This internet site is likely to be viewed by many with a minimal understanding of the operation of the valve, therefore I believe it best to keep the following explanations rather basic. Those that already have a good understanding of valve theory will no doubt already have found suitable sources of further technical detail. I will give suggested onward research pointers at the end of this section.
Basic Operation Of A Valve.
When discussing valve matters often the term thermionic valve is used. This is because heat is an essential requirement to the operation of a valve in a radio receiver, and most other functions. The internal elements of a valve are made of a metal, and within that metal are electrons moving in circular paths in the conductive metal. These electrons have a negative charge. A valve has a glass (or sometimes metal) encasement, this envelope around the metal conductor can be used to create a vacuum in the space in the valve. If the metal is then heated to a sufficient level the speed of the electrons movement within the metal conductor is increases, until eventually the electrons shoot out of the conductor. If we now name that piece of metal the cathode, it can be said that the increase in temperature has caused the electrons to shoot out from the cathode, and return to the cathode again in due course. The heating of the cathode will be caused by the application of volts from a battery, or from another suitable power supply. The speed at which the electrons leave the cathode is determined by the heat of the metal, the hotter the metal - the faster the electrons will leave.
If another metal is introduced into the glass envelope, then a method of control of the electrons can be procured. This new piece of metal is called the anode. Another battery or suitable power supply is used, the positive side of the battery is connected to the anode metal, and the negative side of the battery can be connected to the cathode. Now when power is applied to the cathode, the negatively charged electrons fly off as before, but instead of eventually returning to the cathode, they are attracted to the positively charged anode. The higher the voltage of the anode, the more electrons having left the cathode will be attracted to it. Obviously the maximum number of electrons attracted to the anode is limited to the number leaving the cathode. It may be possible to further increase the number of electrons leaving the cathode by heating the metal to a higher temperature, through application of a higher voltage. This scenario too has a maximum rate of electron departure, also known as emission. The important thing to note is that the flow of electrons is in one direction only, namely from the cathode to the anode. For those electrons to return to the cathode they have to pass from the anode, out of the valve and through the battery or power source, then back into the valve onto the cathode. Therefore this device may be considered to be a simple valve that is able to pass current in one direction only, ie allowing current flow in one direction only. This device is known as a diode valve, and is the basis of detection when used in a radio circuit.
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triode valve on the next page
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