Table of Contents

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