Pentode Press

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The power amp transforms the carefully crafted signal voltages served up by the preamp stages into the punch and sizzle that defines a guitar amplifier's character. It is in the power stage that the designer orchestrates the nuances of touch and tone in an environment of vacuum tubes and circuit components being pushed to their limits.

Written for electronic engineers and professional amp builders, Guitar Amplifier Power Amps moves beyond simplistic advice and cookbook solutions to present a complete guide to the theory and operation of single-ended and push-pull power amplification. From the phase inverter input to the loudspeaker output, every aspect of circuit design is rigorously explained and thoroughly explored. Find out what happens when Class A₁ and Class AB₁ circuits are overdriven to distortion levels never imagined by Frederick Terman or the Radiotron Designer's Handbook. Discover step-by-step how to design phase inverters and power amps to achieve your specific design goals.

The dynamics of power amp distortion create the heart and soul of an amplifier. Guitar Amplifier Power Amps helps you get the most out of it.

Chapter 1. Introducton

Chapter 2. Pentodes and Beam Power Tubes

• Pentodes..........................................................5
• Beam Power Tetrodes...............................................7
• Plate Characteristic Curves.......................................8
• Power Tubes versus Voltage Amplification Tubes...................11
• Low Plate Voltage Effects........................................12
• Performance Differences Between Pentodes and Beam Power Tetrodes.13
• Plotting Curves for a Specific Screen Voltage....................16
• Variability of Tube Characteristics..............................18

Chapter 3. Plate and Screen Circuit Design

• The Basic Steps of Power Amp Design..............................19
• The Common-Cathode Amplifier.....................................19
• The DC Operating Point...........................................20
• Using Triode-Connected Curves....................................21
• Vacuum Tube Response to AC signals...............................23
• Cutoff and Saturation............................................23
• Setting the DC Operating Point...................................25
• The AC Load Line.................................................26
• Optimum Load Line for Pentodes...................................30
• Screen Dissipation...............................................33
• Maximum Power and Headroom.......................................35
• DC Grid Bias Voltage.............................................37
• Fixed Bias.......................................................37
• Cathode Bias.....................................................38
• Practical Aspects of Using Cathode Bias..........................39
• Cathode Degeneration.............................................40
• Selecting the Bypass Capacitor Value.............................42
• The Output Transformer...........................................42
• The Screen Grid-Stopper Resistor.................................44
• Plate Circuit Design Procedure for Single-Ended Amplifiers.......45

Chapter 4. Grid Circuit Design

• A Basic Grid Circuit.............................................47
• Preamp Output Impedance..........................................48
• Equivalent Grid Circuit for Audio Frequencies....................49
• Middle-Range Frequency Response..................................51
• Low-Frequency Response...........................................51
• High-Frequency Response..........................................53
• Measuring Parasitic Capacitance..................................54

Chapter 5. Parallel Tubes and Parasitic Oscillation

• Parallel Tubes for More Power....................................57
• Parasitic Oscillation............................................58
• The Effect of RF Suppression on Audio-Frequency Distortion.......59

Chapter 6. Push-Pull Power Amps

• How a Push-Pull Amplifier Works..................................61
• Class A Push-Pull Operation......................................63
• Class B Push-Pull Operation......................................65
• Class AB Push-Pull Operation.....................................66
• Guitar Amplifiers - In a Class All Their Own.....................67
• Power Supply Voltage Excursion...................................68
• Estimating Power Supply Voltage Sag Based on Current Load........68
• Design Strategies for Dealing with Class AB Power Supply Sag.....73
• Drawing Composite Characteristic Curves..........................75
• The Load Line....................................................78
• Class AB Power Output............................................79
• Plotting the Effective Load Line for One Tube....................80
• Computing the Current Load and Plate Dissipation.................81
• Cathode Bias for Push-Pull Power Amps............................83
• The Screen Grid-Stopper Resistor.................................84
• The Effects of Mismatched Components.............................84

