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Locked-Seam Cathodes

Mullard Outlook, Vol. 11 No. 8, January 1962.
    
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All Mullard frame-grid valves for television receivers now use locked-seam cathodes, and eventually almost all Mullard television valves will incorporate the new cathode.

The normal way of making cathodes is by drawing tubing down to a very small size and then forming the cathode from a cut length of tubing. With the locked-seam cathode, a nickel strip is formed into the required cathode shape in one operation. Experience shows that the latter process results in a much more consistent product, both chemically and dimensionally, which in turn results in valves with much more consistent emission and reduced spreads in characteristics.

Cathode Coating

The source of electrons in a modern television valve is the substance forming the coating on the cathode. When the cathode is heated, this coating makes available a supply of electrons, the process being known as thermionic emission.

Substances which have been found particularly suitable for cathode coatings are the carbonates of barium, strontium and calcium, which are normally used in carefully balanced mixtures in the present-day valves. During the period of evacuation of the completed valve, the carbonates are decomposed to the oxides. This process is brought about by heating the cathodes to a high temperature, and further heat treatment after the valve has been sealed results in the partial reduction of these oxides to produce some free barium. Cathodes in this state are active and capable of producing the supply of electrons necessary in the television valve.

Trace Elements

To assist this process, use is made of carefully controlled amounts of trace elements which are introduced into the cathode tube at the raw-material stage. In some cases, however, these elements have side effects which have to be limited. For example, magnesium, which is among the most effective of these elements, is fairly volatile under the running conditions of a normal cathode, and results in deposits on the cooler part of the structure which can cause poor insulation between the electrodes of the valve. The higher the concentration of the element, the greater will be the amount of evaporation. The quantities of trace elements used should therefore not be greater than are absolutely necessary for the ready activation of the cathode coating.

Emissive Life of Cathodes

The emissive coating is very unstable chemically, and traces of gases, which are evolved from the other components during normal usage of the valve, tend to destroy the coating and thus shorten the emissive life of the cathode. To sustain the emissive properties, therefore, it is necessary to have a slow replacement of the free barium throughout the life of the valve. This continued reduction within the coating requires a higher concentration of trace elements than is ideal, with a consequent increase in the amount of evaporation of trace elements from the cathode. A very high standard of cleanliness of all components, including the cathode, is thus a prime requisite in order that good emissive life and good inter-electrode insulation can both be achieved.

Tubular Cathodes

Until recently, tubular cathodes have been used exclusively in indirectly heated valves. The manufacture of these entails the drawing of nickel tubing through diamond dies until the required diameter is achieved, and then pressing or forming the drawn tube into the appropriate shape.

The drawing process particularly requires the use of large quantities of lubricants, and these produce a high level of contamination. The lubricant becomes so deeply embedded in the nickel that intensive cleaning, including the removal of a layer of the surface of the tube by etching, does not restore fully the desired cleanliness of the surface.

A further process to which tubular cathodes are subjected is one of annealing. This can have the undesired effect of producing a layer of nickel near the surface of the tube in which the concentration of active trace elements is lower than in the body of the tube. Thus it will be seen that to counter the above effects and to achieve a satisfactory level of emission from tubular cathodes, a higher concentration of trace elements in the nickel of the cathode is required than is desirable from considerations of inter-electrode insulation.

Locked-Seam Cathodes

Mullard have recently introduced a method of manufacturing cathodes which does not involve annealing, drawing or forming. Instead of nickel tubing, the new method uses nickel strip. This strip is fed into an automatic machine which first cuts the strip into precise lengths. Each length is then folded into the required shape and the seam is locked by tightly interleaving the edges of the strip.

In the absence of the drawing process, heavy lubrication and successive annealing procedures are not required, and deeply embedded impurities and oxidation of the magnesium are avoided. Thus contamination of the surface of locked-seam cathodes is slight, washing in methyl alcohol is sufficient to produce a surface suitable for coating, and the concentration of trace elements used in the nickel strip can be lower than in the tubular cathodes. It is thus possible to maintain good emission life with a locked-seam cathode while at the same time reducing the deposits. This can result in better insulation, less grid hum modulation and smaller capacitance effects due to accumulation of deposits.

Cathode Dimensions

Because of the closeness of the grid and cathode in frame-grid valves, the thickness of the coated cathode must be controlled to an accuracy of one or two ten-thousandths of an inch. If the uncoated cathode is not manufactured to such narrow limits, it will be necessary to select the cathode into close-tolerance groups and to vary the thickness of coating applied to each group to produce final cathodes all having thicknesses Within the specified limits.

The shape and size of tubular cathodes are set by the processes of drawing and pressing which these cathodes undergo. The tubing is drawn to the required diameter, compressed lengthwise to produce the fixing rim and then partially flattened to give an approximately rectangular cross-section. The dimensions produced by these processes are controlled to within a thousandth of an inch, but are not sufficiently accurate for the cathode to be sprayed without first being selected into closer-tolerance groups.

The locked-seam process however, dispenses with drawing and pressing, and forms the cathodes by a series of accurately controlled folding operations. The precision achieved is such that selection before spraying is not necessary. The locked-seam technique thus obviates a troublesome process, and offers an important saving in production time.

Incorporation in Mullard Valves

Locked-seam cathodes are now used by Mullard in all their television frame-grid valves, and will eventually be incorporated in nearly all their television valves. The locked-seam cathode has resulted in better inter-electrode insulation in the valve and is also helping to produce a negligible failure rate from emission faults over the period of life expected in a television receiver under normal conditions of operation.

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