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Tubes, Discrete Solid State Devices, and Integrated Circuits 99

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Tubes, Discrete Solid State Devices, and Integrated Circuits 99 Tubes, Discrete Solid State Devices, and Integrated Circuits 99 12.1 Tubes Filament Cathode Grid In 1883, Edison discovered that electrons flowed in an Beam Eye-tube evacuated lamp bulb from a heated filament to a sepa- Plate forming deflection rate electrode (the Edison effect). Fleming, making use plates plate of this principle, invented the Fleming Valve in 1905, but when DeForest, in 1907, inserted the grid, he Photo Cold Gas cathode opened the door to electronic amplification with the cathode filled Audion. The millions of vacuum tubes are an outgrowth Figure 12-1. Tube elements and their designation. of the principles set forth by these men.1 It was thought that with the invention of the tran- 12.1.2 Tube Types sistor and integrated circuits, that the tube would disap- pear from audio circuits. This has hardly been the case. There are many types of tubes, each used for a partic- Recently tubes have had a revival because some golden ular purpose. All tubes require a type of heater to permit ears like the smoothness and nature of the tube sound. the electrons to flow. Table12-2 defines the various The 1946 vintage 12AX7 is not dead and is still used types of tubes. today as are miniature tubes in condenser microphones and 6L6s in power amplifiers. It is interesting that many Table 12-2. The Eight Types of Vacuum Tubes feel that a 50 W tube amplifier sounds better than a diode A two-element tube consisting of a plate and a cath- 250 W solid state amplifier. For this reason, like the ode. Diodes are used for rectifying or controlling the phonograph, tubes are still discussed in this handbook. polarity of a signal as current can flow in one direction only. triode A three-element tube consisting of a cathode, a control grid, and a plate. This is the simplest type of tubes 12.1.1 Tube Elements used to amplify a signal. tetrode A four-element tube containing a cathode, a control Vacuum tubes consist of various elements or electrodes, grid, a screen grid, and a plate. It is frequently referred Table 12-1. The symbols for these elements are shown to as a screen-grid tube in Fig. 12-1. pentode A five-element tube containing a cathode, a control grid, a screen grid, a suppressor grid, and a plate Table 12-1. Vacuum Tube Elements and Their hexode A six-element tube consisting of a cathode, a control grid, a suppressor grid, a screen grid, an injector grid, Designation and a plate. filament The cathode in a directly heated tube that heats and heptode A seven-element tube consisting of a cathode, a con- emits electrons. A filament can also be a separate trol grid, four other grids, and a plate. coiled element used to heat the cathode in an indi- rectly heated tube. pentagrid A seven-element tube consisting of a cathode, five grids, and a plate. cathode The sleeve surrounding the heater that emits elec- trons. The surface of the cathode is coated with beam- A power-output tube having the advantage of both the barium oxide or thoriated tungsten to increase the power tetrode and pentode tubes. Beam-power tubes are emission of electrons. tube capable of handling relatively high levels of output power for application in the output stage of an audio plate The positive element in a tube and the element amplifier. The power-handling capabilities stem from from which the output signal is usually taken. It is the concentration of the plate-current electrons into also called an anode. beams of moving electrons. In the conventional tube control grid The spiral wire element placed between the plate the electrons flow from the cathode to the plate, but and cathode to which the input signal is generally they are not confined to a beam. In a beam-power tube applied. This element controls the flow of electrons the internal elements consist of a cathode, a control or current between the cathode and the plate. grid, a screen grid, and two beam-forming elements that are tied internally to the cathode element. The screen grid The element in a tetrode (four element) or pentode cathode is indirectly heated as in the conventional (five element) vacuum tube that is situated tube. between the control grid and the plate. The screen grid is maintained at a positive potential to reduce the capacitance existing between the plate and the control grid. It acts as an electrostatic shield and prevents self-oscillation and feedback within the 12.1.3 Symbols and Base Diagrams tube. suppressor The grid like element situated between the plate grid and screen in a tube to prevent secondary electrons Table 12-3 gives the basic symbols used for tube emitted by the plate from striking the screen grid. The suppressor is generally connected to the circuits. The basing diagrams for various types of ground or to the cathode circuit. vacuum tubes are shown in Fig. 12-2. 