Chapter 7. Distortion Characteristics at Full Power

• Harmonic Distortion..............................................87
• Calculating Percent Harmonic Distortion..........................90
• Intermodulation Distortion.......................................95
• Controlling Harmonic Content.....................................95
• Rectification Effects............................................98
• Single-Ended versus Push-Pull Distortion.........................99
• Class AB Distortion: Fixed Bias versus Cathode Bias..............99

Chapter 8. Distortion in an Overdriven Power Amp

• An Overview.....................................................102
• Headroom........................................................103
• The Cushioning Effect...........................................104
• Bottoming.......................................................105
• Positive Grid Voltage Effects...................................105
• Driving a Power Tube Grid Positive with a High-Impedance Source.109
• Clipping and Clamping...........................................112
• A Different Perspective: How the Circuit Responds over Time.....114
• Bias Excursion and Recovery.....................................118
• The Recovery Phase..............................................120
• Bias Recovery Time versus Bass Response.........................121
• An Example of Grid Bias Excursion...............................124
• The Grid Bias Excursion Ratio...................................127
• Bias Excursion Time.............................................130
• A Summary of Bias Excursion Formulas............................131
• Grid Bias Supply Considerations.................................132
• Grid Bias Supply Voltage Excursion and Recovery.................133
• Bias Excursion for Cathode-Bias versus Fixed-Bias Designs.......135
• Controlling the Dynamics of Bias Excursion......................138
• Bias Excursion and Recovery for Some Vintage Amplifiers.........138
• The Tonal Effects of Overdriving a Power Amp....................139

Chapter 9. Crossover Distortion, Blocking, and Blackout

• Crossover Distortion............................................143
• Blocking Distortion.............................................143
• Minimizing the Likelihood of Blocking Distortion................146
• Class AB: Fixed Bias versus Cathode Bias........................146
• Blackout........................................................147

Chapter 10. The Marshall Model 1967 Head

• Pentode-Operated Pentodes.......................................149
• Triode-Operated Pentodes........................................150
• Ultra-Linear Power Amplifiers...................................153

Chapter 11. Real-World Output Transformers

• Ideal Single-Ended Transformers.................................155
• Ideal Push-Pull Transformers....................................156
• DC Magnetization Current........................................157
• Hysteresis Losses...............................................158
• Middle-Range Transformer Losses.................................160
• Low-Frequency Transformer Response..............................163
• High-Frequency Transformer Response.............................164
• Total Response..................................................164
• Transformer Power Rating and DC Current Effects.................166
• Output Transformer Distortion...................................166
• How Real-World Characteristics Affect Power Amp Design..........167

Chapter 12. Real-World Loudspeaker Impedance

• Nominal Impedance...............................................169
• Resonant Frequency and Beyond...................................170
• An Example - The Jensen C12R-8..................................172
• Estimating the Nominal Impedance of a Loudspeaker...............172
• How Loudspeaker Impedance Affects Power Amp Design..............173

Chapter 13. Paraphase Inverters

• The Common-Cathode Triode Amplifier.............................177
• Computing the Resistor Values...................................179
• Frequency Response..............................................180
• The Gibson GA-20T Inverter......................................180
• Overdriving and Distortion......................................181

chapter 14: The Concertina Phase Splitter

• The Concertina Phase Splitter, an Overview......................183
• The DC Circuit..................................................184
• Maximum Output Voltage Swing....................................187
• The AC Circuit..................................................189
• Phase Splitter Output Impedance for Arbitrary Loads.............192
• Overdriving and Distortion......................................197
• Nonlinear Distortion Effects....................................199
• Summary of Important Concertina Features........................200

Chapter 15: The Long-Tailed-Pair Phase Inverter

• The DC Circuit..................................................201
• The AC Circuit..................................................204
• The Common-Grid Circuit.........................................208
• The Common-Cathode Circuit......................................209
• Voltage Gain Imbalance..........................................211
• Output Impedance................................................212
• Overdriving and Distortion......................................212
• Maximum Output Voltage Swing....................................213
• Adding a Second Signal Input....................................214
• Adding Negative Feedback and a Presence Control.................214
• Voltage Gain and Input Impedance for Negative Feedback..........218
• Comparing the Concertina to the Long-Tailed Pair................219

