Teach Yourself Electricity and Electronics, 5th edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 29

1. In any tube, the cathode emits electrons. In a cathode-ray tube (CRT), the cathode is sometimes called the electron gun because it emits electrons at relatively high speeds. The correct choice is (d).

2. The plate of a vacuum tube corresponds to the collector of a bipolar transistor or the drain of an FET. Here, the correct choice is (a).

3. Mercury vapor was once used in gas-filled rectifier tubes. These days, such tubes are rare because of the toxic nature of mercury. The correct choice here is (b).

4. We can use a neon bulb, a specialized type of gas-filled two-electrode tube, as a functioning component in an audio-frequency (AF) relaxation oscillator. The correct choice is (c).

5. In a CRT, the anodes serve to accelerate the electrons, that is, to increase their speed. The correct choice is (a). The anodes can also focus the electrons so that they travel in a well-defined beam on their way through the deflecting fields to the screen.

6. One of the most significant advantages of an indirectly heated cathode (from a convenience standpoint, anyhow) is the fact that we can use AC to heat it up. The correct choice is (a).

7. Dark noise occurs in photomultiplier tubes designed to detect dim visible light, or to measure its intensity at low levels. This noise, caused by electrons that come from the photocathode even when no light strikes it, limits the maximum sensitivity (signal-to-noise ratio) that we can get from the device. The correct answer is (c).

8. Diode tubes were once commonly used as the rectifiers in high-voltage power supplies. The correct choice is (d). We'll still occasionally see diode tubes in that application, but semiconductor diodes have largely replaced them except in situations where the voltage is so high that semiconductor devices don't work well.

9. A tube with five grids has seven elements in total, counting the cathode and the plate. We call a seven-element tube a heptode, so the correct choice is (a).

10. A suppressor grid, inserted between the screen grid and the plate of a multigrid vacuum tube, reduces or eliminates secondary electron emissions from the plate. A tetrode has no suppressor, but a pentode does. Therefore, if we're experiencing problems that we can trace to secondary electrons, we can likely mitigate the trouble by switching to a pentode tube (with specifications similar to those of the existing tetrode, of course). The answer is (b). We would get no relief by doing action (a) alone. If we tried (c) or (d) we'd likely damage the tube, some external components, or both!

11. A pentode tube always has three grids: The control grid, the screen grid, and the suppressor grid. The correct choice is (c). A pentode might have either a directly heated cathode or an indirectly heated cathode, so neither (a) nor (b) works exclusively here. Dynodes are characteristic of photomultipliers, a category of tube that we don't normally think of in terms of the "triode-tetrode-pentode" paradigm, so (d) doesn't work.

12. Interelectrode capacitance limits the maximum frequency at which a tube can operate. It can also make a tube likely to oscillate in a circuit designed only for signal amplification, especially near the upper end of its usable frequency range. Interelectrode capacitance, all by itself, has nothing to do with UV radiation, excessive power gain, or excessive plate current, so none of the choices (a), (b), or (c) will work. That leaves us with (d), "None of the above," as the correct answer.

13. In a magnetron tube, a device designed to produce energy at UHF and microwave frequencies, the cavity size affects the output frequency. The correct choice is (a).

14. A voltage between the deflecting plates in an electrostatic CRT causes the electron beam to change direction. The correct choice is (a).

15. Certain gas-filled diode tubes exhibit constant voltage drop between the cathode and the anode regardless of the current. The correct choice is (c).

16. Image orthicon camera tubes exhibit fairly fast responses to dim image movement. The correct choice is (c). These days, we don't find camera tubes in video and imaging equipment very often (unless it's old!); solid-state cameras such as the charge-coupled device (CCD) have largely supplanted tubes in these applications.

17. If we apply an increasingly negative voltage to the control grid of a triode, tetrode, or pentode vacuum tube, the plate current decreases. Eventually, if we make the negative grid bias large enough, the plate current will drop to zero, and we'll have a condition of cutoff. The correct choice is (c).

18. Some pentode tubes contain an internal short circuit between the suppressor grid and the cathode. The correct choice is (a).

19. In an electromagnetic CRT, a current in the deflecting coils produces a magnetic field that bends the electron beam at right angles to the magnetic lines of flux. The correct choice is (c).

20. If we apply AC to the filament of a tube with a directly heated cathode, the fluctuating current in the filament can cause unwanted modulation of the output signal. The correct choice is (b). Filament overheating is no more likely with applied AC than with applied DC at the same effective (RMS) voltage, so (a) is wrong. Choices (c) and (d) are irrelevant. Magnetic fields and UV radiation can emanate from some vacuum tubes, but these phenomena rarely if ever pose a problem, and they'd no more likely result from filament DC than from filament AC.