MCQs in Vacuum Tubes Part II

This is the Multiples Choice Questions Part 2 of the Series in Vacuum Tubes as one of the Electronics Engineering topic. In Preparation for the ECE Board Exam make sure to expose yourself and familiarize in each and every questions compiled here taken from various sources including but not limited to past Board Exam Questions in Electronics Engineering field, Electronics Books, Journals and other Electronics References.

Multiple Choice Questions Topic Outline

• MCQs in Vacuum Tubes

The Two-Parts Series

Following is the list of multiple choice questions in this brand new series:

Vacuum Tubes MCQs

PART 1: MCQs from Number 1 – 50                        Answer key: PART I

PART 2: MCQs from Number 51 – 100                   Answer key: PART II

Continue Practice Exam Test Questions Part 2 of the Series

51. Which can be used for proper high frequency amplification?

• A. Triode
• B. Tetrode
• C. Pentode
• D. Diode

52. The indirectly heated cathode of the diode is coated with

• A. thoriated tungsten
• B. nickel
• C. carbon
• D. strontium or barium oxide

53. What started the conduction in a cold cathode tube?

• A. Thermionic emission
• B. Natural sources
• C. Secondary emission
• D. Thermal sources

54. Which emitter is most commonly used in the tubes of a radio receiver?

• A. Tungsten
• B. Oxide coated
• C. Thoriated tungsten
• D. Constantan

55. What is the real measure of valve’s amplification capability?

• A. Plate resistance
• B. Transconductance
• C. Amplification factor
• D. Gain

56. Field emission is utilized in the mechanism of

• A. vacuum tubes
• B. gas-filled tubes
• C. mercury pool devices
• D. TV picture tubes

57. A vacuum tube is normally operated in the temperature saturation region.

• A. To protect against filament aging
• B. To keep the tube envelope hot
• C. To disperse the space charge
• D. Keep the tube envelope cold

58. Plate saturation results when

• A. filament voltage is too high
• B. space-charge region is depleted
• C. plate temperature is too low
• D. space-charge region is saturated

59. When the control grid of a triode is operated with positive potential with respect to cathode

• A. the grid resistance decreases
• B. the grid may overheat
• C. the plate current decreases sharply
• D. the grid resistance increases

60. What is the solid state equivalent of cold cathode diode?

• A. Zener diode
• B. LED
• C. Varactor
• D. Photodiode

61. The nose in a gas-filled tube is ____ that of a vacuum tube.

• A. the same as
• B. less than
• C. more than
• D. very much smaller than

62. What is the phase difference of the output and input voltage of a grounded-cathode amplifier?

• A. 90°
• B. 360°
• C. 270°
• D. 180°

63. A grid controlled vacuum tube acts as

• A. an amplifier
• B. a controlled switch
• C. a rectifier
• D. an oscillator

64. The filament voltage is a direct measure of

• A. filament temperature
• B. amplification
• C. plate temperature
• D. filament resistance

65. The equation that defines the ac plate resistance of a vacuum tube?

• A. ∆E_b/∆E_c
• B. ∆I_b/∆E_c
• C. ∆E_b/∆E_b
• D. ∆E_c/∆I_b

66. The transconductance of a pentode is ______ a triode.

• A. more than that of
• B. about the same as for
• C. less than that of
• D. not comparable to that of

67. The electrons emitted by a thermionic emitter are called

• A. free electrons
• B. thermionic electrons
• C. loose electrons
• D. bound electrons

68. The unit of work function of metals.

• A. Joules
• B. Watt-hour
• C. Electron-volt
• D. Watt

69. What is the typical operating temperature of an oxide coated cathode?

• A. 750°
• B. 500°
• C. 1200°
• D. 1000°

70. What is the amount of additional energy required to emit an electron from the surface of a metal?

• A. Surface barrier
• B. Threshold level
• C. Work function
• D. Potential

71. Oxide coated emitters have electron emission of ______ per watt of heated power.

• A. 5-10 A
• B. 50-100 A
• C. 50-150 mA
• D. 150-1000 mA

72. What is the pentagrid converter?

• A. A tube with a total of five electrodes
• B. A tube with a total of five grids
• C. A tube that can be used for frequency conversion
• D. A tube that requires twice as such plate voltage as a single triode

73. Which emission is most widely used in practice?

• A. Field
• B. Secondary
• C. Thermionic
• D. Photo

74. What is the work function of an oxide coated cathode?

• A. 4.0 electron-volts
• B. 2.63 electron-volts
• C. 4.52 electron-volts
• D. electron-volts

