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True-to-type valves by new methods in Hammersmith factory.
When the Catkin valve made its sudden appearance in May last, and showed at a glance that there could be something new under the sun, it fully warranted the eager interest aroused on all sides. There had been achieved a radical departure from orthodox methods of manufacture; the MO Valve Co., producers of the new valve, had abandoned the traditional lamp form of construction and had struck out with a design which was not only original but sound.
Strangely new designs involve strangely new methods of manufacture, and it is with such methods that this article concerns itself. A visit to the Hammersmith works where Osram (Catkin) valves are made reveals a new world to the man familiar with the valve manufacturing methods of the last fifteen years.
The essential differences between the Catkin and the glass valve will bear repetition. They are as follows:-
- The abolition of a glass pinch and its substitution by a mica insulated steel clamp.
- The abolition of bent and welded wires in the electrode support system and substitution of strong, straight supports which are rigidly held in the steel clamp.
- The use of a new form of circular seal, round the circumference of which the leading in wires are spaced.
- The possibility of eliminating the glass bulb and substituting an air cooled anode forming the exterior envelope of the valve. (This principle has been adopted for many years with success on large transmitting valves dissipating the equivalent of many kilowatts in heat.)
- The abolition of capping paste and substitution of a rubber ring in at metal shell.
- The substitution of a ventilated metal outer container in place of the metallised bulb.
A complete set of components for an AC screen-grid Catkin.
Let us see how these various processes are carried out. The accompanying illustration shows the complete set of components for the AC screen-grid valve. The mixture of strontium and barium oxide normally used as an electron emitting material coated on the nickel-plated cathode in indirectly heated valves is improved upon in the Catkin. The cathode can thus he run at a lower temperature, allowing the use of larger inter-electrode clearances and leading to greater uniformity. An illustration opposite shows the cathodes being sprayed. The cathode is supported axially in the control grid by two mica insulators, the central holes of which locate the cathode, while the grid wires pass through other holes. The control grid, similar to that used in glass valves, is welded to straight support wires which, after passing through the holes in the mica, are slightly flattened to retain the mica in position.
Sectional photograph showing the rubber ring method of capping. The right-hand picture shows the control grid, cathode and heater assembly with steel-mica clamp.
Now comes the insertion of the tungsten heater inside the cathode. The heater is specially prepared steel and mica clamp is now pressed around the electrode assembly. This finished clamp is shown in the photograph. Copper-clad nickel-steel wires are welded to the electrode supports below this clamp, and we now have a complete electrode assembly for an MH4 triode valve, the unit being ready for insertion into the copper anode.
The construction does not vary very much in the case of different types. With the screen-grid valves VMS4 and MS4B the top mica fits inside the top of the screen while the bottom mica fits inside and abuts against the shoulder of the extended cylindrical skirt, thus rigidly determining the position of the grid cathode assembly within the screen. The screen itself is hexagonal, and has solid sides to give the minimum grid-anode capacity. Another mica insulator is new attached to the top of the screen and the completed assembly passes within the copper anode with its glass foot tube.
Whatever the type of valve this method of construction obviously ensures great accuracy in the registration of the various electrodes.
The copper anodes of Catkin valves are solid drawn from copper sheet, and the dimensions can thus be maintained within much closer limits than by the usual method of stamping and welding blanks on thin material. A large factor of safety at working temperatures is secured by reason of the fact that the copper is of sufficient thickness to withstand atmospheric pressure at the high temperatures used in the exhausting processes.
The only glass used in a Catkin valve is the glass foot tube employed to provide the necessary insulation at the lead-in wires.
How the cathodes are sprayed in Osram Catkin AC valves. Right: One of the jigs used to ensure accuracy of spacing in assembling the electrodes.
In joining the glass to the copper anode the open end of the anode is slightly flared and the edge tapered where it is joined to the glass; this thin edge and the end of the glass tube are made red hot in a gas flame and then brought together. In this way a join is formed which, after suitable annealing in a special electrical oven, is both vacuum tight and very strong. In spite of the differences in expansion of the glass and the copper, this join will stand large changes in temperature owing to the ductility of the thin copper flange. The electrode assembly is now inserted into the anode, and the open end of the glass tube is closed by sealing into it a specially shaped exhaust tube not unlike a mushroom. Such tubes are shown in the accompanying photograph.
Pumping and Seasoning
Preparatory to sealing off. Catkin valves entering the baking oven on a round table pump.
Copper anode, glass foot tube and mushroom-shaped exhaust tube are carefully gauged and graded to ensure accurate joining on the machines, which are quite novel in design and construction. The advantage of this annular seal is that the spacing between the lead-in wires is the maximum attainable for a valve of given diameter, and the insulation between them is thus perfect. Now follows the pumping process. Any evolution of gas during the life of a valve will lead to failure, and it is thus essential to remove all the occluded gases. To do this thoroughly each part of the valve must be heated to a much higher temperature during pumping than it attains while operating. Fortunately, however, the construction, with its air-cooled metal anode, permits of the application of much higher temperatures than can be applied with glass valves, thus making possible a higher degree of evacuation.
After sealing off from the pump the valve has to he capped before going through the seasoning and ageing processes. In a new method of capping evolved for the Catkin valve, the base is attached by means of a compressed rubber ring. Rubber has the property of adhering more and more firmly to the glass as time goes on. And this method removes the risk of loose bases. The rubber mounting also helps to protect the valve from external vibration and consequent microphonic noise.
An unshielded specimen with a metal shield ready for fitting.
The photograph above shows the method. The metal shell has a thin rubber ring held between the groove and the flanged top under which the valve is inserted in the press. A Bakelite insert carrying the usual pin arrangement is introduced at the other end of the shield, which is spun over so as to retain it permanently in position. When one of these pins forms the anode terminal, connection is made by a wire brought down inside the rubber from a metal ring gripping the anode. In the screen-grid valve, where the anode terminal is at the top, this takes the form of a brass cup which makes direct and permanent contact with the anode. In those valves in which the external metal container is omitted, the anode is coated with an external layer of black enamel, which is heat resisting and a perfect insulator. In operation the anode runs at a perfectly safe temperature with or without this enamel coating, but the coating, being black, naturally increases the cooling. The majority of valves, however, require the metal shield to provide electrostatic screening.
In the Catkin a metal shield is placed over the entire valve and spun on a base so as to fix it permanently. The sides of the shield are perforated to allow free circulation of air to the anode, and it is claimed that this shield gives more perfect screening than sprayed metal, while the sides, of course, protect the valve from damage.
Each valve is submitted to stringent tests for filament continuity and current anode current, mutual conductance, insulation, etc., and the feature of these tests is the uniformity of results which is inevitable by reason of the precise and rigid manner of construction.
If the listener is happy to have an unbreakable valve so is the packing department at Hammersmith. Elaborate packing is now unnecessary and the cartons can be much reduced in size. The set-maker is happy because no separate valve padding is needed, and, indeed, the set can be despatched complete with Catkin valves ready in their correct positions.
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