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The hexode and heptode
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In some older radio and TV receivers, tubes with four or five grids were sometimes used. These tubes had six and seven elements, respectively, and were called hexode and
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544 Electron tubes heptode. The usual function of such tubes was signal mixing. The schematic symbol for a 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, because solid-state components are used for signal mixing nowadays. You might elicit a raised eyebrow if you talk about a pentagrid converter or a heptode mixer. And you can be sure an engineer is kidding you if (s)he mentions an octode or nonode.
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Vacuum 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, FETs and, ICs) can do everything that tubes ever could, with greater efficiency and using lower voltages and currents. Some electronics hobbyists like to work with antique radios. There is a certain charm in a broadcast receiver that takes up as much space as, and that weighs as much as, a small refrigerator. It brings back memories of a time when drama was broadcast on local AM stations, complete with whining heterodynes and static from summer thundershowers. The action was not rendered on high-resolution, color video, but instead was envisioned in listeners minds as they sat around the radio on a bare, varnished hardwood floor. Tube type, antique broadcast/shortwave receivers were about twice as bulky as necessary, because compactness was not a major concern. Such radios were 10 to 100 times bigger and heavier than their modern semiconductor counterparts. The high (and potentially lethal) voltages caused dust to accumulate via electrostatic precipitation from the air, giving the radio s innards a greasy, gritty film. If the above scenarios, possibilities, and hazards appeal to you, perhaps you d like to 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.
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Radio-frequency power amplifiers
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The most common application of vacuum tubes in modern technology is in RF amplifiers, especially at very high frequencies and/or power levels of more than 1kW. Two configurations are employed: grounded cathode and grounded grid.
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A simplified schematic diagram of a grounded-cathode RF power amplifier, using a pentode 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 C cannot be used. The input impedance of a grounded-cathode power amplifier is moderate; the plate impedance is high. Impedance matching between the amplifier and the load (usually an antenna) is obtained by tapping the coil of the output tuned circuit, or by using a transformer. In the example of Fig. 29-5, transformer output coupling is used.
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29-5 A grounded-cathode RF power amplifier.
Grounded grid
Grounded-cathode RF power amplifiers can (and sometimes do) oscillate, unless they are neutralized or provided with some negative feedback. The oscillation usually occurs at some frequency far removed from the operating frequency. Known as parasitic oscillation, it can rob the amplifier of output power at the desired frequency, as well as causing interference to communications on the frequency or frequencies of oscillation. Neutralization can be a tricky business. When the tube is replaced, the neutralizing circuit must be readjusted. The adjustment is rather critical. Improved stability, without the need for neutralization, can be obtained by grounding the grid, rather than the cathode, of the tube in an RF power amplifier. The grounded-grid configuration requires more driving power than the groundedcathode scheme. While a grounded-cathode amplifier might produce 1 kW of RF output for 10 W input, a grounded-grid amplifier needs about 50 W to 100 W of driving power to produce 1 kW of RF output. A simple grounded-grid RF power amplifier is shown in Fig. 29-6. The cathode input impedance is low, and the plate output impedance is high. The output impedance is matched by the same means as with the grounded-cathode arrangement. In Fig. 29-6, a transformer is used between the plate circuit and the load. Plate voltages in the circuits of Figs. 29-5 and 29-6 are given only as examples. The amplifiers shown would produce perhaps 75 W to 150 W of RF power output. An amplifier with 1 kW of RF output would have a plate voltage of 2 to 5 kV, depending on the tube characteristics and the class of amplifier operation. Typical grounded-grid linear amplifiers are operated in class-AB or B. If linearity is not important (as in CW, FSK, or FM operation), then class-C provides improved efficiency, although the driving power requirement increases, so that 100 W or even 200 W of RF input is necessary to get 1 kW of RF output.
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