An in-depth understanding of preamp circuits is essential to creating a guitar amplifier design that stands out from the crowd. It is here that the designer gets a first crack at setting the amp's gain, frequency response, and distortion limits. Written for electronic engineers and professional amp builders, Guitar Amplifier Preamps moves beyond simplistic advice to present a complete guide to the theory and operation of triode and pentode voltage amplification. From the guitar pickup to the second stage grid, every aspect of circuit design is rigorously explained and thoroughly explored using real-world examples from Ampeg, Fender, Gibson, Laney, Marshall, Matchless, Orange, and Vox.

The frequency shaping and distortion created in the first amplification stage affect the entire signal chain. Guitar Amplifier Preamps helps you get the most out of it.

Richard Kuehnel is a member of the Circuits and Systems Society of the Institute of Electrical and Electronic Engineers.

The Vacuum-Tube Diode 3 The Vacuum-Tube Triode 4 Characteristic Curves, Load Line, and DC Operating Point 7 Voltage Amplification 9 Plate Dissipation 10 Removing DC from the Output 11 Self-Biasing 12 Equivalent Circuit for Audio Frequencies 15 Combining Two Self-Biasing Circuits 19 Plate Resistance, Transconductance, and Amplification Factor 21 The Input Load of the Next Stage 24

Series Impedance Networks 29 Shunt Impedance Networks 31 Inverted-L Networks 33 Gamma Networks 36 Pi Networks 39 T Networks 42 Converting Between T and Pi Networks 44 Double Inverted-L Networks 45 Gamma-Pi Networks 48 Three Factors that Constrain Input Circuit Design 49

Pickup Lumped Impedance Characteristics 53 Pickup Distributed Impedance Characteristics 55 Guitar Volume and Tone Controls 55 Guitar Cable Characteristics 58 Designing the Preamp to Match the Guitar 61 The Impact of Guitar Characteristics on Amplifier Design 61

Miller Capacitance 63 The Complete AC Circuit 65 Approximate Gain for Middle-Range Frequencies 67 Low-Frequency Gain 68 High-Frequency Gain 71 Simplifying Response Calculations for Complicated Circuits 74

The Implications of Increasing Preamp Gain 75 The Effect of the Plate Resistor on Nonlinear Distortion 78 Practical Example: A Preamp with a 390k Plate Resistor 80

Screen Grids and Screen Circuits 85 Adjusting Characteristic Curves to a New Screen Voltage 88 AC Equivalent Circuit 88 Nonlinear Distortion 90 Approximate Gain for Middle-Range Frequencies 93 Low-Frequency Response 93 High-Frequency Response 94 Screen Circuit Impedance 97

Using a Cathode Resistor without a Bypass Capacitor 101 Cathode Impedance 102 Bypass Capacitor Size 103 Cathode Degeneration as Negative Feedback 105 The Effects of Negative Feedback 107

The AC Load Line 109 Determining Headroom 111 How the DC Operating Point Affects Nonlinear Dynamics 114 Second Harmonic Distortion 115 Intermodulation Distortion 120

DC Operating Point and Load Line 121 Estimating the AC Parameters Graphically 124 Unloaded Gain, Miller Capacitance, and Cathode Degeneration 125 Strategy for Determining Frequency Response 126 The Input Circuit 126 Bright Channel Input Network Response 127 Normal Channel Input Network Response 128 Miller Capacitance for the Second Stage 130 Grid-to-Grid Gain 131 Middle-Range Frequency Response 132 Low-Frequency Response 133 High-Frequency Response 135 Effect of the Volume Control on High-Frequency Response 137 Closing the Ultra-Low Switch 138 Closing the Ultra-Hi Switch 141 The Total Response of the Ampeg B42X Preamp 146 Headroom 148

Dual-Triode Output Impedance 153 The DC Operating Point 154

Lead and Wiring Resistance 157 Skin Effect 159 Parasitic Inductance and Capacitance 159 Dielectric Materials 161 The Effect of Parasitic Capacitance on the Preamp Circuit 162 Wiring and Lead Inductance 163 Carbon Composition Resistors 168 Other Types of Resistors 169 Frequency Characteristics of Carbon Composition Resistors 170 Distributed Capacitance Between Wires and Leads 173 The Implications of Distributed Capacitance for Resistors 175 A Real-World Example 176 Lead Inductance 179 Comparison with a Metal Film Resistor 182 Noise Specific to Carbon Composition Resistors 183 Carbon Composition Resistor Nonlinearity 184

Leakage and Dielectric Absorption 187 Resistive and Reactive Characteristics 188 Typical Capacitor Specifications 194 The Effects of a Capacitor's Impedance on a Preamp 195

Microphonic Noise 199 Tube Noise 199 Thermal Noise 201 Noise Figure 202 Cascaded Preamp Stages 202 Volume Control Placement 203

A Cheap Tube Amp for the Masses 207 Voltage Gain 208 Reducing Thermal Noise 210 Headroom and Distortion Characteristics 216 Power Supply Adjustments 217 Input Circuit Design 219

New Design Constraints 223 Preamp Output Voltage Swing at Full Power 224 Power Amp Requirements 224 The New Design 225 The Pentode Stage 226 The Coupling Capacitor Value 231 The Triode Stage 231 Adding Treble Boost 235 Input Circuit Design 236 Coupling to the Power Amp 237 Headroom and Distortion Characteristics 237 The Final Design 239

Series Resonant Circuits 243 Parallel Resonant Circuits 244

References 269 Index 273

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