Vacuum tubes

What is vacuum tube

Generally, vacuum refers to a space where charged particles such as electrons, protons, neutrons and all other matter are absent. In other words, vacuum is nothing but the empty space. 
Vacuum tube is an electronic device that controls the flow of electrons in a vacuum. It is also called as electron tube or valve. John Ambrose Fleming developed the first vacuum tube in 1904. Fleming’s diode allows the flow of electric current in only one direction (from cathode to anode) and blocks the electric current in another direction (from anode to cathode). In 1906, American electrical engineer Lee De Forest invented Audion vacuum tube. 
The invention of vacuum tubes has produced a new branch of engineering called electronics. In early days, vacuum tubes are used in television, radios, radar, electronic computers, and amplifiers. However, after the development of semiconductor devices, the usage of vacuum tubes in the electronic devices was reduced. Now-a-days, most of the electronic devices (computers, television, radar etc.) made from vacuum tubes are replaced by the semiconductor devices such as diodes, transistors, and integrated circuits.
Vacuum tubes are huge and occupy large amount of space. However, the construction and operation of vacuum tubes is easy to understand. Vacuum tubes are made from the materials such as glass and ceramics. Vacuum tubes are mostly depends on the thermionic emission process to emit the free electrons. In the thermionic process, heat is used to emit the free electrons. Vacuum tube that emits the free electrons by the application of heat is called thermionic valve or thermionic tube. 
A vacuum tube consists of cathode (also called as filament), anode (also called as plate), and electrode (also called as grid). Cathode is an electron emitter that emits the free electrons whereas anode is an electron collector that collects the free electrons. 
A vacuum tube consists of cathode (also called as filament), anode (also called as plate), and electrode (also called as grid).
Grid or electrode controls the electric current or flow of electrons between anode and cathode. The free electrons that are emitted by the cathode are attracted towards the anode or plate. These free electrons carry the electric current while moving from cathode to anode.

Directly heated and indirectly heated cathode

In the thermionic tubes, the cathode is heated electrically to a desired temperature to emit the free electrons from the metal surface. This can be done in two ways: by directly heating the cathode or indirectly heating the cathode. 
If the heat or heating electric current is passed directly to the cathode that emits the free electrons, the cathode is said to be a directly heated cathode or directly heated emitter. In the directly heated cathode, the cathode itself is the heating element or filament. Hence, the heat required to emit the free electrons from the metal surface is less compared to the indirectly heated cathode. 
Vacuum tube symbol for directly and indirectly heated cathode
If the heat or heating electric current is passed indirectly to a cathode that emits the free electrons, the cathode is said to be an indirectly heated cathode or indirectly heated emitter. 
In the indirectly heated cathode, there is no electrical connection between the cathode and the heater. Hence, the cathode itself is not a heating element. The heating electric current is passed through the heater or filament and the cathode is heated indirectly. Hence, the amount of heat required to emit the free electrons from the metal surface is more compared to the directly heated cathode.

Types of vacuum tubes

Vacuum tubes are generally classified into four types:

Advantages and disadvantages of vacuum tubes 

Advantages of vacuum tubes 

  1. Vacuum tubes are replaced easily. 
  2. Vacuum tubes can works at high temperature without any damage. 
  3. Vacuum tubes produce superior sound quality. 

Disadvantages of vacuum tubes

  1. Vacuum tubes are huge compared to the semiconductor devices such as diodes, transistors, and integrated circuits. 
  2. Vacuum tubes generate more heat. 
  3. High voltages are required to operate the vacuum tubes. 
  4. Vacuum tubes consume more power. 
  5. High cost.
  6.  Failure rate is high. 
  7. Vacuum tubes occupy more space than the transistors.