Vacuum Tubes - 100 MCQs

Vacuum tubes were the primary electronic components before the invention of transistors. They are evacuated glass or metal containers used to control the flow of electrons.

Below are 100 multiple-choice questions (MCQs) categorized by tube types and fundamental principles.

Section 1: Fundamentals & Thermionic Emission

1. The process of electron emission from a heated surface is called:

   • A) Photoelectric emission

   • B) Secondary emission

   • C) Thermionic emission (Correct)

   • D) Field emission

2. The material most commonly used for the filament in a vacuum tube is:

   • A) Copper

   • B) Tungsten (Correct)

   • C) Aluminum

   • D) Silver

3. An oxide-coated emitter is used because it:

   • A) Has a high work function

   • B) Can withstand high voltage

   • C) Emits electrons at low temperatures (Correct)

   • D) Is very cheap

4. Which emitter has the longest life span?

   • A) Oxide-coated (Correct)

   • B) Thoriated-tungsten

   • C) Pure tungsten

   • D) Carbon

5. A "hard" vacuum tube refers to a tube with:

   • A) A metal shell

   • B) A very high vacuum (no gas) (Correct)

   • C) Gas-filled at high pressure

   • D) A ceramic base

6. The "work function" of a material is the energy required to:

   • A) Heat the filament

   • B) Move an electron from the surface (Correct)

   • C) Ionize a gas

   • D) Accelerate a proton

7. Directly heated cathodes have a warm-up time of:

   • A) Almost zero (Correct)

   • B) 10 seconds

   • C) 1 minute

   • D) 5 minutes

8. In an indirectly heated cathode, the heater is:

   • A) Part of the cathode

   • B) Electrically insulated from the cathode (Correct)

   • C) Connected to the plate

   • D) Connected to the grid

9. The most common shape for an indirectly heated cathode is:

   • A) A flat plate

   • B) A cylinder (Correct)

   • C) A wire loop

   • D) A sphere

10. The Richardson-Dushman equation relates:

    • A) Plate voltage to plate current

    • B) Temperature to emission current (Correct)

    • C) Grid voltage to gain

    • D) Vacuum level to pressure

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Section 2: Vacuum Diodes

11. A vacuum diode has how many electrodes?

    • A) 1

    • B) 2 (Correct)

    • C) 3

    • D) 4

12. The two principal electrodes in a diode are:

    • A) Cathode and Grid

    • B) Cathode and Plate (Correct)

    • C) Plate and Grid

    • D) Filament and Screen

13. A vacuum diode acts as a:

    • A) Bidirectional switch

    • B) Unidirectional switch (Correct)

    • C) Amplifier

    • D) Oscillator

14. In a vacuum diode, conduction occurs only when the plate is:

    • A) Negative with respect to the cathode

    • B) Positive with respect to cathode (Correct)

    • C) At zero potential

    • D) Grounded

15. The primary application of a vacuum diode is:

    • A) Amplification

    • B) Rectification (Correct)

    • C) Oscillation

    • D) Voltage regulation

16. Child’s Law (I = KV^{3/2}) defines the relationship in:

    • A) Saturation region

    • B) Space-charge limited region (Correct)

    • C) Temperature-limited region

    • D) Breakdown region

17. The "Space Charge" is a cloud of:

    • A) Protons near the plate

    • B) Electrons near the cathode (Correct)

    • C) Gas ions in the tube

    • D) Neutrons in the vacuum

18. The plate resistance (rp) of a vacuum diode is:

    • A) Delta Vp / Delta Ip (Correct)

    • B) Vp / Ip

    • C) Ip / Vp

    • D) Constant for all voltages

19. What happens to plate current when plate voltage is increased beyond the saturation point?

    • A) It increases linearly

    • B) It increases exponentially

    • C) It remains constant (Correct)

    • D) It drops to zero

20. A vacuum diode cannot be used as an:

    • A) Rectifier

    • B) Detector

    • C) Amplifier (Correct)

