The Manufacture of C.R. TUBES

The following artical, originally published in the August 1952 issue of Practical Television magazine, gives an insight into the CRT manufacturing process of the time, though it does sound a bit like an E.M.I. advert !

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THE rapid growth of television in this country has made the cathode-ray tube almost as familiar an item to the general public as, say, the loudspeaker of a radio set.

The way in which these tubes are made, the host of involved and intricate processes necessary, and the complicated and ingenious machinery required in their manufacture provides a fascinating story.

A striking example of the application of modern mass production methods to the manufacture of cathode-ray tubes is to be found in the E.M.I, factories plant.

To follow the many processes entailed in the making of Emiscope tubes, an outline of the sequence of operations, and descriptions of some of the numerous items of automatic equipment installed in the plant, are given in the following notes, with appropriate illustrations of certain parts of the process.

Bulb and Neck Welding

The familiar glass envelopes of the tube arrive at the E.M.I, factory in two component parts : the screen and bulb, and the cylindrical neck. Skilled glassworkers operating specially designed lathes weld the parts together to make the complete tube envelopes.

It is worthy of note that in spite of supply difficulties only hard glass is used in the manufacture of these tubes. Hard glass, because of its high resistance to thermal shock, enables much more rigorous and exacting processing to be performed than is possible with normal glass.

Screen Settling

Once the complete tube envelope is assembled, the next step is to apply the fluorescent screen to the face of the tube, but before this is done it is of vital importance that the glass is suitably prepared to receive the fluorescent materials. This is achieved by shaking and rotating the tube so that a quantity of marble chips inserted in the bulb thoroughly cleanses the glass face.

Prior to the advent of the rotary shaking machine this extremely essential operation was performed manually. For the operators employed on this task it was a very strenuous and lengthy business. The tube had to be shaken and rotated at the same time so that the marble chips removed all contaminants from the glass face.

The E.M.I, rotary shaker now carries out this complex process automatically. Its ingenious mechanism not only faithfully reproduces the duplex shaking and rotary action necessary for efficient cleaning, but furthermore, at the pull of a lever the marble chips are ejected and the tube placed in position to receive the injections of acid and demineralised water which complete the operation. Before they are used again for the cleansing of other tubes the marble chips are thoroughly rewashed.

The decontaminated tubes are now almost ready to have the fluorescent screen deposited or " settled " on the glass face. This " settling " process is one of the most intricate in the manufacture of picture tubes and was originally carried out by hand. The process was a lengthy one requiring a high degree of skill, and in order to achieve a high output a large number of operators had to be employed on it. Recent expansions of the E.M.I, plant and the introduction of new manufacturing techniques have lead to the development of the three giant automatic screen settling conveyors now installed in the factory. They are the only machines of this type in Great Britain. Each measures about 60ft. long and 15ft. high and is attended by only three operators.

Before the tubes are settled they are given an initial rinse in a concentrated solution of caustic soda, followed by another in demineralised water. Overhead conveyors then carry them from the rinsing bays to the automatic screen settlers.

There they are placed on the moving conveyor belt which, as it travels, carries the tubes past the dispensing heads, where the correct amount of fluorescent screen material, (pre-mixed with demineralised water) is injected. The tubes travel slowly along to the other end of the conveyor to give the screen powder time to settle and then a film of nitro-cellulose is added, which settles on the surface of the demineralised water. (The nitro-cellulose is introduced to provide a suitably receptive surface for the aluminising film which is added later. It is removed during a subsequent stage of manufacture by a baking process.)

At either end of the platform is a large drum. As the tubes, now containing the screen deposit, pass round the circumference of the drum at the far end of the conveyor the demineralised water is automatically tipped out, leaving the nitro-cellulose to settle evenly on the fluorescent screen. After the liquid has been removed the tubes are dried by jets of warm air and are then taken off the conveyor ready for the next stage of manufacture, that of aluminising. One of the settling conveyors' has been fitted experimentally with an automatic drying conveyor, which runs in a pit beneath it. From. here the dried tubes are transported to, the aluminising bays.

The annealing process and aluminising table [21K] Tubes being prepared to receive the screen coating [17K]
The annealing process and
the circular aluminising table.
Tubes being prepared to receive
the screen coating.


The aluminising of C.R. tubes is a comparatively recent innovation, which E.M.I. were the first to perfect in this country. They have been manufacturing them on a production scale since 1949.

