well blogger In electronics, a vacuum tube, electron tube (in North America), thermionic valve, or just valve (elsewhere, especially in Britain), is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often tubular in form. Many devices called vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes); as distinct from the hard vacuum type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence thermionic.

Vacuum tubes were critical to the development of electronics technology, which drove the expansion and commercialization of radio broadcasting, television, radar, high-fidelity sound reproduction, large telephone networks, modern types of digital computer, and industrial process control. Some of these applications pre-dated electronics, but it was electronics that made them widespread and practical; electronics has driven mechanical computers such as slide-rules to the point of obsolescence.

For most purposes, the vacuum tube has been replaced by solid-state semiconductor devices such as transistors and solid-state diodes: for most applications, they are smaller, more efficient, more reliable, and cheaper—either as discrete devices or as integrated circuits. However, tubes are still used in specialized applications: for engineering reasons, as in high-power radio frequency transmitters; or for their aesthetic appeal, as in modern audio amplification. Cathode ray tubes are still used as display devices in television sets, video monitors, and oscilloscopes, although they are being replaced at various rates by LCDs and other flat-panel displays. A specialized form of the electron tube, the magnetron, is the source of microwave energy in microwave ovens and some radar systems
for more inf check source http://en.wikipedia.org/wiki/Vacuum _tube

Answered by sanjay , an ibibo Guru, at 4:30 PM on May 26, 2008

well blogger01, Devices in which a stream of electrons is controlled by electric and magnetic fields have many applications in electronics. Because a vacuum must be provided in the form of an evacuated enclosure in which the electrons can move without collisions with gas molecules, these devices were called vacuum tubes or electron tubes in the US, and thermionic valves in Britain.
Vacuum tubes were critical to the development of electronics technology, which drove the expansion and commercialization of radio broadcasting, television, radar, high fidelity sound reproduction, large telephone networks, modern types of digital computer, and industrial process control. Some of these applications pre-dated electronics, but it was electronics that made them widespread and practical; electronics has driven mechanical computers such as slide-rules to the point of obsolescence.

Answered by Romi , an ibibo Master, at 6:23 PM on May 21, 2008

Blogger01,
Vacuum tubes are also known as Valves and previously before the solid state devices era, were used for rectification, voltage stabilisation, surge suppressors, pre amplifiers and power amplifiers etc. Even the All India Radio station and their transmitter and repeaters used it. It mainly consisted of Heater, Cathode, Anode and gates. The number of gates increased on the basis of whether it is Tetrode, Pentode or Heptode. Valves are still manufactured in limited quantity mainly in China, Chekoslovakia and Poland. The heater, Anode, Cathode, Gates were put in a vacuum tube of Glass and the connections coming out through bottom and top. Sometimes the MIL category had metal encapsulations. The manufacturers were RCA, Edisson, Marconi, BEL etc. I have seen German auto radio with miniature valves and portable radio with 22V battery and miniature valves. During 2nd world war, His Master's Voice introduced World Radio in India with Long Wave, Medium Wave, Short wave and thn followed by Kings Murphy and Phillips. All were with valves.Grundig had spool recorders which used valves. Still now Valves are popular with people who want quality reproduction of sound. Quad EL86 output stage is used in the output stage of hybrid amplifiers, using IC's as pre amplifiers. I have seen 60KW Pentode Valve being used for 10 KV high voltage generation for years without failure. For high Power valves used special liquid or forced air cooling. Some of the smaller valves are EL81, EF81, EF86, ELF81, ELF86, EL86 etc. Then the transistors came in from Mullard such as OC70, OC71 etc and the age of solid state electronics started.

Answered by kanchan , an ibibo Advisor, at 12:18 AM on May 21, 2008

they were used previously in place of transistors....now they are extinct.

Answered by mayank at 8:11 PM on May 20, 2008

In electronics, a vacuum tube (or thermionic valve outside North America) is a device generally used to amplify, or otherwise modify, a signal
a vacuum tube, electron tube (in North America), thermionic valve, or just valve (elsewhere, especially in Britain), is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often tubular in form. Many devices called vacuum tubes are filled with low-pressure gas

Vacuum tubes were critical to the development of electronics technology, which drove the expansion and commercialization of radio broadcasting, television, radar, high-fidelity sound reproduction, large telephone networks, modern types of digital computer, and industrial process control.

