In this article, we published the vacuum tubes multiple choice answers for electrical and electronics engineers.
Vacuum Tubes Questions
The below list provides all the multiple-choice questions (MCQ) related to vacuum tubes.
Answers are available at the bottom of the article.
1. The cathode heating time of a thermionic glass diode is ______ that of a vacuum diode.
| A) | much less than |
| B) | much more than |
| C) | not related to |
| D) | the same as |
2. Which thermionic emitter has the highest opening temperature?
| A) | Eureka |
| B) | Tungsten |
| C) | Thoriated-tungsten |
| D) | Oxide coated |
3. The voltage on the suppressor grid of a pentode is generally
| A) | positive with respect to cathode |
| B) | zero with respect to cathode |
| C) | positive with respect to cathode |
| D) | at zero potential |
4. The internal resistance after ionization of a gas-filled tube is
| A) | zero |
| B) | high |
| C) | low |
| D) | very high |
5. Which valve has the lowest amplification factor?
| A) | Triode |
| B) | Tetrode |
| C) | Diode |
| D) | Pentode |
6. In directly heated cathode, filament and cathode are
| A) | made of alloys |
| B) | same components |
| C) | made of metals |
| D) | separated components |
7. Valves in a radio receiver generally employ ______heated cathodes.
| A) | nickel |
| B) | oxide |
| C) | indirectly |
| D) | directly |
8. A voltage amplifier is designed to have
| A) | high µ and low RL |
| B) | low µ and high RL |
| C) | high µ and RL |
| D) | high rp and low RL |
9. For a given plate voltage, if the negative potential on the control grid of the triode is increased, the plate current
