Corona-free top cap terminal assembly for electron discharge devices

Abstract

An electron discharge tube having a corona-free top cap terminal assembly comprising a metallic top cap conductively bonded to the outer surface of the tube envelope.

Description

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatus for manufacturing electrical devices and, more particularly, to a method and apparatus for making a top cap terminal assembly for electron discharge tubes.

Conventionally, the top cap terminal for high voltage electron discharge tube comprises a cup-shaped metallic member cemented to the top of the glass envelope, for enclosing the tube electrodes, with the rim of the cup-shaped member disposed adjacent the envelope. Viscous fluid, paste, and preformed cements are known types of bonding agents for fixing the top cap terminal to the envelope. Electrical connection between the top cap and the anode is provided via a lead wire extending through the envelope which is welded or soldered to an aperture in a top wall of the top cap. The envelopes for such tube are typically provided with a conductive getter flash or deposit on the inside surface of the envelope to enhance the vacuum therein. The flash is usually disposed on the inside surface of the envelope opposite the top cap.

Heretofore, such tubes have suffered from the problem that electrical noise is generated during the use of such devices in television receivers. It was discovered that the primary source of such electrical noise was a corona or electrical discharge between the rim of the top cap and the envelope at points where the rim was spaced from the surface of the envelope. A secondary source of electrical discharge has been found to be between the anode lead wire and the getter deposit on the inner surface of the envelope opposite the top cap. This deposit is usually in electrical contact with the lead wire during the early life of the tube, but the contact can become broken because the deposit is thin and will deteriorate over the life of the tube.

In addition to the electrical noise created by the corona, it has been found that the envelope, which is conventionally made of glass, will be severely damaged during the life and operation of such devices and can result in the tube eventually becoming defective.

The primary source of this problem appears to have been recognized and discussed in U.S. Pat. No. 3,727,088 of Allgaier issued on Apr. 10, 1973. The Allgaier patent suggests that this source of the problem can be solved by maintaining the getter assembly at anode potential.

Other solutions which have been suggested are the provision of a conductive paint or coating on the external surface of the envelope adjacent the rim of the top cap, thereby to provide a conductive path between the top cap rim and the envelope. This was found to eliminate the corona problem but was found to be unsatisfactory because of the extra cost and labor required to put the conductive paint on the surface. Furthermore, the paint was found to require careful application to prevent the paint from being applied to the surfaces of the top cap and envelope to be bonded. If the paint is applied to these surfaces, a defective or weak bond can be produced.

Accordingly, it is an object of this invention to provide an electron discharge tube with an improved top cap terminal assembly, thereby avoiding the problems associated with the prior art.

It is another object of this invention to provide an electron discharge tube having a top cap terminal assembly conductively bonded to the tube envelope.

It is still another object of this invention to provide a top cap terminal which can be conductively bonded to the tube envelope in a simple and inexpensive manner.

Other objects of the invention will be apparent from the following description and accompanying claims.

SUMMARY OF THE INVENTION

In accordance with this invention, an electron discharge tube is provided comprising a tube envelope with a protuberance thereon, a cup-shaped top cap terminal disposed about said protuberance with the rim of the top cap disposed adjacent the outer surface of said envelope, said top cap being bonded to said envelope by an electrically conductive cement, thereby to provide an electrically conductive path between said top cap and said envelope.

In accordance with another aspect of the invention, a method for attaching a cup-shaped top cap terminal to an electron discharge tube comprises providing an electrically conductive cement, disposing said cement and said cap over an exterior surface of the tube envelope, so that an electrically conductive bond can be formed between said surface and said cap.

The invention will be better understood after consideration of the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view of a fragmentary axial section of an electron discharge tube with a top cap terminal constructed in accordance with this invention.

FIGS. 2A and 2B illustrate the steps of attaching a top cap terminal to a tube envelope in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, and more particularly to FIG. 1, there is shown an electrical element 11, which may be an anode or other electrode, of a sealed discharge device tube type having a top cap 13 and a lead wire or top cap wire 15 projecting from an opening 17 in the top surface of the top cap 13. The tube type may otherwise be of any conventional construction, such as that shown and described in U.S. Pat. No. 2,857,544 of J. F. Stephens, patented Oct. 21, 1958, and assigned to the assignee of this application. The invention, it will be understood, is equally applicable to any other tube type wherein a top cap terminal or electrode connection is required or desired.