    • D) Switch

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Section 3: Vacuum Triodes

21. A triode has how many electrodes?

    • A) 2

    • B) 3 (Correct)

    • C) 4

    • D) 5

22. The third electrode added to a triode is called the:

    • A) Screen grid

    • B) Suppressor grid

    • C) Control grid (Correct)

    • D) Anode

23. The control grid is usually placed:

    • A) Between the cathode and the plate (Correct)

    • B) Outside the plate

    • C) Behind the cathode

    • D) On top of the plate

24. In normal operation, the control grid is maintained at a potential that is:

    • A) Positive with respect to the cathode

    • B) Negative with respect to cathode (Correct)

    • C) Equal to plate voltage

    • D) Higher than plate voltage

25. The amplification factor (μ) of a triode is defined as:

    • A) Delta Vp / Delta Vg at constant Ip (Correct)

    • B) Delta Ip / Delta Vg

    • C) Delta Vg / Delta Vp

    • D) rp ✖ gm

26. Transconductance (gm) is the ratio of:

    • A) Delta Ip to Delta Vg (Correct)

    • B) Delta Vp to Delta Ip

    • C) Delta Vp to Delta Vg

    • D) Vg to Ig

27. The relationship between μ, r🇵, and gm is:

    • A) μ = rp / gm

    • B) μ = rp  ✖  gm (Correct)

    • C) gm = μ  ✖  rp

    • D) rp = μ  ✖  gm

28. If the negative potential on the control grid is increased, the plate current:

    • A) Increases

    • B) Decreases (Correct)

    • C) Remains constant

    • D) Becomes infinite

29. "Cut-off" grid voltage is the voltage at which:

    • A) Plate current becomes zero (Correct)

    • B) Plate current is maximum

    • C) The tube explodes

    • D) The grid starts emitting electrons

30. Triodes are mainly used as:

    • A) Rectifiers

    • B) Voltage Amplifiers (Correct)

    • C) High-power regulators

    • D) Only in computers

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Section 4: Tetrodes and Pentodes

31. A Tetrode has how many electrodes?

    • A) 3

    • B) 4 (Correct)

    • C) 5

    • D) 6

32. The purpose of the "Screen Grid" in a tetrode is to:

    • A) Increase secondary emission

    • B) Reduce grid-to-plate capacitance (Correct)

    • C) Reduce heat

    • D) Protect the cathode

33. The negative resistance region in a tetrode characteristic is caused by:

    • A) Primary emission

    • B) Secondary emission (Correct)

    • C) Thermal noise

    • D) Gas ionization

34. A Pentode has how many electrodes?

    • A) 4

    • B) 5 (Correct)

    • C) 3

    • D) 7

35. The fifth electrode in a pentode is called the:

    • A) Screen grid

    • B) Control grid

    • C) Suppressor grid (Correct)

    • D) Accelerator grid

36. The suppressor grid is usually connected to the:

    • A) Plate

    • B) Control grid

    • C) Cathode (Correct)

    • D) Power supply positive

37. Which tube has the highest amplification factor?

    • A) Diode

    • B) Triode

    • C) Tetrode

    • D) Pentode (Correct)

38. The primary purpose of the suppressor grid is to:

    • A) Increase plate current

    • B) Eliminate secondary emission effects (Correct)

    • C) Cool the tube

    • D) Filter noise

39. A pentode is considered a:

    • A) Constant voltage device

    • B) Constant current device (Correct)

    • C) Linear resistor

    • D) Passive component

40. Which tube is least affected by the Miller effect?

    • A) Triode

    • B) Pentode (Correct)

    • C) Diode

    • D) Gas diode

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Section 5: Gas-Filled Tubes

41. A gas-filled tube contains a small amount of:

    • A) Oxygen

    • B) Inert gas or mercury vapor (Correct)

    • C) Carbon dioxide

    • D) Hydrogen

42. Ionization in a gas tube occurs when:

    • A) The heater is turned off

    • B) Electrons collide with gas atoms (Correct)

