In a tube, the electron-emitting electrode is the cathode. The cathode is usually heated by means of a wire filament, similar to the glowing element in an incandescent bulb. The heat drives electrons from the cathode. The cathode of a tube is analogous to the source of an FET, or to the emitter of a bipolar transistor. The electron-collecting electrode is the anode, also called the plate. The plate is the tube counterpart of the drain of an FET or the collector of a bipolar transistor. In most tubes, intervening grids control the flow of electrons from the cathode to the plate. The grids are the counterparts of the gate of an FET or the base of a bipolar transistor.
Directly Heated Cathode
At A, schematic symbol for diode tube with directly heated cathode. At B, symbol for diode tube with indirectly heated cathode. At C, simplified rendition of the construction of a diode tube
In some tubes, the filament also serves as the cathode. This type of electrode is called a directly heated cathode. The negative supply voltage is applied directly to the filament. The filament voltage for most tubes is 6 V or 12 V dc. It is important that dc be used to heat the filament in this type of tube, because ac will tend to modulate the output. The schematic symbol for a diode tube with a directly heated cathode is shown in above figure A.
Indirectly Heated Cathode
In many types of tubes, the filament is enclosed within a cylindrical cathode, and the cathode gets hot from the IR radiated by the filament. This is an indirectly heated cathode. The filament normally receives 6 V or 12 V ac or dc. In an indirectly heated cathode arrangement, ac does not cause modulation problems, as it can with a directly heated cathode tube. The schematic symbol for a diode tube with an indirectly heated cathode is shown in above figure B.
Because the electron emission in a tube depends on the filament or heater, tubes need a certain amount of time to warm up before they can operate properly. This time can vary from a few seconds (for a small tube with a directly heated cathode) to a couple of minutes (for massive power-amplifier tubes with indirectly heated cathodes). The warm-up time for a large tube is about the same as the boot-up time for a personal computer.
In a gas-filled tube, the cathode does not have a filament to heat it. Such an electrode is called a cold cathode. Various chemical elements are used in gas-filled tubes. In fluorescent devices, neon, argon, and xenon are common. In gas-filled voltage-regulator (VR) tubes, mercury vapor is used. In a mercury-vapor VR tube, the warm-up period is the time needed for the elemental mercury, which is a liquid at room temperature, to vaporize (approximately 2 minutes).
The plate, or anode, of a tube is a cylinder concentric with the cathode and filament, as shown in above figure C. The plate is connected to the positive dc supply voltage. Tubes operate at plate voltages ranging from 50 V to more than 3 kV. These voltages are potentially lethal. Technicians unfamiliar with vacuum tubes should not attempt to service equipment that contains them. The output of a tube-type amplifier circuit is almost always taken from the plate circuit. The plate exhibits high impedance for signal output, similar to that of an FET.
The flow of current can be controlled by means of an electrode between the cathode and the plate. This electrode, the control grid (or simply the grid ), is a wire mesh or screen that lets electrons pass through. The grid impedes the flow of electrons if it is provided with a negative voltage relative to the cathode. The greater the negative grid bias, the more the grid obstructs the flow of electrons through the tube.
Schematic symbols for vacuum tubes with grids: triode (A), tetrode (B), pentode (C), hexode (D), and heptode (E).
A tube with one grid is a triode. The schematic symbol is shown at A in above figure. In this case the cathode is indirectly heated; the filament is not shown. This omission is standard in schematics showing tubes with indirectly heated cathodes. When the cathode is directly heated, the filament symbol serves as the cathode symbol. The control grid is usually biased with a negative dc voltage ranging from near 0 to as much as half the positive dc plate voltage.
A second grid can be added between the control grid and the plate. This is a spiral of wire or a coarse screen, and is called the screen grid or screen. This grid normally carries a positive dc voltage at 25 to 35 percent of the plate voltage. The screen grid reduces the capacitance between the control grid and plate, minimizing the tendency of a tube amplifier to oscillate. The screen grid can also serve as a second control grid, allowing two signals to be injected into a tube. This tube has four elements, and is known as atetrode. Its schematic symbol is shown at B in above figure.
The electrons in a tetrode can bombard the plate with such force that some of them bounce back, or knock other electrons from the plate. This so-called secondary emission can hinder tube performance and, at high power levels, cause screen current so high that the electrode is destroyed. This problem can be eliminated by placing another grid, called the suppressor grid or suppressor, between the screen and the plate. The suppressor repels secondary electrons emanating from the plate, preventing most of them from reaching the screen. The suppressor also reduces the capacitance between the control grid and the plate more than a screen grid by itself. Greater gain and stability are possible with a pentode, or tube with five elements, than with a tetrode or triode. The schematic symbol for a pentode is shown at C in above figure. The suppressor is often connected directly to the cathode.
Hexode and Heptode Tubes
In some older radio and TV receivers, tubes with four or five grids were sometimes used. These tubes had six and seven elements, respectively, and were called hexode and heptode. The usual function of such tubes was signal mixing. The schematic symbol for a hexode is shown at D in above figure; the symbol for a heptode is at E. You will not encounter hexodes and heptodes in modern electronics, because solid-state components are used for signal mixing.
In a vacuum tube, the cathode, grid(s), and plate exhibit interelectrode capacitance that is the primary limiting factor on the frequency range in which the device can produce gain. The interelectrode capacitance in a typical tube is a few picofarads. This is negligible at low frequencies, but at frequencies above approximately 30 MHz, it becomes a significant consideration. Vacuum tubes intended for use as RF amplifiers are designed to minimize this capacitance.