75. A directly heated cathode has ____ warm-up time

• A. zero
• B. large
• C. small
• D. very large

76. Which thermionic emitter has the highest opening temperature?

• A. Oxide coated
• B. Tungsten
• C. Thoriated-tungsten
• D. Eureka

77. The internal resistance after ionization of a gas-filled tube is

• A. low
• B. very high
• C. high
• D. zero

78. One advantage of a mercury vapor diode over the high rectifier is

• A. its higher peak inverse voltage rating
• B. its reduced RF interference effect
• C. its lower voltage drop when the plate current is flowing
• D. the elimination of the need for a warm up period

79. The screen grid is used to

• A. increase the capacitance between the second grid and the plate
• B. decrease the capacitance between the control grid and the plate
• C. reduce the secondary emission effect
• D. lower the tube’s plate resistance

80. A tube tester is used to check a triode’s transconductance, which is the ratio of

• A. a small change in cathode current to the corresponding small change in grid current
• B. a small change in plate current to the corresponding small change in grid current
• C. a small change in plate voltage to the corresponding small change in plate current
• D. a small change in plate voltage to the corresponding small change in plate current

81. Which emitter is used in high voltage (>10 kV) applications?

• A. Tungsten
• B. Oxide coated
• C. Thoriated-tungsten
• D. Constantan

82. Which of the following is a desirable characteristic of an emitter?

• A. Large work function
• B. Small work function
• C. Very large work function
• D. Very small work function

83. 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

84. Secondary emission effects are undesirable in

• A. vacuum tubes
• B. gas-filled tubes
• C. ICs
• D. transistors

85. What would cause the plate current to increase in a pentode tube?

• A. A short circuit between the plate and the screen grid
• B. An open circuit in the lead that is connected to the
• C. A short circuit between the suppressor grid and the cathode
• D. A short circuit between the control grid and the cathode

86. In directly heated cathode, filament and cathode are

• A. separated components
• B. same components
• C. made of metals
• D. made of alloys

87. What is provided by transformer coupling?

• A. Impedance matching
• B. Step-up in voltage
• C. Good frequency response
• D. Stability of gain

88. What is one advantage of a pentode tube over a triode?

• A. Lower input impedance
• B. Lower output impedance
• C. Less noise internally generated
• D. Less control grid to plate capacitance

89. The load resistance R in a triode amplifier should be nearly _____ for good amplification.

• A. ½ r_p
• B. 3 r_p
• C. r_p
• D. 2 r_p

90. A voltage amplifier is designed to have

• A. high µ and R_L
• B. low µ and high R_L
• C. high r_p and low R_L
• D. high µ and low R_L

91. What transformer secondary voltage is utilized in a center-tap circuit?

• A. One-half
• B. One-third
• C. full
• D. One-eighth

92. Class C amplifiers are used as ____ amplifiers.

• A. audio-frequency voltage
• B. radio-frequency
• C. audio-frequency power
• D. audio-frequency current

93. The typical application of a cold cathode tube is as a

• A. diode
• B. tetrode
• C. triode
• D. pentode

94. Vacuum tube rectifiers are ____ affected by the changes in temperatures.

• A. not
• B. highly
• C. greatly
• D. severely

95. The internal resistance of a gas-filled tube is _____ that of a vacuum tube.

• A. the same as
• B. less than
• C. more than
• D. dependent

96. The ionization potential in a gas diode depends upon

• A. plate current
• B. size of the tube
• C. cathode construction
• D. type and pressure of gas

97. When the gas pressure in a gas-filled diode is increased, its PIV rating

• A. remains unchanged
• B. decreases
• C. increases
• D. becomes infinite

98. Ionization of cold cathode diode takes place at ______ plate potential compared to hot cathode gas diode.

• A. the same
• B. much lower
• C. much higher
• D. zero

99. A cold cathode diode is used as _____ tube.

• A. a rectifier
• B. a regulating
• C. a power-controlled
• D. an amplifying

100. For a conventional vacuum tube used in the UHF band

• A. The electron transit time becomes critical
• B. The distance between the control grid and the plate must be increased
• C. The physical size of the tube must be increased
• D. Only a pentode can be used because of noise effects

101. 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. gas current
• B. gas discharge
• C. plasma current
• D. plasma discharge

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