Answered by Om Prakash , an ibibo Master, at 6:30 PM on May 20, 2008

In electronics, a vacuum tube, electron tube (inside North America), thermionic valve, or just valve (elsewhere); is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often not tubular in form. Many devices called vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes); as distinct from the hard vacuum type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence thermionic.

Answered by Shobhit , an ibibo Master, at 6:23 PM on May 20, 2008

A vacuum tube (also called an electron tube or, in the UK, a valve ) is a device sometimes used to amplify electronic signals.
A vacuum tube consists of arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope. Although the envelope is classically glass, power tubes often use ceramic and metal. The electrodes are attached to leads which pass through the envelope via an airtight seal. On most tubes, the leads are designed to plug into a tube socket for easy replacement.

The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal plate inside the envelope, if the plate (also called the anode) is positively charged relative to the filament (or cathode). The result is a flow of electrons from filament to plate. This cannot work in the reverse direction because the plate is not heated and does not emit electrons. This very simple example described can thus be seen to operate as a diode: a device that conducts current only in one direction. The vacuum tube diode conducts conventional current from plate (anode) to the filament (cathode); this is the opposite direction to the flow of electrons (called electron current).

Vacuum tubes operate primarily on the function of the heat gradient difference between the hot cathode and the cold anode. Because of this operating requirement, vacuum tubes are inherently power-inefficient; enclosing the tube within a heat-retaining envelope of insulation would allow the entire tube to reach the same temperature, resulting in electron emission from the anode that would counter the normal one-way current flow. Because the tube requires a vacuum to operate, convection cooling of the anode is typically not possible. Instead anode cooling occurs primarily through black-body radiation and conduction of heat to the outer glass envelope via the anode mounting frame. Cold cathode tubes do exist but are used primarily in lighting systems, where unidirectional power regulation is not the functional purpose of the tube.[citations needed]

The vacuum tube is a voltage-controlled device, with the relationship between the input and output circuits determined by a transconductance function. The solid-state device most closely analogous to the vacuum tube is the JFET, although the vacuum tube typically operates at far higher voltage (and power) levels than the JFET.

Answered by jaivir , an ibibo Master, at 4:08 PM on May 20, 2008

In electronics, a vacuum tube, electron tube (inside North America), thermionic valve, or just valve (elsewhere); is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often not tubular in form. Many devices called vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes); as distinct from the hard vacuum type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence thermionic.

Answered by Nishu's , an ibibo Master, at 3:30 PM on May 20, 2008

Hello,


Devices in which a stream of electrons is controlled by electric and magnetic fields have many applications in electronics. Because a vacuum must be provided in the form of an evacuated enclosure in which the electrons can move without collisions with gas molecules, these devices were called vacuum tubes or electron tubes in the US, and thermionic valves in Britain.

The electrons move from the cathode (K), the negative electrode, to the anode or plate (P), the positive electrode. Conventional current is in the opposite direction. The electrons are liberated at the cathode by heat--thermionic emission--or as a result of bombardment by positive ions, which can cause emission of electrons or even heat the cathode the required amount for thermionic emission. All receiving tubes employ thermionic emission, though we will note certain examples of cold cathodes in special cases.

Answered by Sharad Singh , an ibibo Master, at 3:24 PM on May 20, 2008

hi blog a vacuum tube, electron tube (in North America), thermionic valve, or just valve (elsewhere, especially in Britain), is a device used to amplify, switch, otherwise modify, or create an electrical signal by controlling the movement of electrons in a low-pressure space, often tubular in form. Many devices called vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes); as distinct from the hard vacuum type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence thermionic.

Vacuum tubes were critical to the development of electronics technology, which drove the expansion and commercialization of radio broadcasting, television, radar, high fidelity sound reproduction, large telephone networks, modern types of digital computer, and industrial process control. Some of these applications pre-dated electronics, but it was electronics that made them widespread and practical; electronics has driven mechanical computers such as slide-rules to the point of obsolescence.

Answered by laks , an ibibo Master, at 3:17 PM on May 20, 2008