100 Chapter 12 Table 12-3. Tube Nomenclature 12.1.4 Transconductance C Coupling capacitor between stages Transconductance (gm) is the change in the value of Cg2 Screen grid bypass capacitor plate current expressed in microamperes (µA) divided Ck Cathode bypass capacitor by the signal voltage at the control grid of a tube, and is Ebb Supply voltage expressed by conductance. Conductance is the opposite Eff Plate efficiency of resistance, and the name mho (ohm spelled back- Ep Actual voltage at plate ward) was adopted for this unit of measurement. Siemens (S) have been adopted as the SI standard for Esg Actual voltage at screen grid E Output voltage conductance and are currently replacing mhos in o measurement. E Signal voltage at input sig The basic mho or siemen are too large for practical Eg Voltage at control grid usage; therefore, the term micromho (µmho) and micro- Ef Filament or heater voltage siemens (µS) is used. One micromho is equal to one- If Filament or heater current millionth of a mho. I Plate current p The transconductance (gm) of a tube in µmhos may Ik Cathode current be found with the equation I Screen-grid current sg ∆I I Average plate current p pa gm = ------------- Ebb held constant (12-1) ∆Esig Ipac Average ac plate current Ika Average cathode current where, ∆I is the change of plate current, Isga Average screen grid current p g Transconductance (mutual conductance) ∆Esig is the change of control-grid signal voltage, m E is the plate supply voltage. mu Amplification factor (µ) bb P Power at screen grid sg For example, a change of 1 mA of plate current for a Pp Power at plate change of 1 V at the control grid is equal to a transcon- P-P Plate-to-plate or push-pull amplifier ductance of 1000 µmho. A tube having a change of Rg Grid resistor 2 mA plate current for a change of 1 V at the control Rk Cathode resistor grid would have a transconductance of 2000 µmho. Rl Plate-load impedance or resistance g = I × 1,000 (12-2) Rp Plate-load resistor m pac Rsg Screen-dropping resistor where, g is the transconductance in micromho or Rd Decoupling resistor m microsiemens, rp Internal plate resistance Ipac is the ac plate current. Vg Voltage gain 12.1.5 Amplification Factor Amplification factor (µ) or voltage gain (Vg) is the ratio of the incremental plate voltage change to the control- electrode voltage change at a fixed plate current and Diode Triode Tetrode Pentode or Beam sheet-beam power constant voltage on all other electrodes. This normally is the amount the signal at the control grid is increased in amplitude after passing through the tube. Tube voltage gain may be computed using the Pentagrid Eye tube Gas-filled Photo tube High-voltage converter rectifier rectifier equation ∆Ep Vg = ---------- (12-3) ∆Eg Duo-diode Dual-triode Two-section Full-wave triode rectifier where, Figure 12-2. Basing diagrams for popular tubes. Vg is the voltage gain, Tubes, Discrete Solid State Devices, and Integrated Circuits 101 ∆Ep is the change in signal plate voltage, close-fitting metal tube shield around the glass envelope ∆Eg is the change in the signal grid voltage. connected to the cathode terminal. Generally, the capac- itance is measured with the heater or filament cold and If the amplifier consists of several stages, the amount with no voltage applied to any of the other elements. of amplification is multiplied by each stage. The gain of an amplifier stage varies with the type tube and the interstage coupling used. The general equation for 180° 180° 0° voltage gain is + E0 Esig Vgt = Vg1Vg2…Vgn (12-4) 0° where, R R p Vgt is the total gain of the amplifier, sg Vg1, Vg2, and Vgn are the voltage gain of the individual stages. B+ Triode tubes are classified by their amplification A. Polarity reversal of the signal between the factor. A low-µ tube has an amplification factor less elements of a pentode vacuum tube. I + than 10. Medium-µ tubes have an amplification factor p E from 10–50, with a plate resistance of 5 Ω–15,000 Ω. p High-µ tubes have an amplification factor of 50–100 Isg + E with a plate resistance of 50 kΩ–100 kΩ. + sg E0 Esig Ik + Rg 12.1.6 Polarity Ek R Polarity reversals take place in a tube. The polarity R p R sg reversal in electrical degrees between the elements of a k self-biased pentode for a given signal at the control grid is shown in Fig.
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