The top cap wire 15 fits loosely within the opening 17 of the top cap 13 during assembly and fabrication of the tube; and when the fabrication of the tube is completed, the top cap wire 15 should be firmly, tightly, and conductively secured to the top cap 13. The cap 13 is a metallic, cup-shaped member of any suitable material, such as brass, and is secured to the envelope 21 of the tube by an electrically conductive cement 19. The cement 19, which, in accordance with this invention, includes a graphite or other conductive particle filler for good electrical conductivity, is preferably any general type of epoxy cement which can be preformed in a partially cured, non-tacky state. Such conductive epoxy cements are well known and are marketed by Amicon Corporation under the trademark "Uniset" as product No. SE-902-31.

In FIG. 2A and in accordance with the illustrated embodiment, the cement 19 is preformed in the shape of an annular ring 25 which can be slid over a protuberance 24 on the outer surface of envelope 21. After disposing the preformed epoxy cement 25 in place, the top cap 13 can be threaded over the wire 15 with the rim portion 27 thereof ultimately disposed adjacent a portion of the outer surface of envelope 21 surrounding the protuberance 24. The top cap 13 (FIG. 2B) then can be heated by any conventional means 29, such as a gas burner, to soften the annular ring 25 into a viscous, tacky state so that, upon curing or hardening, an electrically conductive bond is formed between the top cap rim 27 and the outer portion of envelope 21. It can be seen that the outer portion of envelope 21 forms acute angle with the rim 27 so that the cement 19 can flow in its viscous, tacky state into the tapered space formed therebetween so as to insure that good electrical contact between the rim 27 and the outer portion is formed. Accordingly, it can be seen that this construction will avoid the corona problems noted in some prior art constructions in a simple and inexpensive manner.

It will be understood by those skilled in the art that the cement need not be applied in a preformed, non-tacky state and further that conductive cements other than epoxy-based cements can be used within the scope of this invention.

After the top cap 13 has been conductively bonded to the envelope 21, the portion of the wire 15 which extends beyond a projection 25 in the upper portion of the top cap 13 is broken or cut off. The remaining portion of lead 13 extending into the projection 25 is then welded in accordance with the method set forth in U.S. Pat. No. 3,192,610 of Keating, issued on July 6, 1965, and assigned to the assignee of this application, the disclosure of which is incorporated herein by reference.

Although a particular embodiment of the subject invention has been described, many modifications may be made, and it is understood that the appended claims are intended to cover all such modifications that fall within the true spirit and scope of the invention.

Claims

What I claim is new and desire to secure by Letters Patent of the United States is:

1. An electron discharge tube comprising:

a. an evacuated envelope having a top wall portion with an external protuberance thereon;

b. at least one electrode sealed within said envelope;

c. a lead wire extending from said electrode, said wire passing through and sealed in said protuberance;

d. a cup-shaped top cap terminal disposed over said protuberance with the rim of said top cap disposed adjacent an outer surface of said top wall portion, and said top cap electrically connected to said lead; and

e. an electrically conductive cement disposed between said top cap, said protuberance, and said surface providing an electrically conductive path between said top cap rim and said surface and bonding said top cap to said protuberance and said top wall portion.

2. The tube of claim 1 wherein said cement is a mixture of epoxy and conductive particles.

3. A method for attaching a cup-shaped top cap terminal on an electron discharge tube, said tube comprising a plurality of electrodes in an envelope, a lead wire extending from one of said electrodes through said envelope and said top cap, said method comprising:

a. providing an electrically conductive cement; and

b. disposing said cement and said cap over a portion of the exterior surface of the envelope through which the lead extends so as to permit the formation of an electrically conductive bond between said cap and said portion.

4. The method of claim 3 wherein said cement is provided in a preformed, non-tacky state, said method further comprising heating said portion, said cement and said cap to form said electrically conductive path thereamong.

5. The method of claim 4 wherein said portion has a protuberance through which said lead extends; said cement is provided in a preformed shape of an annular ring; and said cement and said cap are disposed around said protuberance prior to heating.