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A valve amplifier or tube amplifier is a type of electronic amplifier that makes use of vacuum tubes to increase the power and/or amplitude of a signal. They are typically (but not exclusively) use for sound or radio frequency signals.
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. Some special function 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.
Probably the most important member of the family of vacuum devices we refer to as "tubes" is the vacuum triode. This is the simplest vacuum device that is capable of amplification; i.e. providing an output signal greater than, and roughly proportional to, an applied input signal. In this respect it is functionally analogous to the ubiquitous transistor.
Physically, a tube operates by heating the "cathode", one of the electrodes inside the vacuumized envelope. This causes electrons in the cathode to attain enough velocity to leave the surface, forming a "space charge" around the cathode. The positively-charged "plate" (also sometimes called "anode"), usually surrounding and concentric with the cathode, attracts this space charge, causing an electron current to flow through the space between the cathode and plate. If the plate is negatively charged, the space charge is repelled and no current flows; this is how the vacuum diode works.
In what's called a vacuum triode, there is another electrode called the "grid", placed between the cathode and the plate, consisting of a spiral screen of fine wire. The grid is normally biased negatively, so that it does not act as a secondary plate and draw current. Small changes in voltage on the grid cause significantly greater changes in electron current through the tube from cathode to plate, resulting in amplification.
The vacuum inside the envelope must be as perfect, or "hard", as possible. Any gas atoms remaining might be ionized at operating voltages, and will conduct electricity between the elements in an uncontrolled manner. This can lead to erratic operation or even catastrophic destruction of the tube and associated circuitry. Unabsorbed free air sometimes ionizes and becomes visible as a pink-purple glow discharge between the tube elements.
The solar wind is a stream of charged particles—a plasma—ejected from the upper atmosphere of the sun. It consists mostly of electrons and protons with energies of about 1 keV. The stream of particules varies in temperature and speed during time. These particles are able to escape the sun's gravity, in part because of the high temperature of the corona, but also because of high kinetic energy that particles gain through a process that is not well-understood.