A thermionic valve (or vacuum tube) is a device that controls electric current flow in a high vacuum between electrodes to which a high voltage has been applied.

The original valves looked much like incandescent light bulbs, which is hardly surprising because it's how they were discovered.

In 1880, inventor William Joseph Hammer, noticed that the early bulbs blackened with age and William figured that electrons were 'boiling' off the filament, hitting the glass and turning it black. To prove his theory he made a special bulb with a third electrode, to which he could attach an instrument to measure the current. He reasoned that if the current flowed between the two ends of the filament, it would also flow to this third electrode - it worked. It was later realised that this could function as a diode (a device that passes current in one direction but not the other), and allowed the conversion of AC to DC.

The Triode valve was invented in 1906 by engineer Lee de Forrest by putting a metal grid between the cathode and the anode, allowing signal amplification. This kicked-off the electronic revolution, because it made all kinds of gadgets possible, from amplifiers to radios to transmitters and later to TVs and the Cathode Ray Tube, an earlier display screen technology used before LCDs and LEDs.


There are two main types of valve :-

• Directly heated is where the heated filament emits electrons towards the anode. These normally warmed up in a couple of seconds. The filament (cathode) to anode voltage was typically 50 ~ 150 volts DC. Because of the low voltage requirements, equipment using these types of valves could be run from batteries.
  
  
• Indirectly heated is where the cathode emits electrons when it is heated separately by a small heating filament. These normally took about 20 seconds to warm up. The filament was normally run from a 6.3 volt AC supply taken from a transformer. The cathode to anode voltage was typically 100 ~ 400 volts DC also taken from a transformer but rectified by an EZ80 double diode valve and smoothed by a capacitor-resistor combo.