Electron Tubes Interview Questions and Answers:

1. What is an electron tube?

Ans. An electron tube is an electronic device in which conduction of electrons takes place in an evacuated or gas-filled enclosure.

2. What are vacuum tubes?

Ans. Vacuum tubes are those tubes in which gas is removed so that the residual gas pressure is very low (of the order of 10^-8 atmosphere).

3. How does an ac plate resistance of a diode differ from its dc plate resistance?

Ans. The resistance offered by a diode to alternating current is known as ac plate resistance while the resistance offered by a diode to direct current is known as dc plate resistance. AC plate resistance is defined as the ratio of a small change in plate voltage across a diode to the resulting change in plate current i.e. r_p = ΔE_p/ΔI_p. The dc plate resistance is defined as the ratio of dc plate voltage across the diode to the plate current at a given point. DC plate resistance is not constant but depends on the operating point on the plate characteristic.

4. Why is the control grid in a triode, placed nearer to the cathode than to plate?

Ans. Control grid, being nearer to the cathode than the plate, controls the plate current more effectively.

5. Why is triode cannot be used as a rectifier?

Ans. The triodes are usually designed for very small values of plate current and, therefore, cannot be used for current ranges for which diode rectifiers are usually designed. However, for low capacities triode can be used for rectification.

6. Why is a control grid of a tube always kept at negative potential?

Ans. Control grid of a tube is always kept at negative potential so that it may not draw current and consume power.

7. Pentode is better than tetrode. Why?

Ans. The pentode eliminates the distortion of the plate characteristics and retains all the advantages of tetrodes like low grid-to-plate capacitance, high amplification factor, and large power output.

8. What are the possible causes of failure of an electron tube?

Ans. The possible causes of failure of an electron tube arc loose elements (due to improper soldering), failure of filament (may occur due to excessive filament current or due to gradual molecule loss from filament wire) and gassy tube (due to leakage in the tube envelope).

9. How do gas-filled tubes differ from vacuum tubes?

Ans. A gas-filled tube is essentially a vacuum tube filled with a small amount of some inert gas (such as argon, neon, helium, krypton or xenon) at low pressure (varying from 10 mm of mercury to 50 mm of mercury).

10. What is a thyratron?

Ans. Thyratron is a hot cathode gas-filled triode. Inert gases used are argon, hydrogen, neon or mercury vapour. Before the tube fires, its characteristics arc similar to those of a vacuum triode. After firing of the tube the grid loses control over plate current completely and plate current is limited by the external resistance in the plate circuit i.e. the thyratron behaves like a diode.

11. Why the thyratron is usually filled with hydrogen?

Ans. The characteristics of hydrogen thyratron are stable and the devices are fast.

12. Why fast acting devices do not use mercury vapour?

Ans. The fast acting devices do not use mercury vapour as the mercury vapour has the disadvantage of characteristics varying with temperature and it takes time in deionization.