How does a vacuum tube convert alternating current into direct current?

A vacuum tube converts alternating current (AC) into direct current through a process known as rectification. This is achieved using a two-electrode vacuum tube, often called a thermionic valve or diode, which contains a heated cathode and a positively charged anode.21

The conversion works based on the following principles:

• Electron Emission: When the cathode is heated, it emits electrons.12
• Unidirectional Flow: If a positive voltage is applied to the anode, the emitted electrons are attracted to it, allowing current to flow through the tube.12
• Blocking Reverse Current: If the polarity of the alternating voltage reverses and the anode becomes negatively charged, the electrons are repelled, and no current flows.12

Because the tube only conducts when the anode is positive, it effectively blocks one half of the AC cycle, resulting in a pulsating direct current. This ability to permit electron flow in only one direction makes the vacuum tube a fundamental rectifier in electronic circuits.32