Construction

• The vacuum tube rectifier includes a heater that heats a cathode, or a self-heating cathode called a filament. The cathode is coated with a very small amount of Thorium or a coating that consists of barium and strontium oxides. The anode, or plate as it is commonly called, is the final element. In some rectifiers there are two separate plates which allow the vacuum tube to be configured as a full-wave rectifier.
  
  

Theory of Operation

• When voltage is applied to the filament or heater, it heats the cathode causing it to emit electrons. When the anode voltage becomes positive relative to the cathode, the tube begins to allow current to flow. When the voltage reverses and the anode voltage becomes negative relative to the cathode, the current flow stops. In essence the vacuum tube has become a rectifier, allowing current to flow in one direction only, just like a solid state device.
  
  

Other Configurations

• Vacuum tubes were also built with dual cathodes and a single anode for use in full wave rectifiers with a center tapped transformer. More complex tubes included two sets of cathodes and anodes so that multiple rectifiers could be configured with a single tube.
  
  

Continuing Applications

• Vacuum tubes are becoming the relics of the 20th century. Some high-voltage applications still utilize vacuum tubes but the cost of high-voltage solid state devices is becoming more competitive. One of the last groups to give up the vacuum tube ship is the guitarists. While solid state electronics have taken over most applications, vacuum tubes are still the device of choice for many guitar amps.
  
  

A Fabled History

• The vacuum tube rectifier has morphed through many shapes, sizes and applications since John Fleming, but the Krytron developed by EG&G during World War II was probably the most exotic. The Krytron was a very high-speed switch capable of switching thousands of volts and thousands of amps in radar applications during the war. Time magazine reported in 1985 that 8,100 Krytrons, which reportedly had applications in nuclear technology, had been illegally sold to Israel by a California engineer. None were recovered.