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Osram Miniature Valves

The following has been taken from an introductory chapter to an Osram Valves data book.
    
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Contents
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The Making of a Modern Miniature Valve.
The Glass 'Button'.
Electrode Preparation and Assembly.
Precision Tooling.
High Vacuum Technique.
Final Processes.

The Making of a Modern Miniature Valve.

Early valves were all hand-made. This was possible when the numbers needed were small and when comparatively wide tolerances could be allowed in the positioning and spacing of their electrodes, which, to modern eyes, seem of such large size and of such simple nature.

The valve of today, with it's often complex system of tiny parts, could not be made by hand. The degree of accuracy called for in the making and assembling of it's components is as high as that needed for building the finest of watches.

To produce valves of the highest grade, it is first necessary to design and build machinery of extreme precision, capable of handling small, delicate parts and of turning out finished work with a maximum variation in it's dimensions equal to less than half the thickness of a single sheet of the thinnest India paper.

Two examples of miniature construction may be given. The filaments of the latest types of battery valves are so fine that they are barely visible to the unaided eye; such filaments are coated with an electron-emissive material in such a way that every portion of their tiny surface is evenly covered and they must be mounted under exactly the right tension to ensure that the finished valve is not microphonic. The other example is the control grid of valves, which must be wound under closely regulated tension with a diameter which must not vary by more than one thousandth of an inch from the standard measurement.

The assembly of a typical modern valve will be described, but it is first worth while explaining why the Osram works lays so much stress upon precision in every stage of the making of the valve.

To give it's user good service, a valve must possess many qualities difficult to achieve. It must conform within close tolerances to the standard characteristics of it's type; it must remain consistent during a working life, with neither a softening of it's vacuum nor a serious falling off in emission; it must not be microphonic nor prone to other kinds of 'noisiness'; it must be reasonably robust and not easily damaged either by insertion into and removal from it's holder, or by the vibration and the minor shocks which may be expected to come it's way in normal use.

The main objects of all the research behind Osram valves and the precision methods employed in their manufacture are to provide the user with the best and most reliable valve available to perform any duty that he may require of it.

It is because of this policy that every valve that leaves the Osram works (and not just a percentage of each batch made) is subjected to two separate and comprehensive series of severe tests. The user is thus assured that each valve can be relied upon to give the best possible performance during it's working life.

The large, somewhat pear-shaped valve, with 'silvered' bulb and a moulded cap, is rapidly becoming obsolete. It's place is being taken in radio receiving and television sets, as well as in many other types of electronic apparatus, by the miniature, glass-based valve, which, besides being much smaller, has many other important advantages.

For example, the miniature valve has no cemented-on cap to work loose, and it's inter-electrode capacitances are in many cases considerably lower than in earlier types of valve.

The Glass 'Button'.

The first stage in the making of a valve of the B7G or miniature base types is the placing of the pins in a jig, with that maximum tolerance of one thousandth of an inch which is the basis of Osram valve manufacturing technique. This process is done by a machine.

The pins look simple enough, though they embody the results of a great deal of metallurgical research, undertaken to ensure that they give the user the best possible service. They must be of exactly the right tensile strength. What would happen if the valve was handled with too great a pressure on pins which were too soft needs no explanation. If, however, they were a little over-hard there would be a risk of cracking the glass base when the valve was pressed home in the socket.

Every pin must also be thoroughly cleaned because any dirt on the part inside the bulb might lead to faulty joints, with consequent poor performance, and 'noise'.

After the pins have been sealed in the base and cooling has taken place, a finished 'button' (a glass dish moulded exactly to size and containing seven or nine pins) is delivered from the machine.

Electrode Preparation and Assembly.

Other parts of the valve assembly are also made by precision machines.

The grids, for instance, are small spirals of fine wire, with the turns secured to vertical supporting wires. it has already been mentioned that any finished grid must comply with very close tolerances, but the story may not end there.

In mains operated power valves, the control grid may run at a high temperature and in such cases would itself emit electrons, which would cause a most undesirable flow of grid current, were not it's emissive qualities reduced by treating it, for example, by gold plating. Grid temperatures are even higher in television line-scan pentodes, in which that of the screen grid may reach 800°C. Even gold plating would be of no use here; but other new and very effective processes have been developed as the result of research.

The anode of a valve has to be treated in a special way by a carbonising process to ensure it's being able to dissipate sufficient heat to remain at a temperature consistent with efficient working.

The cathodes in mains valves (and the filaments in those of the battery type) must be subjected to very carefully controlled application of emissive material. It is essential, if the valve is to perform well and consistently, that the surface of each cathode should be evenly coated and that no area should be left uncovered.