| A) | remains the same |
| B) | decreases |
| C) | becomes zero |
| D) | increases |
10. What are the two principal electrodes in every tube?
| A) | Plate and control grid |
| B) | Cathode and screen grid |
| C) | Screen grid and control grid |
| D) | Plate and cathode |
11. The nose in a gas-filled tube is _____ that of a vacuum tube.
| A) | more than |
| B) | very much smaller than |
| C) | the same as |
| D) | less than |
12. The unit of work function of metals.
| A) | Watt-hour |
| B) | Electron-volt |
| C) | Joules |
| D) | Watt |
13. For a conventional vacuum tube used in the UHF band
| A) | The electron transit time becomes critical |
| B) | The physical size of the tube must be increased |
| C) | Only a pentode can be used because of noise effects |
| D) | The distance between the control grid and the plate must be increased |
14. A pentode is a ______ device.
| A) | bilateral |
| B) | constant voltage |
| C) | linear |
| D) | constant current |
15. What is the typical life span of an oxide-coated emitter?
| A) | 1000 hours |
| B) | 500 hours |
| C) | 200 hours |
| D) | 10000 hours |
16. When the gas pressure in a gas-filled diode is increased, its PIV rating
| A) | remains unchanged |
| B) | decreases |
| C) | becomes infinite |
| D) | increases |
17. The PIV of hot cathode gas diode is _______ the equivalent vacuum diode.
| A) | the same as that of |
| B) | more than |
| C) | independent that of |
| D) | less than |
18. The _______ voltage should be reduced to zero to stop conduction in a thyratron.
| A) | grid |
| B) | heater |
| C) | filament |
| D) | plate |
19. What is the real measure of valve’s amplification capability?
| A) | Amplification factor |
| B) | Transconductance |
| C) | Gain |
| D) | Plate resistance |
20. What operation results in severest distortion?
| A) | Class AB |
| B) | Class A |
| C) | Class C |
| D) | Class B |
21. What is provided by transformer coupling?
| A) | Good frequency response |
| B) | Impedance matching |
| C) | Step-up in voltage |
| D) | Stability of gain |
22. The grid to palate capacitance is least in ________ valve
| A) | diode |
| B) | tetrode |
| C) | triode |
| D) | pentode |
23. Field emission is utilized in the mechanism of
| A) | TV picture tubes |
| B) | gas-filled tubes |
| C) | vacuum tubes |
| D) | mercury pool devices |
24. Oxide coated emitters have electron emission of ________ per watt of heated power.
| A) | 5-10 A |
| B) | 150-1000 mA |
| C) | 50-150 mA |
| D) | 50-100 A |
25. The equation that defines the dc plate resistance of a vacuum tube.
| A) | Ib2 x Eb |
| B) | Ib / Eb |
| C) | Eb / Ib |
| D) | Ib x Eb |
26. The screen grid is used to
| A) | lower the tube’s plate resistance |
| B) | increase the capacitance between the second grid and the plate |
| C) | reduce the secondary emission effect |
| D) | decrease the capacitance between the control grid and the plate |
27. A hard tube is defined as a tube with
| A) | a metal envelope |
| B) | a gas in the envelope |
| C) | a tungsten filament |
| D) | no gas in the envelope |
28. Which of the following is a desirable characteristic of an emitter?
| A) | Large work function |
| B) | Very small work function |
| C) | Small work function |
| D) | Very large work function |
29. A thyratron can be used as
| A) | an attenuator |
| B) | a controlled switch |
| C) | an amplifier |
| D) | an oscillator |
30. Which provides the best frequency response?
| A) | RC coupling |
| B) | Impedance coupling |
| C) | Transformer coupling |
| D) | Direct coupling |
31. What is the pentagrid converter?
| A) | A tube that can be used for frequency conversion |
| B) | A tube with a total of five electrodes |
| C) | A tube with a total of five grids |
| D) | A tube that requires twice as such plate voltage as a single triode |
32. Which generates the least noise in operation?
| A) | Octode valve |
| B) | Pentode valve |
| C) | Triode valve |
| D) | Tetrode valve |
33. Which can be used for proper high frequency amplification?
| A) | Diode |
| B) | Pentode |
| C) | Triode |
| D) | Tetrode |
34. A directly heated cathode has ______ warm-up time
| A) | zero |
| B) | very large |
| C) | small |
| D) | large |
35. What started the conduction in a cold cathode tube?
| A) | Secondary emission |
| B) | Thermionic emission |
| C) | Thermal sources |
| D) | Natural sources |
36. The typical application of a cold cathode tube is as a
| A) | triode |
| B) | diode |
| C) | tetrode |
| D) | pentode |
37. A vacuum diode acts as a _______ switch.
| A) | controlled |
| B) | unidirectional |
| C) | omnidirectional |
| D) | bidirectional |
38. Which emitter is most commonly used in the tubes of a radio receiver?
| A) | Constantan |
| B) | Oxide coated |
| C) | Tungsten |
| D) | Thoriated tungsten |
39. The equation that defines the ac plate resistance of a vacuum tube?
| A) | ΔIb / Ec |
| B) | ΔEb / Ec |
| C) | ΔEb / Eb |
| D) | ΔEc / Ib |
40. A vacuum diode can be used as
| A) | an amplifier |
| B) | an oscillator |
| C) | a regulator |
| D) | a rectifier |
41. Direct coupling is used for _______ amplification.
| A) | audio-frequency |
| B) | ultra high frequency |
| C) | very low frequency |
| D) | radio-frequency |
42. Ionization current which is a positive-ion current produced by collision between electrons and residual gas molecules in an electron tube is also called
