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Chemical Highlights of Tube Manufacturing

  1. HomeKnowledge BaseChemical Highlights of Tube Manufacturing

Chemical Highlights of Tube Manufacturing

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LOCK-IN TUBE PARTS. Illustrated are the basic parts to make a 7A7 tube. Bulb and base are not shown and the header on the right is already assembled in 10 parts.

Most people may not realize that the chemical purity of the various metals, insulators, and ceramic coatings used in vacuum tube manufacture is of the greatest importance in making tubes that will operate satisfactorily for hundreds of hours. A brief description of some of the chemical and physical considerations involved in vacuum tube design will explain how they function, and account for changes in characteristics that may take place over long periods of time.

The Emitter

Probably the most important part of a tube is the emitter. In all modern receiving tubes this is either a wire or a cathode sleeve coated with a mixture containing barium and strontium salts which when properly processed form the critical mixture of alkaline earth metals and their oxides, which are responsible for the actual emission. This coating is very susceptible to “poisons,” particularly sulphur and iron oxide. Very little of these in the base metal or in the spray will spoil the tube initially, and if the bulb or outer elements contain these “poisons” they eventually reach the cathode coating and impair tube performance or life. Platinum and its alloys were used early in the industry but eventually nickel of sufficient purity became available which together with a few nickel alloys for special tubes is almost universally used today.

The Grids

The materials used for the small grid wires are molybdenum or one of the various nickel-chrome-iron alloys. The side rods are generally nickel or copper. Emission from the grid would give undesired characteristics so that in tubes which may have the grids operating at a fairly high temperature it is necessary to “poison” the surface without using a material which would eventually drift over to the emitter. Manganese alloyed with the nickel is one such way, a carbon coating another, and for some types gold-plating has even been resorted to.

The Plates

The plates and larger sheet metal shields, beam confining plates, etc., have commonly been made of nickel because of its purity and ease of processing without introducing impurities. Recently, however, the high production schedules for tubes and the scarcity of nickel have forced the development of iron pure enough for the purpose. This is protected while in stock by nickel plating or a coating of specially purified oil which has to be removed before use by thorough washing with chemicals. With this material, the closest possible control of all manufacturing processes is even more necessary than before. The use of iron or nickel plated iron is still unsatisfactory for cathodes.

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HEATING THE PLATE ON THE EXHAUST MACHINE. The water-cooled copper coil shown around the tube carries the high frequency current to make the plate red hot.

The Heater

In Sylvania cathode type tubes the heater wire is usually tungsten. No other metal or alloy has yet been found having better characteristics for this application. The insulating coating used to keep the heater from shorting to the cathode sleeve is critical as to purity also, but for a different reason. In the exhaust process the high temperature of the tungsten will cause it to react chemically with all but a very few of the usual ceramic materials. Quartz was originally used but was discarded in favor of very finely ground aluminium oxide. This is generally mixed with a binder and the wire is passed through the mixture by machine and the coating is baked on. The binder is later broken down chemically and driven off, leaving a sintered coating of the oxides to form an insulating layer.

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WIRE COATING MACHINE. This machine is used for coating heater wire. The wire is drawn through the mixture of insulating oxides and the coating is baked on in passing through the electric oven.

Micas and Miscallaneous Parts

Before the war all Sylvania tubes used the best grade of India mica, but, now that this cannot be obtained in quantity, special procedures have been found that enable American and Brazilian micas to be used with equal satisfaction. For many tubes it is necessary to apply a roughening surface coat of an insulating spray in order to reduce the effect of metal vapors that may condense upon the mica surface. Getter tabs, hook wires, support rods, and connectors all have their own particular problems. The getter itself is an interesting chemical problem. In incandescent lamps, phosphorus is used as a getter because of its affinity for oxygen, but in tubes with coated emitters phosphorus is a poison, and besides, a more active material which will combine with gases other than oxygen is necessary. Magnesium was the first material used. It is well known as the material used for flash powder and photo flash lamps. A small piece of the metal is fastened on some part where it can be vaporized later. In recent years a better gettering action has been obtained by using a mixture of magnesium, aluminum and barium, the exact proportions varying with the requirements of the tube. The mixtures containing more barium give a darker color deposit and do not spread over the bulb as much as those with more magnesium. These getters of course must be kept stored in vacuum or they will be worthless and the tubes which have had the getter applied may only be allowed to stand a limited time before exhausting or the air will have destroyed the getter.

