• Vacuum Tube Amplifier"Head-Fi Meet 2007 San Jose" is Copyrighted by Flickr user: Vincent? (Vincent Liu) under the Creative Commons Attribution license.
  
  

Description

• A vacuum tube consists of a sealed glass bulb that has had the air pumped out. The size of the tube varies from a few millimeters to about 60 cm tall. The glass is attached to an insulating plastic plug on the bottom, called the base. The tube contains a set of metal plates connected to pins that run through the base. The pins and base are designed to plug into a socket. They're keyed so the tube can be plugged in in only one orientation, preventing misconnection. The manufacturer prints an identifying part number on the glass or the base.
  
  

Diode

• The simplest tube, called a diode, has a metal plate and a wire filament. An electric current heats the filament so it emits electrons. The filament is called the cathode. The electrons flow through the vacuum and are collected on the plate, called the anode. This constitutes another electric current, separate from the one that heats the cathode. The electrons flow only one way. This action, called rectification, is necessary for converting alternating current (AC) power to direct current (DC).
  
  

Triode

• The triode tube builds on the diode, adding a metal grid between the cathode and anode. A negative voltage on the grid repels some of the electrons coming off the cathode, decreasing the current to the anode. If you add an AC signal to the negative grid voltage, the anode current will be an amplified version of it. Triodes allowed electronic amplification of signals, leading to developments in audio, radio and television.
  
  

X-Ray Tube

• An X-ray tube is similar to a diode; the anode is angled to direct radiation out of the tube and towards a target. These tubes operate at 20,000 volts or higher. The high voltages are necessary to produce X-rays. Higher voltages create more penetrating X-rays.
  
  

Other Tubes

• High-power tubes used in radio broadcasting use metal or ceramic instead of glass. The largest tubes available in 2009 can handle more than one megawatt of power.
  
  Other specialty tubes have been made using small amounts of different gases, including mercury vapor, hydrogen and helium. The gases change the tube's conduction characteristics, making it useful for high-power switching.