Special purpose tubes are designed to operate or perform functions beyond the capabilities of the basic tube types discussed in prior tutorials. The special purpose tubes covered in this section will include multi-unit, multi-electrode, beam power, power pentode, variable-mu, uhf, cold cathode, thyratrons, and cathode-ray tubes.
Because of their great versatility, the four basic tube types (diode, triode, tetrode, and pentode) covered in previous tutorials have been used in the majority of electronic circuits. However, these types of tubes do have limits, size, frequency, and power handling capabilities.
MULTI-UNIT AND MULTI-ELECTRODE TUBES
One of the probems associated with electron tubes is that they are bulky. The size of an electron tube circuit can be decreased by enclosing more than one tube within a single envelope, as mentioned in a previous tutorial. There is a large variety of tubes that can be combined into this grouping of special purpose tubes called MULTI-UNIT tubes. The picture below illustrates the schematic symbols of a few of the possible combinations found in multi-unit tubes.
Typical multi-unit tube symbols.
An important point to remember when dealing with multi-unit tubes is that each unit is capable of operating as a separate tube. But, how it operates, either as a single tube or as a multi-unit tube, is determined by the external circuit wiring. When you analyze the schematic of a circuit, simply treat each portion of a multi-unit tube as a single tube, as shown in the next illustration below.
Multi-unit tube Identification.
Another type of special-purpose tube is the MULTI-ELECTRODE tube. In some applications, tubes require more than the three grids found in conventional tubes. In some cases, up to seven grids may be used. These types of tubes are called multi-electrode tubes and are normally classified according to the number of grids they contain. An example of this is illustrated below. Here, you see a tube with five grids; hence, its name is "pentagrid." The application of these tube types is beyond the scope of this module, but because multi-electrode tubes have been commonly used you should be aware of their existence.
Pentagrid multi-electrode tube.
Beam-power, power pentode tubes, and electron flow in a power pentode
Variale-mu-tubes, cutoff in a conventional tube, and grid operation in a remote cut-off tube.
Special-UHF-Tubes and limitations of transit time.
Gas-Filled-Tubes, electrical conduction in gas-diodes, and the gas-triode.
Cathode-ray-tubes and the electron gun
The CRT Screen