The aluminising process consists of depositing a microscopically thin film of aluminium (about 5 millionths of a centimere thick) on the face and bulb of the tube, and to spread this film evenly over the tube is a very delicate and refined operation, calling for great care and accuracy. This film acts as a "mirror" to the light produced on the fluorescent screen by the impact of the beam of electrons shot from the electron gun, and by reflecting it in a forward direction towards the viewer, and not allowing any to escape back into the tube, greatly increases the brightness of the picture. Further-more, whilst being substantially transparent to the light-producing electrons, the aluminium film effectively prevents any ions that may be present in the tube from reaching the screen and causing the well-known ion burn.

To accommodate the vast numbers of aluminised tubes now being turned out, an automatic rotary aluminising machine has been devised, capable of performing this operation on a mass-production basis. Three of these machines —they are the only ones of their type in the country—are now in operation in the E.M.I, factory and between them are capable of aluminising nearly 100 tubes an hour.

The rotary aluminiser consists of a series of head units, each fitted with a sillicone vacuum pump on a rotating circular platform. Tubes are loaded on to the aluminiser and a filament carrying an aluminium pellet of carefully pre-determined size and weight is inserted into each tube. As the circular platform rotates, the pumps evacuate all gases from the tubes until a vacuum is obtained. An electric current is then passed through the filament so that the aluminium pellets evaporate, and, due to the vacuum conditions inside the tubes, spread a microscopic film of aluminium evenly over the faces and bulbs of the tubes. Certain of the head units of one of the aluminisers have been specially equipped to provide for aluminising the neck of the tube in addition to the screen and bulb to enable the graphite coating, normally applied, to be dispensed with.

At this step it is necessary to remove the organic process materials, including the nitro-cellulose film, from the tubes. This is achieved by a rotary pre-bake oven, in which they are heated to a temperature of 450 deg. C.

Inserting the Electron Gun

The glass bulbs, complete with fluorescent screen and aluminising film are now ready to be fitted with the electron "gun" assembly, which shoots the beam of electrons on to the fluorescent screen. This done, the glass "pinch," through which the pins connected to the various gun electrodes protrude, is then sealed on, and the tube has almost assumed its final appearance.

Evacuation and Sealing

The next stages in the manufacture of Emiscope cathode-ray tubes—those of sealing the tubes on to the vacuum pumps, evacuating the tube envelopes, baking, degassing the electrodes and finally sealing up the tube —are all performed by a huge automatic pumping machine. This amazingly complicated apparatus consists of 28 trucks, each equipped to accommodate two tubes. These trucks travel round an oval-shaped track, and as they progress, skilled operators carry out their various processes.

The first of these processes is that of sealing the tubes on to the vacuum pumps so that the air can be evacuated from the envelopes. Then as the tubes travel slowly round the pumping operation takes place. While the pumping is in progress the tubes enter a baking oven where they are heated to a temperature of 500 deg. C. to remove gas from the glass bulb and walls. As the tubes emerge from the oven a high-frequency current from an eddy-current heater is applied to the modulator to outgas it effectively. No actual contact with the metal is made, the high-frequency current from the heater work coil being induced in it from outside the tube neck. It is interesting to note that the high-frequency heaters employed in both this operation and the getter firing process which follows are produced in the E.M.I. factories as part of their wide range of industrial electronic equipment. A cathode surface is then "formed" by passing a voltage through the tube heater.

The evacuated tubes are "sealed off" from the vacuum pumps and are then fitted with their anode connecting caps. E.M.I, have developed a novel method of performing this operation quickly, simply and with no risk of damage to the tube. Into the hollow anode cap is inserted a brickette of thermo-plastic conductive cement. The cap is then placed into a heating element which is attached to an electrically heated soldering iron, and applied over the anode contact protruding from the bulb of the tube. The heat causes the cement to adhere firmly to the glass and to the anode contact electrical connection between the cap and the contact being secured by means of the conducting properties of the cement.

Ageing and Testing

The finished cathode-ray tubes are " aged " and tested on yet another amazing piece of apparatus which might justifiably be mistaken by the laymen for a model railway. It consists of 55 self-contained test cabinets, each carrying a tube, which travels round on an oval-shaped track. By the time they reach the end of the track the ageing process is complete and the tubes are then meticulously tested for brightness, focus, degree of vacuum, deflection linearity, etc., to ensure that all tubes leaving the factory measure up to the exacting standards of quality and reliability required of Emiscope tubes.

Modulator outgassing process [9K] Tubes en route to the ageing and testing process [15K]
Modulator Outgassing process "Railway" carrying the tubes for
the ageing and testing process.

Page copyright ©
J.Evans 2003
Last updated
7th January 2003