    • C) The vacuum is too high

    • D) The plate is negative

43. A gas-filled diode is also known as a:

    • A) Triode

    • B) Phanotron (Correct)

    • C) Thyratron

    • D) Pentode

44. The main advantage of a gas diode over a vacuum diode is:

    • A) Higher vacuum

    • B) Lower internal voltage drop (Correct)

    • C) Smaller size

    • D) No heater required

45. A Thyratron is a:

    • A) Gas-filled triode (Correct)

    • B) Vacuum pentode

    • C) High-speed transistor

    • D) Type of battery

46. Once a Thyratron fires, the grid:

    • A) Still controls the current

    • B) Loses control of the current (Correct)

    • C) Increases the current

    • D) Turns the tube off

47. Cold-cathode tubes operate based on:

    • A) Thermionic emission

    • B) Glow discharge (Correct)

    • C) Laser beams

    • D) Friction

48. A typical application of a cold-cathode gas tube is:

    • A) Audio amplifier

    • B) Voltage regulation (Correct)

    • C) Radio receiver

    • D) Microwave oven

49. The color of the glow in a gas tube depends on:

    • A) The plate voltage

    • B) The type of gas used (Correct)

    • C) The cathode material

    • D) The glass thickness

50. An Ignitron is a high-power gas tube that uses a:

    • A) Tungsten cathode

    • B) Mercury pool cathode (Correct)

    • C) Oxide cathode

    • D) Carbon grid

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Section 6: Applications & Characteristics

51. Which tube generates the least noise?

    • A) Pentode

    • B) Tetrode

    • C) Triode (Correct)

    • D) Hexode

52. A "Magic Eye" tube is used for:

    • A) Power rectification

    • B) Tuning indication (Correct)

    • C) X-ray production

    • D) Audio output

53. The "Inter-electrode capacitance" in a triode is highest between:

    • A) Cathode and Plate

    • B) Grid and Plate (Correct)

    • C) Heater and Cathode

    • D) Grid and Cathode

54. At very high frequencies (UHF), vacuum tubes fail because of:

    • A) Too much heat

    • B) Electron transit time (Correct)

    • C) Low voltage

    • D) Glass breakage

55. Which tube is used as a frequency converter?

    • A) Pentagrid converter (Correct)

    • B) Triode

    • C) Diode

    • D) Tetrode

56. A Klystron is a vacuum tube used for:

    • A) Audio amplification

    • B) Microwave frequencies (Correct)

    • C) DC regulation

    • D) Battery charging

57. The "Getter" in a vacuum tube is used to:

    • A) Connect the pins

    • B) Absorb residual gas (Correct)

    • C) Heat the cathode

    • D) Support the plate

58. Plate dissipation is the:

    • A) Power lost as heat at the plate (Correct)

    • B) Power used by the heater

    • C) Useful output power

    • D) Input signal power

59. Class C amplifiers using vacuum tubes are mostly used in:

    • A) High-fidelity audio

    • B) Radio transmitters (Correct)

    • C) Hearing aids

    • D) Small computers

60. The solid-state equivalent of a vacuum diode is:

    • A) BJT

    • B) PN Junction Diode (Correct)

    • C) MOSFET

    • D) Capacitor

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Section 7: Quick Review MCQs (61-100)

