SUMMARY

The following summary covers the main points of this chapter. Study it to be sure you understand the material before you begin the next chapter.

MULTI-UNIT TUBES were developed to reduce the size of vacuum tube circuits. Incorporating more than one tube in the same envelope allowed the size of a vacuum tube circuit to be reduced considerably. While a single envelope may contain two or more tubes, these tubes are independent of each other.

MULTI-ELECTRODE TUBES were developed to extend the capability of conventional tubes. In some cases, a multi-element tube may contain up to seven grids. These types of tubes are normally classified by the number of grids they contain.

POWER PENTODES are used as current or power amplifiers. Power pentodes use in-line grid arrangements. In this manner, more electrons can reach the plate from the cathode. In effect, this lowers plate resistance and allows maximum conduction through the tube.

BEAM-POWER TUBES are also used as power amplifiers. In addition to the in-line grid arrangement, beam-power tubes use a set of negatively charged beam-forming plates. The beam-forming plates force electrons that would normally be deflected from the plate back into the electron steam and, thus, add to the number of electrons the tube can use for power amplification.

UHF TUBES are special-purpose tubes designed to operate at ultrahigh frequencies between 300 MHz and 3000 MHz with minimum effect from transit time limitations. Among these are acorn tubes, and doorknob tubes, lighthouse tubes, and oilcan tubes.

PLANAR TUBES have their plates and grids mounted parallel to each other. Because of their planar construction, they can handle large amounts of power at uhf frequencies.

GAS-FILLED TUBES contain a small amount of gas that ionizes and reduces the internal resistance of the tubes. Because of this, gas-filled tubes can handle relatively large amounts of power while maintaining a constant voltage drop across the tube.

COLD-CATHODE TUBES lack heaters or filaments and, therefore, do not use thermionic emission. Instead, a voltage potential applied across the tube causes the internal gas to ionize. Once ionization has occurred, the voltage drop across the tube remains constant, regardless of increased conduction.

The CRT is a special-purpose tube that has the unique ability to visually display electronic signals. The CRT uses the principles of electrostatic attraction, repulsion, and fluorescence. Because of its unique ability, the CRT makes up the heart of many types of test equipment that you will become familiar with during your career in electronics.