Valves and Tubes
The 19 and 23-inch television CRTs are now in almost universal use, but the new bonded-face tubes are only used by a few manufacturers. Brimar had, side-by-side and displaying identical pictures, the three versions of the bonded-face tube using glass, Diakon and matt glass face-plates. A simple and direct comparison with a hand- lamp quickly demonstrated the reflection-free properties of the last type. The diagram shows the simple mounting arrangement.
Simple mounting in cabinet is a feature of bonded-faceplate television Tube. (Brimar).
Useful for stereo tape-recorder level indication and for FM use is the double EMM802 (Brimar) `magic eye'. This has two independent sections governing the illumination of the fluorescent stripe on the bulb. The lighted areas `grow' from the ends of the bulb towards the middle as bias is applied to the control grids so that, by adding a rudimentary scale, two voltages can be compared. Another Brimar valve, the double triode 13D7, is designed for use in low-level low-noise preamplifiers. It has an amplification factor of 140 and can provide an average grid-hum level of only 3 μV-slightly more than low-noise AF pentodes - with a total valve gain of over 5,000.
From Mullard comes the PY33 mains rectifier valve for television, a valve which produces 10 V higher output than its predecessors running under the same conditions. The necessary close spacing of anode and cathode is made possible by a technique giving a very smooth, hard cathode surface: this involves the `pressing' of the emissive materials, instead of the usual spraying or dipping. Another valve from Mullard is a triode-heptode, type ECH84, which is particularly suitable for sync separation in negative-modulation TV receivers. Noise pulses often rise well above the sync-pulse level of the signal and, with a normal separator, give spurious sync outputs and block off the proper pulses. One way of avoiding this is to use a dual-control valve, such as a heptode, the noise pulses being separated and used to cut off the sync-separator electron stream, so preventing the third grid, to which video and sync signals are applied, from drawing current and thus blocking the following sync pulses.
Thorn-AEI, who last year as Ediswan-Mazda introduced the first variable-μ television frequency-changer (30C17) this year showed a range of new valves for television including a cascode double-triode giving the highest gain achieved so far from a single valve of this type (30Ll7). Slope is 15 mA/V, using frame-grid construction, and to overcome instability an arrangement which harks back to early days has been adopted. Two cathode connections are necessary, thus a ten-pin base would be necessary to allow earthing of the screening separately from the earthed grid. To avoid the introduction of a new base the screening is connected to one heater pin which must be isolated and bypassed to RF New video-amplifier valves (6F28 and 30FL12) allow for 405/625 working as an anode load as low as 4.7 kΩ can be used while not dropping the peak-to-peak VF output below 100 V at 190 V HT.
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