The cathodes of indirectly heated miniature valves are very small. Several different shapes are used, each suited to the design of a particular type of valve. Inside the cathode is the heater, a minute coil of wire, suitably shaped for each particular cathode. The heaters must also be sprayed, in this case with a thin coating of material which has high insulating qualities at fairly high temperatures. Once more the interests of the user supplied the motives of the research to produce the best insulating substance and of the manufacturing methods which ensure that the coating is continuous and complete.

If the insulating was less effective and the application to the heater less carefully controlled the valve would have considerable 'hum'. Owing to the high quality of their heater-cathode insulation, Osram miniature mains valves can justly claim to achieve a very high standard in this respect.

Precision Tooling.

The electrodes are held firmly and securely in position by the insertion of there supports into holes of precisely the right size, and positioned to within one thousandth of an inch in two mica discs, known as the top and bottom micas.

To avoid any possible microphonic tendencies it is essential that the grid supports shall be rigid. For this reason the dimensions of the holes must be such as to produce what is known as an 'interference' fit for the supports.

The discs themselves are of mica, treated to ensure the highest possible degree of inter-electrode insulation. Very small leakages would lead to valve 'noises' which, however slight, would be uneconomical. Uneconomical? Yes, because the valve would be rejected and become a loss when subjected to it's 'passing out' tests. Nothing is allowed to impair the reputation of Osram valves for maintaining the highest standard of freedom from 'noise' which is practicable in a commercial product.

Throughout the various assembly processes minutely accurate jigs and gauges are used to ensure the exact positioning of every component part of the valve.

The electrode connecting wires are spot-welded to their respective pins. To ensure perfect joints of great strength and low resistance, each wire is cleaned by special methods and the spot-welding process is minutely controlled.

The electrode assembly is not yet quite complete. Above the control grid is placed a carbonised radiator, the purpose of which is to keep the working temperature of the grid reasonably low. To reduce the anode-grid capacitance each mica is provided with a metal screen connected to the cathode.

High Vacuum Technique.

There is something further to be done before the bulb can be placed over the electrode assembly and sealed to the button.

No matter what precautions are taken, considerable numbers of molecules of gas remain in the interstices of the metal parts. That there are innumerable such holes and crannies in even the most highly finished metal surface is difficult to realise until such a surface is seen under a high powered microscope. If nothing was done to remove them, the gasses would be liberated when the metal parts became hot and would impair the vacuum. To overcome this difficulty a substance known as a 'getter' forms one of the essential components of a valve. and a component of special form and location containing the getter is welded to the valve structure.

After completion of assembly and the meticulous inspection of every part the tightly fitting bulb with its long thin exhaust tube is pushed on and the valve is sealed onto the button.

Pumping is the next process. After the bulb has been sealed on and pumped, the valve is brought into an intense high frequency magnetic field, intense enough to induce heavy currents in its metal parts and make them become red hot. The heat drives out the trapped gasses and at the culminating point the barium getter volatilises. Just before the valve leaves the pump the upper part of the tube is sealed off, leaving the 'pip' at the top of the bulb.

Final Processes.

The valve has now it's final shape and general appearance; but much remains to be done before it can pass from factory to stock-room as a valve certain to give the user the best possible service.

It's next process is ageing, one which consists of (a) in developing the emission of the cathode or filament by operating them for carefully controlled times at temperatures (equally carefully controlled) in excess of those of ordinary working conditions; and (b) in running the valve for a sufficient period under conditions which have been found by research and long experience to ensure stability.

After it has been aged, the valve is ready to undergo the stringent structural, static and dynamic tests awaiting it.

Static tests follow, the number varying according to the type of valve. Any valve which does not conform within prescribed tolerances with the published characteristics of it's type is rejected and every individual Osram valve has to survive every test.

For the final 'noise' test the valve is placed in an instrument which feeds it's output, highly amplified, to a loudspeaker. It's bulb is now given a succession of sharp, quick raps with a rubber hammer and any response from the loudspeaker means rejection of the valve.

The valve now goes into stock; but when it is withdrawn from stock to fulfil an order, it's performance is not taken for granted. Far from it. The valve goes to a separate department for visual examination and for a complete retest.

Osram valves have built up through the years a reputation for consistent high quality; with the coming of the miniature, the steps taken to ensure maintenance of this reputation are even more vigorous. This becomes even more essential, as with the advent of television and complex electronic applications the numbers of valves in any one piece of apparatus may be considerable, and the prevention of failure due to a valve fault proportionately more important.

This was written in the early 1960's

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