| A) | plasma discharge |
| B) | gas discharge |
| C) | gas current |
| D) | plasma current |
43. What would cause the plate current to increase in a pentode tube?
| A) | An open circuit in the lead that is connected to the |
| B) | A short circuit between the suppressor grid and the cathode |
| C) | A short circuit between the plate and the screen grid |
| D) | A short circuit between the control grid and the cathode |
44. Before ionization, a gas-filled tube has a ______ resistance.
| A) | zero |
| B) | very high |
| C) | very small |
| D) | small |
45. The peak inverse voltage of a diode is defined as the maximum allowable
| A) | positive voltage applied across the load resistor |
| B) | positive voltage to plate with respect to cathode |
| C) | negative voltage across the load resistor |
| D) | negative voltage applied to plate with respect to cathode |
46. Which tube generates the greatest noise?
| A) | Pentode |
| B) | Tetrode |
| C) | Triode |
| D) | Diode |
47. Plate saturation results when
| A) | space-charge region is depleted |
| B) | space-charge region is saturated |
| C) | filament voltage is too high |
| D) | plate temperature is too low |
48. What is the typical value of ac plate resistance for a triode?
| A) | 1000 KΩ |
| B) | 10 Ω |
| C) | 1000 Ω |
| D) | 100 KΩ |
49. Ionization of cold cathode diode takes place at ______ plate potential compared to hot cathode gas diode.
| A) | the same |
| B) | zero |
| C) | much higher |
| D) | much lower |
50. What transformer secondary voltage is utilized in a center-tap circuit?
| A) | One-third |
| B) | One-eighth |
| C) | One-half |
| D) | full |
51. The screen grid potential is kept _____ plate potential
| A) | somewhat lower than |
| B) | same as |
| C) | somewhere higher than |
| D) | at zero potential with respect to |
52. What is the typical operating temperature of an oxide-coated cathode?
| A) | 1000° |
| B) | 750° |
| C) | 500° |
| D) | 1200° |
53. A vacuum diode acts as a rectifier because of its _______ conduction.
| A) | omnidirectional |
| B) | unidirectional |
| C) | isotropic |
| D) | bidirectional |
54. Once a thyratron is fired, its control grid ______ over palate current.
| A) | exercises rough control |
| B) | becomes helpless |
| C) | loses all control |
| D) | exercises fine control |
55. What is the solid-state equivalent of thyratron?
| A) | FET |
| B) | UJT |
| C) | SCR |
| D) | BJT |
56. Vacuum tube rectifiers are ______ affected by the changes in temperatures.
| A) | greatly |
| B) | not |
| C) | highly |
| D) | severely |
57. A triode is normally operated with control grid at ______ potential with respect to cathode.
| A) | positive |
| B) | zero |
| C) | negative |
| D) | high positive |
58. The anode-cathode potential of a gas-filled tube at which gas de-ionizes and stops conduction is called ______ potential.
| A) | extinction |
| B) | striking |
| C) | ionizing |
| D) | reverse |
59. The internal resistance of a gas-filled tube is ______ that of a vacuum tube.
| A) | less than |
| B) | more than |
| C) | dependent |
| D) | the same as |
60. A vacuum tube is a ______ device.
| A) | linear |
| B) | bilateral |
| C) | exponential |
| D) | non-linear |
61. Which emission is most widely used in practice?
| A) | Photo |
| B) | Secondary |
| C) | Thermionic |
| D) | Field |
62. A vacuum diode can be used as
| A) | an oscillator |
| B) | a rectifier |
| C) | an amplifier |
| D) | a regulator |
63. Which of the following defines the amplification factor of a vacuum tube?
| A) | ΔEc/ΔEb |
| B) | ΔEb/ΔEc |
| C) | ΔIb/ΔEb |
| D) | ΔEb/ΔIc |
64. The electrons emitted by a thermionic emitter are called
| A) | free electrons |
| B) | loose electrons |
| C) | loose electrons |
| D) | thermionic electrons |
65. What is the unit of transconductance?
| A) | Ohm |
| B) | Siemen |
| C) | Volt |
| D) | Siemen/m |
66. A tube tester is used to check a triode’s transconductance, which is the ratio of
| A) | a small change in plate voltage to the corresponding small change in plate current |
| B) | a small change in cathode current to the corresponding small change in grid current |
| C) | a small change in plate current to the corresponding small change in grid current |
| D) | a small change in plate voltage to the corresponding small change in plate current |