Chapter 16: Negative Feedback

• A Generalized Negative Feedback System..........................221
• Loop Gain.......................................................222
• A Second Look at Cathode Degeneration...........................223
• Negative Feedback from the Output Transformer Secondary.........224
• Frequency Response with Negative Feedback.......................226
• Other Feedback Effects..........................................228
• A Handy Formula for the Long-Tailed-Pair Phase Inverter.........229
• Stability.......................................................230
• Motorboating....................................................234
• An Example of Negative Feedback Design..........................237

Chapter 17: A Step-by-Step Single-Ended Design Example

• The Basic Steps of Single-Ended Design..........................243
• Selecting the Tube and the Screen Voltage.......................244
• Selecting the Idle Plate Voltage................................244
• Estimating the Cutoff Grid Voltage and Plate Current............245
• Setting the DC Operating Point..................................246
• Designing the Cathode Bias Circuit..............................246
• Selecting the Output Transformer Primary Impedance..............248
• Determining the Plate Circuit Operating Conditions..............250
• Computing the Harmonic Distortion at Full Power.................252
• Designing the Grid Circuit......................................254
• The Final Power Amp Design......................................254

Chapter 18: A Step-by-Step Class AB Parallel Push-Pull Design Example

• Accounting for Power Supply Voltage Sag.........................257
• Selecting the Output Transformer Impedance......................259
• Determining Output Power and Voltage Gain.......................261
• Selecting a Screen Resistor.....................................262
• Plotting the Composite Characteristic Curves....................262
• Plotting the Effective Load Line for One Tube...................262
• Computing the Average Plate and Screen Current at Full Power....264
• Computing the Plate and Screen Dissipation......................268
• Computing the Power Supply Voltage Sag at Full Power............268
• Determining the Zero-Signal Characteristics.....................270
• Calculating Third Harmonic Distortion at Full Power.............272
• Selecting the Grid Resistor Value...............................273
• Applying Preamp Constraints.....................................273
• A Paraphase Design..............................................274
• Selecting the DC Operating Point................................276
• Computing the Resistor Values...................................278
• A Concertina Design.............................................279
• Selecting the DC Operating Point................................280
• Examining the Concertina's Nonlinearity.........................280
• Determining the Other Resistor Values...........................283
• A Long-Tailed-Pair Design.......................................283
• Tail Resistance and the DC Load Line............................284
• Selecting the DC Operating Point................................285
• Examining the Long-Tailed-Pair's Nonlinearity...................287
• Balancing the Voltage Gains.....................................287
• The Final Phase Inverter Design.................................288
• Computing the Coupling Capacitor Value..........................289
• Selecting the Grid-Stopper Resistor Value.......................289
• Computing Bias Excursion and Recovery...........................291
• The Final Power Amp Design......................................291
• Some Last Words About Class AB Design...........................293

Chapter 19: Epi-Log

Appendices A-G: Vintage Power Amps Listed by Tube Type and Operating Class. Tube Data Sheets

• EL84/6BQ5 Power Amps............................................297
• GE 6BQ5 Data Sheet..............................................298
• 6V6/6AQ5 Power Amps.............................................305
• GE 6V6GT Data Sheet.............................................307
• 7027 Power Amps.................................................313
• RCA 7027 Data Sheet.............................................314
• 7591 Power Amps.................................................323
• Sylvania 7591A Data Sheet.......................................324
• EL34/6CA7/KT77 Power Amps.......................................329
• Philips EL34 Data Sheet.........................................331
• 6L6/5881/KT66 Power Amps........................................339
• Marconi KT66 Data Sheet.........................................342
• 6550/KT88 Power Amps............................................353
• GE 6550A Data Sheet.............................................354

Appendix H: Derivation of Additional Formulas for the Long-Tailed Pair

• Voltage Gains...................................................363
• Output Impedance................................................365

References

Index

Publisher: Pentode Press
800 5th Ave. #101-147
Seattle, WA 98104-3191
Book Format: 7-inch by 9-inch paperback
Pages: 376
ISBN: 978-0976982241
Publication Date: January 22, 2008