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Valves at the 1953 Show.

Wireless World, October, 1953.
    
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Field Marshal Viscount Montgomery, who opened the Earls Court Radio Show, inspecting the sound reproducing equipment in the RIC Control Room. This year's show attracted 295,240 visitors - over 5,000 more than last year.

Most television set makers are now featuring the 17-in rectangular tube in their latest models, so there is no doubt that this size of tube has come to stay. Although there is some talk of a reversion towards the 9-in size (perhaps because owners of big-tube receivers are finding it difficult to pay for tube replacements), it seems fairly certain that the future trend of design will be towards even larger screen diameters. Cathodeon, for example, have just brought out a mammoth rectangular tube with a 27-in screen and a deflection angle of about 90 degrees. It has a tetrode gun and the anode voltage is about 18 kV. EMI are still producing the 21-in metal tube that they introduced two years ago, and we understand that one or two other manufacturers may be coming out with new tubes of this size.

Electron-gun construction of Brimar electrostatically focused CRT's.

A rather more down-to-earth design trend has been the reshaping of existing sizes of tubes so that they take up less space in the cabinet. More and more types are being made with wider deflection angles and consequently shorter lengths. Usually the wide deflection angle is about 70 degrees, but in one experimental 12-in tube made by GEC the reshaping process has been taken even further to give an angle of 90 degrees and a reduction in length of over 5 inches. The deflector coils have to be specially designed for such wide angle scanning, of course, but existing types of output valves can apparently be used in the time bases.

Rectangular tubes are another way of saving space in the cabinet - although Ediswan have pointed out that their new 14-in round tube CRM141 actually gives a picture of greater area than that of a 14-in rectangular. The secret is, of course, a little judicious corner-cutting. On the other hand, GEC take an entirely new line in introducing their 17-in rectangular tube, 7401A, by pointing out that it can be arranged to give a picture with nice square corners, like that of a projection receiver! Incidentally, the Ediswan CRM141 has an improved type of ion-trapping tetrode gun which not only stops the negative ions from getting to the fluorescent screen but also prevents the positive ones from bombarding the cathode and poisoning it. The secret of this is a slanting electrostatic lens formed between the first and final anodes.

Other manufacturers seem to be showing some interest in electrostatic methods of focusing the beam. Indeed, two recent tubes made by Brimar, the C14GM and C17GM, are entirely focused by electrostatic means - a reversion to the early days when all tubes were of this type. They use an extra electrode at near cathode potential inserted between the penultimate and final anodes. The main advantages of this system are its simplicity and avoidance of external focus magnets. Usually, the fineness of electrostatic focusing is about the same as magnetic focusing, but Brimar claim that their electrostatic tubes are actually better. They also say that the focus does not change so much with variations of final anode voltage. A similar electron optical principle is used by Mullard in one of their recent magnetically-focused tubes, the MW43-64, and here the object is to improve the uniformity of focusing over the whole screen. The pre-focusing action of the electrostatic lens formed by the extra electrode makes the beam narrower than in an ordinary tetrode, so there is less deflection de-focusing at the outer edges of the picture. Judging from recent American experience, however, it is questionable whether there will be a complete swing- over to electrostatic focusing. For one thing, the electrode structures of the tubes are rather more difficult and costly to manufacture than those of ordinary tubes.

It seems, too, that manufacturing problems are really at the heart of the old triode-versus-tetrode controversy, which has recently flared up again. Some firms have adopted the tetrode exclusively, while others insist that it is not really any better and continue to produce their latest tubes with triode guns. The main advantage of the tetrode would seem to be that the tube characteristics are not affected by variations in the final anode voltage, so that the picture brightness remains more constant. 0n the other hand, the triode is supposed to give a spot that is less prone to astigmatism, so that the definition of the picture may be somewhat better. It is often stated that because the electron beam of the tetrode is narrower than that of the triode it requires less focusing power and is not so liable to deflection de-focusing. Defenders of the triode, however, reply that there is no reason why its beam should not be made equally narrow. They also say that there is nothing to prevent ion traps from being fitted to triodes - although apparently this is not quite so easy to do as in tetrodes. The fact that the Americans use the tetrode exclusively is not particularly significant - they just didnt happen to think of the triode first.

With the cost of replacing cathode-ray tubes as high as it is at the moment, many people will be interested in a new English Electric scheme for reconditioning their 16-in metal tubes, which normally cost over £22. When a tube fails (beyond the 6 months guarantee period) it can be exchanged for a reconditioned one, carrying a new guarantee, for £12. The reconditioning process consists of cutting off the neck of the tube, fitting a new gun and renewing the fluorescent screen.

Developments in valves this year have not been very spectacular, but there have been some steady improvements in design, particularly in mechanical construction. It is said, in fact, that the technique of reliable valve production is having such a beneficial effect on ordinary valves that there will soon be no need to distinguish between the two kinds. One rather interesting example is to be found in the Osram television booster diode U329. A high heater-to-cathode resistance is needed for this type of valve, so the makers use a thin layer of vacuum - in other words, they space the two electrodes apart. The arrangement is claimed to be much more reliable than the usual insulating ceramics, which are rather prone to electrolytic action.

On the score of electrical efficiency the latest range of Mullard B7G battery valves are a notable achievement, for they have a filament consumption of only 25mA. This firm also have a new B9A pentode, the EF86, intended for audio voltage amplifying stages, which replaces their EF37A or EF40. Its mechanical construction has been designed to avoid microphonic tendencies, and the bi-filar heater reduces hum to a minimum. A similar valve from another manufacturer is the Osram Z729, notable for its low hum-level Of 1.5 μV.

Osram have also produced two high-slope B9A valves, primarily for television applications. The Z309 is a short-base RF pentode intended as a video amplifier and has a slope of 15 mA/V; while the Z719, designed as an RF or IF amplifier, has a slope of 7.4 mA/V. In the Z719 two cathode connections are provided to reduce input circuit damping, while the low anode voltage of 170 V makes the valve particularly suitable for transformerless receivers. For stabilizing the series heater current in these receivers against mains fluctuations, and thereby prolonging the life of the valves and tubes, this firm have introduced a barretter, type 305. With a control range of 40-90 V, it maintains the current within ±5 per cent of the nominal value.

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