Some tubes are obsolete

Chapter Some tubes are obsolete

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • heptode. The usual function of such tubes was signal mixing. The schematic symbol fora hexode is shown at C in Fig. 29-4; the symbol for a heptode is illustrated at D.You’ll probably never hear about these devices in modern electronics, becausesolid-state components are used for signal mixing nowadays. You might elicit a raisedeyebrow if you talk about a “pentagrid converter” or a “heptode mixer.” And you can besure an engineer is kidding you if (s)he mentions an “octode” or “nonode.”Some tubes are obsoleteVacuum tubes aren’t used in receivers anymore, except for picture tubes in TV sets.This is because, at low-signal levels, solid-state components (bipolar transistors, FETsand, ICs) can do everything that tubes ever could, with greater efficiency and usinglower voltages and currents.Some electronics hobbyists like to work with antique radios. There is a certaincharm in a broadcast receiver that takes up as much space as, and that weighs as muchas, a small refrigerator. It brings back memories of a time when drama was broadcast onlocal AM stations, complete with whining heterodynes and static from summer thun-dershowers. The action was not rendered on high-resolution, color video, but insteadwas envisioned in listeners’ minds as they sat around the radio on a bare, varnishedhardwood floor.Tube type, antique broadcast/shortwave receivers were about twice as bulky asnecessary, because compactness was not a major concern. Such radios were 10 to 100times bigger and heavier than their modern semiconductor counterparts. The high (andpotentially lethal) voltages caused dust to accumulate via electrostatic precipitationfrom the air, giving the radio’s “innards” a greasy, gritty film.If the above scenarios, possibilities, and hazards appeal to you, perhaps you’d liketo collect and operate radio antiques, just as some people collect and drive vintage cars.But be aware that replacement receiving tubes are hard to find. When your relic breaks,you’ll need to become a spare-parts sleuth.Radio-frequency power amplifiersThe most common application of vacuum tubes in modern technology is in RF ampli-fiers, especially at very high frequencies and/or power levels of more than 1kW. Twoconfigurations are employed: grounded cathode and grounded grid.Grounded cathodeA simplified schematic diagram of a grounded-cathode RF power amplifier, using a pen-tode tube, is shown in Fig. 29-5. The output circuit is tuned to the operating frequency.The circuit can be operated in class-AB, B, or C. If the amplifier is to be linear, class Ccannot be used.The input impedance of a grounded-cathode power amplifier is moderate; the plateimpedance is high. Impedance matching between the amplifier and the load (usually anantenna) is obtained by tapping the coil of the output tuned circuit, or by using a trans-former. In the example of Fig. 29-5, transformer output coupling is used.544 Electron tubes