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The Cossor Type 3272 television cathode-ray tube.
One of the features which will make this year's exhibition outstanding is the prominence which will be given to television apparatus. Television receivers have previously been shown, it is true, but never before has equipment for high-definition television been shown by so many firms. It is, of course, early days in television as yet, and at the time of writing vision from the Alexandra Palace has not passed the stage of initial tests. This has not deterred research laboratories from developing receivers, however, so that it is to be anticipated that the start of a vision service will not find receivers far behind.
Two different models will be shown by Marconiphone, but the same vision equipment is used in each. The vision receiver is not a superheterodyne but a straight set embodying five stages of HF amplification, giving a total amplification of some 40,000 times. The circuits are pre-tuned to 45 MHz and the output is taken to a diode detector which feeds the vision signals to the cathode-ray tube and the synchronising impulses to the timebases. Two valve-oscillators are employed in this portion of the equipment and generate the necessary scanning voltages for actuating the cathode-ray tube. A special power unit for supplying the necessary operating voltages completes the vision receiver.
In the model 701 a picture of 10 × 8 inches is obtained, it is not viewed directly but through an inclined mirror, for the cathode-ray tube is mounted vertically. For sound reception, an all-wave receiver covering 16.7-53, 4.6-140, 185-560, and 750-2,250 metres, and having an output of 3 Watts, is fitted. In-order to receive the sound accompaniment to television on 41.5 MHz this receiver is employed and derives its input from the second stage of the vision receiver.
The Marconiphone Model 702 ultra-short-wave vision and sound receiver.
The model 702 has the same vision equipment, but the picture size is 9.5 by 8 inches and as in the 701, it is viewed through a mirror. The sound receiver in this model is intended purely for ultra-short-wave reception, and like the vision receiver is fixed tuned, but to 41.5 MHz instead of 45 MHz. It is a superheterodyne having a triode-hexode frequency-changer, one IF stage, a duo-diode-triode detector and LF amplifier and a pentode output valve.
Aerial equipment for ultra-short-wave reception is supplied with the receivers, and consists of wires cut to the correct length for a half-wave dipole, the necessary insulators, and a special screened cable for the connection to the receiver. Where the distance between the receiver and the aerial does not exceed some 50 ft lead-covered bell-wire can be used for the connection - but at greater separations a special high-frequency feeder cable is necessary. The same aerial is used for both sound and vision on ultra-short wavelengths, but in the case of the model 701 an ordinary aerial is required in addition for receiving on the longer wavelengths.
The HMV Model 900 combined television and all-wave sound receiver.
Two vision and sound receivers will be shown by HMV In the case of the Model 900 the sound receiver is an all-wave set covering 7-141 metres continuously in three ranges and 185-560 metres and 750-2,200 metres. The output stage delivers some 3 Watts to the loud speaker and six valves are used in this portion of the equipment. The vision receiver and synchronising gear have a total of fifteen valves, and there are two more in the power unit, The total number of valves is thus twenty-three. The cathode-ray tube has a diameter of 12 in, and is mounted vertically, the picture being viewed through a mirror. The model 901 is similar, but the sound receiver is for ultra-short waves only, both sets are pre-tuned and the total number of valves is twenty-two, there being four in the sound receiver.
The GEC will also be showing television equipment, and it is understood that in this apparatus the tube is mounted horizontally and viewed directly. This course is adopted in the Cossor apparatus, which has a cathode-ray tube having a diameter of about 13 in. A superheterodyne is used for both vision and sound, and the sound receiver also functions on the medium and long wavebands, so that ordinary broadcasting can be enjoyed. The 'Televisor' shown on the Bush Radio stand is also a superheterodyne employing a total of twenty valves. The cathode-ray tube is mounted vertically and covered by a sheet of safety glass, the picture, which has a size of 12 in by 9 in, is viewed through a mirror.
Cathode-Ray Tubes
The New Mullard television cathode-ray tube.