The Exhaust Process

The exhaust process is much more than just sucking the air out of a bulb, for that is just to prepare the ground, as it were, for the necessary chemical changes which will make it into an operating radio tube. After most of the air has been pumped out, the heaters are connected to something more than normal voltage. The binders are first broken down chemically into gases which are then pumped out. A good pump is necessary because if these gases are not removed as fast as released they will cause undesirable interactions to occur and might also cool and condense on the other parts to cause trouble later. The barium and strontium carbonates on the cathode coating, now free of binder, give off carbon dioxide and are converted into the oxides required for emission. About this time a high frequency oscillator is used to make the metal plates red and sometimes even white hot. To most people a piece of metal is “all metal” but to a tube man it is as full of air as a porous brick. Heating these parts red hot drives out the air, but it comes out so slowly that for some high voltage transmitting tubes the exhaust process may last several hours. Careful control of this heat is important because too much will vaporize the metal which would be deposited on the insulation causing leak-age and possibly noise. Following this, the heater coil is moved closer to the getter which flashes and deposits active material on the bulb to react with any gas subsequently released.

Ageing and Testing

When tubes are taken off the exhaust machine they are sometimes usable as they are, but the characteristics are not very stable. The ageing process consists of operating the tube at higher than normal current long enough to be sure that all the emitter coating is chemically changed and to permit gases and impurities still present to come out of hiding. An average time of about half an hour is required for this process in order to be sure that the whole surface of the cathode contains active electrons and that all traces of gas have been cleaned up by the still active getter. Tubes testing good after this are reasonably sure to give the required service, but regular tests are taken and put in operation at full rating for 1000 hours to assure high quality and to give warning in case the chemical or electrical properties of some material has changed.

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TYPICAL LIFE TEST RACK. This shows one of the many life test racks as used in the Salem receiving tube plant. Each rack has a capacity of 800 tubes of 16 different kinds.

This article originally appeared in Sylvania Electric Products Inc.’s “Sylvania News” Vol. 10, No. 12 February-March 1944

In This Section

  • Black Plate Tubes
  • Chemical Highlights of Tube Manufacturing
  • Cryogenic Treatment of Tubes: An Engineer’s Perspective
  • Developments in Trustworthy-Valve Techniques
  • Evolution of the Tube
  • Foil Those Tube Forgers
  • Microphonics
  • Mullard ECC83 (12AX7) Reissue vs Original – A Physical Comparison
  • Mullard ECC83 (12AX7) Reissue vs Original: An Electrical Comparison
  • Noise
  • Oxide Cathode Life: Investigations into the Causes of Loss of Emission
  • Secrets of the Tube Alchemists
  • Signal Tubes
  • Speed, Efficiency & Perfection – Aims That Have Built a Mammoth Factory in 16 Years
  • Subminiature Tubes: The Future of Audio!
  • That’s a Sylvania tube, the print is green, no, it’s blue
  • The ‘Magic Eye’
  • The ’12AT7′ Tube
  • The ’12AU7′ Tube
  • The ’12AX7′ Tube
  • The 12AX7 Tube: The Cornerstone of Guitar Tone
  • The 6SN7GT: The Best General-Purpose Dual Triode?
  • The Accurate BSPICE Tube Models
  • The Cool Sound of Tubes
  • The Inner Workings of Vacuum Tube Buffers
  • The Tube Family Tree – Part 1
  • The Tube Family Tree – Part 2
  • The Tube Family Tree – Part 3
  • Tube Vendors
  • Tubes: The Old Verses the New
  • Vacuum Tubes and Transistors Compared
  • Valve Microphony Part 1: Production of Microphony and Methods of Investigation
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