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What is high power transmitting valves?

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January 11, 2008 5:06PM

A high power transmitting valve is used in a high power transmitter such as a short-wave radio broadcasting transmitter, or a TV transmitter or a radar transmitter etc. High power can mean from less than one kilowatt to hundreds of kilowatts. The 'valve' or 'tube' is a device to amplify electrical signals. Valves were invented at the beginning of the 20th century and were the first way of amplifying an electrical signal. They were the driving force in the development of all aspects of electronic engineering. In the second half of the 20th century transistors and later integrated circuits came to supplant valves. One application of valves that continues to this day is the high power amplification, of radio frequency signals. Although almost all medium wave (broadcast band) and medium power TV transmitters use transistors for amplification, valves are still ised in high power short wave transmitters, some TV transmitters and a special sort of valve is used in microwave transmitters including radar (and also the microwave cooker in a kitchen). In general, a valve comprises a cathode, one or more grids, and an anode all housed in a very high vacuum inside an insulating envelope. The principle of operation is that electrons are forced out of the cathode by heating it with an electrically heated filament rather like the filament in an incandescent lamp. The electron cloud around the cathode is attracted towards the anode by a high voltage connected between the anode (positive) and the cathode (negative). The flow of electrons from cathode to anode can be controlled by an electrostatic field from the grid(s). Thus a change of voltage on the grid will vary the flow of electrons (electric current) flowing from cathode to anode. The valve can be constructed so that a small variation of voltage on the grid can cause a large variation of current between cathode and anode - this is the basis of signal amplification. The electrical efficiency of a valve can be from a few percent up to more than 70% but if it is being used at high power even 70% efficiency will mean that there are high levels (for example 30% of 500 kW) of wasted energy. This wasted energy shows up as heat - so high power transmitting valves typically need some high power cooling systems. Typical cooling systems are air blast cooling from large air blowers and boiling water systems where the heat is used to boil water. It takes a lot of heat to boil a small quantity of water so the water cooling system can be made relatively small. The physical size of valves varies from the size of a pea to as large as a man. High power transmitter valves range in sise from a grapefruit to a beer barrel.