THE VACUUM TUBE

The filament is a heater that glows hot heating up the cathode. The cathode, when hot, emits electrons. The plate attracts electrons from the cathode when energized with voltage. The grid (also known as the valve) is in the path of electrons and controls the flow of electrons.

Amplifying Vacuum Tubes have been around since 1906. Although most functions of the Vacuum Tube have been replaced by solid state devices, the use of Vacuum Tubes in audio remains useful.

Working with vacuum tubes requires special care. Voltages used on audio tube plates can vary from 150VDC to 500VDC and are dangerous. Although the voltages are high, the current drawn by audio tubes used as voltage amplifiers such as a pre-amplifier or mixer amplifier is low. A single stage triode voltage amplifier draws about 4 milliamperes ( .004 amperes ) for the plate high voltage.
When working with high voltage care must be taken not to get too close to high voltage points. If you get close enough (less than 2 inches up to 500V), high voltage can jump out at you. Power amplifier tubes draw much higher current in the area of 200 milliamperes ( .2 amperes ). Extra care must be taken working with power amplifiers since the extra available current increases the risk of arcing.
The filament voltage is low but draws much higher current depending on the tube. Filament voltage is usually around 6.3 volts.

HEAT..... Tubes get hot, heat is required for electron flow. There is also an amount of heat loss dissipated while current is flowing through the tube. Voltage amplifiers used in pre-amplifiers or mixer amplifiers don't get very hot and touching them usually will not hurt you. Power amplifier tubes get very hot and should not be touched at all while operating. You can get a serious burn from an operating power tube. Any tube with a red glowing plate is hot, never touch any tube with a red glowing plate. A red glowing plate on a voltage amplifier tube means something is wrong. Power tubes may glow red slightly at full volume.

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 are called vacuum tubes or electron tubes.
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-- heating the cathode the required amount for thermionic emission. A 2-element vacuum tube, cathode and plate, is called a diode.

A third electrode, the grid, is placed between the cathode and the anode, closer to the cathode. It is made of a spiral of fine wire, so electrons can pass through without hindrance. When it is made negative, it opposes the effect of the anode in creating an electric field, but does not attract any electrons, and so draws no current (except for the positive-ion current mentioned above). If it is made sufficiently negative, it can cut off the plate current entirely. If it is made positive, it can enhance the plate current, but then draws some grid current itself. The grid provides a sensitive control, using negligible power, of the large plate current, so the vacuum tube is a powerful amplifying device. A 3-element vacuum tube, cathode, grid and plate, is called a triode.

In order to provide higher voltage gain, the plate resistance must be reduced. The plate resistance is the result of the effect of plate voltage on the space charge. This effect is not necessary for control, which is provided by the control grid, so what we need is to eliminate the effect of the plate voltage on the space charge. This is done by introducing another grid, the screen grid between the control grid and the plate. If this grid is held at a constant potential, the space charge is "screened" from the effects of changes in plate voltage. The resulting tube is called a screen-grid tetrode.

Although tetrodes worked as expected, they had a serious defect. It happens that speedy electrons colliding with the plate knock out secondary electrons. In a triode, these are rapidly sucked back to the positive plate, and the same happened in a tetrode when the plate potential was higher than the screen grid potential. In normal operation however, especially with large voltage gain, the plate voltage has a large swing, and can become less positive than the screen grid. Now all these secondary electrons (and some of the primary ones, too) are attracted to the screen grid, and there is a definite sag in the characteristic in this region. To prevent this, it is necessary to establish an electric field at the plate that is always directed toward the plate, to suppress the escape of secondary electrons. This is provided by a third grid, the suppressor grid, which is usually connected to the cathode. The tube with three grids: control, screen and suppressor is called a pentode.

Another kind of tube is the voltage regulator. The voltage across a glow discharge depends on the gas and the cathode material, and is almost idependent of the current through the discharge in the "normal glow" region, in which the glow does not completely cover the cathode, and expands to accommodate more current. Tubes were manufactured for voltages of 75, 90, 105 and 150 that were used like Zener diodes, handling up to 40 mA.