WHICH HAS A MORE CONSTANT RESISTANCE WITH CHANGE IN TEMPERATURE RELATIVE TO THE CHANGE IN RESISTANCE OF SAID HEATER WITH CHANGE IN TEMPERATURE.