61. Cathode material for high voltage (X-rays): Tungsten.

62. Tube used in first-gen computers: Vacuum Tube/Valve.

63. Efficiency of a Class A tube amplifier: Low (approx 25%).

64. Effect of gas in a vacuum tube: Causes "gassy" tube/malfunction.

65. Voltage at the screen grid is usually: Slightly less than plate voltage.

66. Beam Power Tube: A modified tetrode with beam-forming plates.

67. Input impedance of a vacuum tube: Very high.

68. A "Duo-diode triode" contains: Two diodes and one triode in one envelope.

69. Secondary emission is: Electrons knocked off the plate by primary electrons.

70. Saturation current depends on: Cathode temperature.

71. Mutual conductance (gm) unit: mho or Siemens.

72. Plate resistance unit: Ohms.

73. Amplification factor ( μ ) unit: No unit (Dimensionless).

74. Flicker noise is prominent at : Low frequencies.

75. Transit time is the time taken by: Electron to travel from cathode to plate.

76. Vacuum tubes are "voltage-controlled" devices. (True)

77. Transistors are "current-controlled" devices. (True)

78. Plate current in a triode is controlled by: Grid voltage.

79. Total electrodes in a Heptode: 7.

80. Vacuum level in a standard tube: 10^{-6} to 10^{-8} mm of Hg.

81. Why use a suppressor grid? To prevent electrons from returning to the screen grid.

82. Does a vacuum tube work on DC or AC? Works on DC (signals can be AC).

83. A cathode ray tube (CRT) is a : Special type of vacuum tube.

84. X-ray tubes operate at : Very high voltages.

85. Filament voltage is usually: 6.3V or 12.6V (Standard).

86. Blue glow in a vacuum tube indicates: Presence of gas (Leaky tube).

87. Space charge limited current is independent of: Temperature (mostly).

88. Triode amplification is limited by: Inter-electrode capacitance.

89. Anode is another name for: Plate.

90. Can a triode be used as a rectifier? Yes, by grounding the grid.

91. Virtual cathode is: The point of minimum potential in the space charge.

92. Microphonics is: Noise caused by the mechanical vibration of tube elements.

93. Soft tube: A tube with some gas intentionally left inside.

94. Cathode Bias: Using a resistor in the cathode circuit to provide grid bias.

95. Push-pull circuit: Uses two tubes to increase power and reduce distortion.

96. Tetrode "kink": The dip in the Ip-Vp curve due to secondary emission.

97. Remote cut-off tube: A tube where the grid turns off the current gradually.

98. Nixie tube: A gas-filled tube used for numeric displays.

99. Magnetron: A tube used to generate microwaves (Radar/Ovens).

100. Vacuum tubes were replaced by: Transistors (Solid State).

Test your Knowledge

Q.1

Thermionic emission from oxide-coated cathodes takes place at a temperature of about ::

(a) 1000 degree C

(b) 2000 degree C

(c) 100 degree C

Q.2

Vacuum tubes are used, instead of solid state devices, where ::

(a) Very high power is handled

(b) very long lifetime is desirable

(c) very low frequency signal is involved

Q.3

The control grid of a triode is usually kept at a negative potential so as to ::

(a) make the grid current zero

(b) reduce the effect of space charge

(c) limit the plate current to safe values

Q.4

The output power of a triode amplifier is greater than the input power. This extra power ::

(a) comes from the input signal source

(b) comes from the d.c. Sources

(c) violate the principle of conservation of energy

Q.5

Temperature-limited current in a diode valve is given by ::

(a) Child-Langmuir Law

(b) Richardson's equation

(c) Ohm's Law

Q.6

Space charge limited current in a diode valve is given by ::

(a) Child-Langmuir Law

(b) Richardson's equation

(c) Ohm's Law

Q.7

gm of a triode valve can be increased by ::

(a) decreasing the cathode-grid spacing

(b) increasing the cathode-grid spacing

(c) decreasing the cathode size

Q.8

µ of a triode valve can be increased by ::

(a) increasing grid wire spacing

(b) increasing grid-plate spacing

(c) increasing the plate current

Q.9

The shape of the plate of a vacuum diode or triode is ::

(a) rectangular

(b) circular

(c) cylindrical

(d) straight

Q.10 

Schottky effect originates from ::

(a) thermionic emission

(b) photoemission

(c) secondary emission

(d) field emission

ANSWER

1.a,  2.a, 3.a, 4.b, 5.b, 6.a, 7.a, 8.b, 9.c, 10.d