67. The output stage of a practical amplifier always employs what coupling?
| A) | RC coupling |
| B) | Direct coupling |
| C) | Transformer coupling |
| D) | Impedance coupling |
68. Class C amplifiers are used as ______ amplifiers.
| A) | audio-frequency voltage |
| B) | radio-frequency |
| C) | audio-frequency power |
| D) | audio-frequency current |
69. A vacuum tube is normally operated in the temperature saturation region.
| A) | Keep the tube envelope cold |
| B) | To protect against filament aging |
| C) | To disperse the space charge |
| D) | To keep the tube envelope hot |
70. What is the work function of an oxide-coated cathode?
| A) | 2.63 electron-volts |
| B) | 4.0 electron-volts |
| C) | 4.52 electron-volts |
| D) | electron-volts |
71. When a thyratron tube has fired one thing that will cause it to stop conducting is
| A) | a more positive on the plate |
| B) | a more negative voltage on the control electrode |
| C) | a more positive voltage on the control electrode |
| D) | a negative voltage on the plate |
72. Saturation in a tube is a condition where an increase in plate voltage will produce
| A) | a decrease in electron emission |
| B) | no appreciable change in plate current |
| C) | an appreciable change in plate current |
| D) | a rise in electron emission |
73. The plate resistance of a tube is mainly due to
| A) | electrodes of the tube |
| B) | space charge |
| C) | vacuum in the tube |
| D) | gas in the tube |
74. What is one advantage of a pentode tube over a triode?
| A) | Lower output impedance |
| B) | Lower input impedance |
| C) | Less control grid to plate capacitance |
| D) | Less noise internally generated |
75. What is the solid-state equivalent of cold cathode diode?
| A) | Photodiode |
| B) | Zener diode |
| C) | Varactor |
| D) | LED |
76. Directly heated cathode require ______ amount of heating
| A) | small |
| B) | large |
| C) | very large |
| D) | very small |
77. The negative resistance characteristics of the tetrode is due to
| A) | plate being positive with respect to cathode |
| B) | secondary emission |
| C) | screen grid being negative with respect to cathode |
| D) | control grid being negative with respect to cathode |
78. What is the amount of additional energy required to emit an electron from the surface of a metal?
| A) | Potential |
| B) | Work function |
| C) | Threshold level |
| D) | Surface barrier |
79. A cold cathode diode is used as ______ tube.
| A) | a rectifier |
| B) | a regulating |
| C) | a power-controlled |
| D) | an amplifying |
80. A grid controlled vacuum tube acts as
| A) | a rectifier |
| B) | an amplifier |
| C) | an oscillator |
| D) | a controlled switch |
81. When the control grid of a triode is operated with positive potential with respect to the cathode
| A) | the grid resistance increases |
| B) | the grid may overheat |
| C) | the plate current decreases sharply |
| D) | the grid resistance decreases |
82. Which emitter is used in high voltage (>10 kV) applications?
| A) | Constantan |
| B) | Tungsten |
| C) | Thoriated-tungsten |
| D) | Oxide coated |
83. The ionization potential in a gas diode depends upon
| A) | cathode construction |
| B) | size of the tube |
| C) | plate current |
| D) | type and pressure of gas |
84. For the same plate voltage, a gas diode can conduct ______ the equivalent vacuum diode.
| A) | one-half the current than |
| B) | more current than |
| C) | same current as |
| D) | less current than |
85. What emitter is used in X-ray tubes?
| A) | Thoriated tungsten |
| B) | Nickel |
| C) | Tungsten |
| D) | Oxide-coated |
86. A vacuum tube will conduct only if its plate is _______ with respect to the cathode.
| A) | negative |
| B) | at zero potential |
| C) | positive |
| D) | at infinite potential |
87. Secondary emission effects are undesirable in
| A) | gas-filled tubes |
| B) | transistors |
| C) | vacuum tubes |
| D) | ICs |
88. Which is the best tube for high-frequency amplification?
| A) | Tetrode valve |
| B) | Diode valve |
| C) | Triode valve |
| D) | Pentode valve |
89. The indirectly heated cathode of the diode is coated with
| A) | carbon |
| B) | strontium or barium oxide |
| C) | nickel |
| D) | thoriated tungsten |
90. For faithful amplification, the control grid should be ______ with respect to cathode.
| A) | at infinite potential |
| B) | positive |
| C) | negative |
| D) | at zero potential |
91. Voltage amplifiers are operated as ______ amplifiers.
| A) | Class AB |
| B) | Class C |
| C) | Class B |
| D) | Class A |
92. For the sample plate dissipation, the output power of a Class B push-pull circuit is nearly ______ that of Class A operation.
| A) | 4 times |
| B) | 5 times |
| C) | 3 times |
| D) | 2 times |
93. The filament voltage is a direct measure of
| A) | filament resistance |
| B) | amplification |
| C) | filament temperature |
| D) | plate temperature |
94. The transconductance of a pentode is ________ a triode.
| A) | less than that of |
| B) | not comparable to that of |
| C) | about the same as for |
| D) | more than that of |
95. One advantage of a mercury vapor diode over the high rectifier is
| A) | the elimination of the need for a warm up period |
| B) | its higher peak inverse voltage rating |
| C) | its reduced RF interference effect |
| D) | its lower voltage drop when the plate current is flowing |
96. The load resistance R in a triode amplifier should be nearly ______ for good amplification.
| A) | 3rp |
| B) | 2rp |
| C) | 1/2 rp |
| D) | rp |
97. Which of the following would have the most effect on decreasing the life of a vacuum tube?
| A) | A grid current that is too low |
| B) | A plate resistance value that is too high |
| C) | Too much of a grid excitation |
| D) | An excessive filament voltage |
98. The actual voltage gain of a triode amplifier is less than µ due to
| A) | voltage drop in dc resistance of the tube |
| B) | voltage drop in ac resistance of the tube |
| C) | grid being negative with respect to cathode |
| D) | plate being positive with respect to cathode |
99. What is the typical plate efficiency of Class A amplifier?
| A) | 50 % |
| B) | 10 % |
| C) | 75 % |
| D) | 30 % |
100. What is the phase difference of the output and input voltage of a grounded-cathode amplifier?
| A) | 270° |
| B) | 360° |
| C) | 90° |
| D) | 180° |
101. When the temperature of an emitter is increased two times, the electron emission
| A) | increases two times |
| B) | increases four times |
| C) | Decreases two times |
| D) | increases several million times |
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