All receivers, whatever their make, are arranged for reception of either the 240 lines 25 frames of the Baird transmissions or the 405 interlaced lines 50 half-frames of the EMI system, the change being accomplished by means of a switch. In addition to the ordinary controls of the sound apparatus, the vision equipment is provided with brilliancy, contrast, and synchronising.controls.
In addition to complete apparatus, much subsidiary gear will be shown, chief among which are cathode-ray tubes. As well as their well-known older tubes, Ediswan will have a new cathode-ray tube for television purposes. This is the type 12H with a screen diameter of 30 cms. It has a heater consuming 1.5 Ampere at 2 Volts AC or DC, and its maximum anode potential is 6,000 Volts. Under normal operating conditions the 3rd anode requires 4,000 Volts, the 2nd 1,200 Volts, and the 1st 150-400 Volts, while about 100-250 Volts grid bias is needed. The maximum modulating voltage required is 20 Volts peak to peak.
Cossor have a wide range of tubes, the largest being the type 3272 with a diameter of 34 cms and requiring 4,500 Volts for its operation. It is of the three anode type, has a black-and-white screen, and costs 15 guineas. A smaller tube, the 3274, requires the same voltage and has a diameter of 25.4 cms; it is priced at 12 guineas. This firm will also have a tube for television transmission and high-speed photography with, a diameter of 13.3 cms; it requires 10,000 Volts.
The Mullard 42-12 television tube has a diameter of 30 cms and requires 3,000-5,000 Volts for the third anode with a potential of 650-1,000 Volts for-the second anode. The first anode needs only 250 Volts with about 80 Volts grid bias. The heater his rated at 4 Volts 1 Ampere, and the tube has a white screen.
In addition to television tubes, there will be many smaller cathode-ray tubes on view which are intended primarily for oscillographic purposes, although some of them can be used for television also. The Mullard 6001 tube is of this type, its operating conditions are similar to those of the 42-12, but it has a diameter of 22 cms and a green screen. Smaller tubes are the 42-G6 and 42-B6 with green and blue screens respectively. They have a diameter of 16 cms and are of the two-anode type requiring a maximum operating potential of 2,000 Volts. A tube of purely oscillographic interest is the 4002, which needs only 1,000 Vots. It has two anodes, a green screen, and a diameter of 9.5 cms.
The Cossor general purpose high vacuum tube is the 3276, with a diameter of 16.2 cms. It has three anodes and needs up to 4,000 Volts the screen is blue with an afterglow of less than 1 μS but it is also available with a sepia screen. It is priced at 8 guineas. Gas focused tubes will also be shown, including the 3237, which has a diameter of 4.5 in, requires 1,500 Volts, and costs £4 15s.
Osram will be showing gas-filled relays which in the smaller types find application in television and other timebases. The GT1 is an example of these. It is indirectly heated, has a control ratio of about 20, and is rated for a maximum anode current of 0.5 Ampere RMS, It is priced at 40s. Ediswan will also have relays of this type on view under the trade name of Thyratron.
Among apparatus for use in conjunction with cathode-ray tubes must be mentioned the Haynes, Radio timebase units. These are available in various forms, and one of them is a double timebase suitable for television purposes. It includes gas-filled relays, and an output of up, to 1,000 Volts is obtainable in push-pull. High voltage mains units with outputs up to 5,000 Volts for operating cathode-ray tubes will also be shown.
Cossor will have a timebase, using hard valves and with a frequency range of 0.25 Hz to 250 kHz and push-pull output power units for operating cathode-ray tubes will also be shown by both these firms.
Complete cathode-ray gear will also be shown by "Cossor including a linear oscillograph with a built in, timebase and mains unit. For use in conjunction with this there will also be a special ganging oscillator. This is in the form of a signal generator with a frequency-range of 90 kHz to 20 MHz and capable of being amplitude modulated at 400 Hz or by an external source. In addition, frequency modulation of ±15 kHZ is obtainable so that a picture of the resonance curve of the receiver under test can be seen in the cathode-ray tube. Other apparatus includes an engine indicator and a cardiograph.
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