Module 6 - Introduction to Electronic Emission, Tubes, and Power Supplies

Navy Electricity and Electronics Training Series (NEETS)
Chapter 1:  Pages 1-51 through 1-56

A TRIODE is basically a diode with a control grid mounted between the plate and the cathode. The control grid gives the triode the ability to amplify signals.

The Operation of a TRIODE depends on the ability of the control grid to either increase or decrease conduction through the tube in response to an ac input signal. The output voltage is developed across the tube between the cathode and plate because of the voltage drop across the plate-load resistor changing as the plate current responds to the input signal.

TUBE BIASING is the process of placing a dc voltage, usually negative, on the grid. Bias has several functions in circuit design. Biasing may be divided into two types: fixed and self. Tubes using fixed bias have a dc voltage applied to their control grids from an external source such as a battery. Self-

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biasing voltages, on the other hand, are derived from current conducting through the tube. The most common types of self-biasing are cathode biasing and grid-leak biasing.

The Class of Operation of an Amplifier is determined by the bias applied to a triode. An amplifier operating as class a conducts continually through the duration of the input cycle. Class AB operation occurs when the amplifier conducts for more than half but less than the entire duration of the input cycle. a class B amplifier conducts for only 50% of the input cycle. The class C amplifier conducts for less than half of the input cycle.

TRANSIT TIME is the time required for electrons emitted by the cathode to reach the plate. Because transit time in a vacuum tube is considerably less than the speed of light, vacuum tube operation is affected at high frequencies.

Interelectrode Capacitance is created by the naturally occurring capacitance between elements in a vacuum tube. One effect of interelectrode capacitance is to feed back a portion of the output

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of a triode to the input. This effect is a prime-limiting factor in applying triodes. It is a major reason why triodes are seldom used - especially at the higher frequencies.

MU and TRANSCONDUCTANCE are measures of tube efficiency. Mu (µ), or amplification factor, is a measure of the amount that plate voltage varies in relation to variation of the input voltage. Mathematically, mu (µ) is expressed as

TRANSCONDUCTANCE, on the other hand, is a measure of the amount of variation of plate current caused by a variation of the input signal. Mathematically, it is expressed as:

TETRODES were developed to compensate for the effects of interelectrode capacitance. Placing a positively charged screen grid between the control grid and plate has the effect of adding a capacitor in series with the capacitance that exists between the control grid and plate. This reduces total capacitance below the value of either capacitor as shown by applying the formula:

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SECONDARY EMIsSION of electrons from the plate is caused by the acceleration of electrons by the screen grid. This causes the performance of a tetrode to be degraded. In addition to reduced amplitude, the output signals become noisy.

PENTODES do not suffer from the effects of secondary emission. This is because a negatively charged suppression grid placed between the screen grid and plate forces any electrons emitted back to the plate.

Answers to Questions Q1. Through Q33.

A1.     By heating it.

A2.     Because the negatively charged electrons are attracted to the positively charged plate.

A3.     Filament and plate.

A4.     Negative.

A5.     Positive.

A6.     Pulsating dc.

A7.     Thoriated-tungsten and oxide-coated metals.

A8.     They reach operating temperatures quickly.

A9.     It serves as a mounting for the tube elements and as the terminal connection to the circuit.

A10.   The linear portion.

A11.    Plate resistant Rp.

A12.    Peak Inverse Voltage (PIV).

A13.    The triode contains a third element called the control rid.

A14.    Because it is closer to the cathode.

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A15.    a plate load resistor RL

A16.    To prevent them from drawing grid current.

A17.    The input signal

A18.    +275 volts.

A19.

    a.    100 volts.

    b.    180º out of phase.

A20.

    a.    Cutoff.

    b.    Saturation.

A21.    Cathode biasing.

A22.    Through the use of a bypass capacitor

A23.    Rkg, the cathode to grid resistance.

A24.    Unequal charge and discharge paths of the coupling capacitor Cc.

A25.

    a.    Class B.

    b.    Class C

    c.     Class A.

A26.    42.

A27.    340 volts.

A28.    The changes in plate current and grid voltage. A29.  240 volts.

A30.    The interelectrode capacitance (cpg) is divided between two series capacitances; thus, cpg is greatly reduced.

A31.    Secondary emission, and noise.

A32.     Secondary emission.

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A33.

    a.    Plate, positive.

    b.    Suppressor grid, negative.

    c.    Cathode, can be negative, positive, or at dc ground potential, depending on biasing type.

   d.    Control grid, negative.

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