A vacuum tube is in a glass capsule of which the air has been extracted, and that takes in its interior metallic electrodes. A simple tube of two elements (diode) is conformed by a cathode and an anode, this last one connected to the positive terminal of a power supply. The cathode (a small metallic tube that is warmed up by means of a filament) releases electrons which they migrate towards him. If an alternating current voltage is applied to the anode, the electrons will flow towards the anode during the positive half-cycle; during the negative cycle of the alternating current voltage, the repelera anode the electrons, preventing that any current happens through tube. partial unloadings the connected diodes in such a way that they only allow the positive half-cycles of an AC voltage (CA) denominate rectifying tubes and are used in the conversion of the AC voltage to DC (CC). When inserting a grid, formed by a metallic thread in spiral, between the cathode and the anode, and applying a negative tension a this grid, is possible to manage to control the electron flow. If the grid is negative it repels, them and then only one small fraction of electrons emitted by the cathode can arrive at the anode. This type of tube, call triode, can be used like amplifier. The small variations of the tension that take place in the grid, like the generated ones by a sound or radio signal, can, consequently, generate great variations in the electron flow from the cathode towards the anode and in the system of circuits connected to the anode.