Pentode Press

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 and cookbook solutions 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.

Chapter 1. Introducton

Chapter 2. Triode Voltage Amplifier Design

• The Vacuum-Tube Diode...........................................3
• The Vacuum-Tube Triode..........................................4
• Characteristic Curves, Load Line, and DC Operating Point........7
• Voltage Amplification...........................................8
• Plate Dissipation...............................................9
• Removing DC from the Output....................................11
• Self-Biasing...................................................11
• Equivalent Circuit for Audio Frequencies.......................14
• Combining Two Self-Biasing Circuits............................17
• Plate Resistance, Transconductance, and Amplification Factor...20
• The Input Load of the Next Stage...............................21

Chapter 3. Input Circuit Design

• Series Impedance Networks......................................27
• Shunt Impedance Networks.......................................30
• Inverted-L Networks............................................31
• Gamma Networks.................................................35
• Pi Networks....................................................38
• T Networks.....................................................41
• Converting Between T and Pi Networks...........................43
• Double Inverted-L Networks.....................................44
• Pi-Gamma Networks..............................................46
• Three Factors that Constrain Input Circuit Design..............47

Chapter 4. Guitar Characteristics

• Pickup Lumped Impedance Characteristics........................51
• Pickup Distributed Impedance Characteristics...................53
• Guitar Volume and Tone Controls................................53
• Guitar Cable Characteristics...................................56
• Designing the Preamp to Match the Guitar.......................59
• The Impact of Guitar Characteristics on Amplifier Design.......60

Chapter 5. Preamp Frequency Response

• Miller Capacitance.............................................61
• The Complete AC Circuit........................................63
• Approximate Gain for Middle-Range Frequencies..................65
• Low-Frequency Gain.............................................66
• High-Frequency Gain............................................68
• Simplifying Response Calculations for Complicated Circuits.....71

Chapter 6. The Selection of a Plate Load Resistor Value

• The Implications of Increasing Preamp Gain.....................73
• The Effect of the Plate Resistor on Nonlinear Distortion.......77
• Practical Example: A Preamp with a 390k Plate Resistor.........77

Chapter 7. Pentode Voltage Amplifier Design

• Screen Grids and Screen Circuits...............................81
• Adjusting Characteristic Curves to a New Screen Voltage........83
• AC Equivalent Circuit..........................................84
• Sharp-Cutoff versus Remote-Cutoff Distortion...................85
• Approximate Gain for Middle-Range Frequencies..................87
• Low-Frequency Response.........................................88
• High-Frequency Response........................................89
• Screen Circuit Impedance.......................................90
• Microphonic Noise..............................................93

Chapter 8. Cathode Degeneration

• Using a Cathode Resistor without a Bypass Capacitor............95
• Cathode Impedance..............................................96
• Bypass Capacitor Size..........................................97
• Cathode Degeneration as Negative Feedback......................99
• The Effects of Negative Feedback..............................101

Chapter 9. Headroom, Distortion, and Noise

• The AC Load Line..............................................103
• Determining Headroom..........................................106
• How the DC Operating Point Affects Nonlinear Dynamics.........109
• Computing Second Harmonic Distortion..........................110
• Intermodulation Distortion....................................114
• Shot Noise....................................................114

Chapter 10. Advanced Design Example: The Ampeg B42X Preamp

• DC Operating Point and Load Line..............................118
• Estimating the AC Parameters Graphically......................120
• Unloaded Gain, Miller Capacitance, and Cathode Degeneration...121
• Strategy for Determining Frequency Response...................122
• The Input Circuit.............................................122
• Bright Channel Input Network Response.........................123
• Normal Channel Input Network Response.........................124
• Miller Capacitance for the Second Stage.......................126
• Grid-to-Grid Gain.............................................127
• Middle-Range Frequency Response...............................128
• Low-Frequency Response........................................128
• High-Frequency Response.......................................132
• Effect of the Volume Control on High-Frequency Response.......133
• Closing the Ultra-Low Switch..................................134
• Closing the Ultra-Hi Switch...................................137
• The Total Response of the Ampeg B42X Preamp...................142
• Headroom......................................................143

Chapter 11. Using Two Triodes in Parallel

• Parallel Triode Design and its Implications...................148
• Dual-Triode Output Impedance..................................149
• The DC Operating Point........................................150

Chapter 12. Real-World Resistors and Their Impact on Preamp Design

• Lead and Wiring Resistance....................................153
• Skin Effect...................................................153
• Parasitic Inductance and Capacitance..........................155
• Dielectric Materials..........................................157
• The Effect of Parasitic Capacitance on the Preamp Circuit.....157
• Wiring and Lead Inductance....................................159
• Carbon Composition Resistors..................................162
• Other Types of Resistors......................................164
• Frequency Characteristics of Carbon Composition Resistors.....165
• Distributed Capacitance Between Wires and Leads...............169
• The Implications of Distributed Capacitance for Resistors.....170
• A Real-World Example..........................................172
• Lead Inductance...............................................175
• Comparison with a Metal Film Resistor.........................175
• Resistor Noise Characteristics................................178
• Noise Specific to Carbon Composition Resistors................179
• Carbon Composition Resistor Nonlinearity......................181

Chapter 13. Real World Capacitors and Their Impact on Preamp Design

• Leakage and Dielectric Absorption.............................183
• Resistive and Reactive Characteristics........................184
• Typical Capacitor Specifications..............................190
• The Effects of a Capacitor's Impedance on a Preamp............191

Appendix A: Series and Parallel Resonance

• Series Resonant Circuits......................................195
• Parallel Resonant Circuits....................................196

Appendix B: Dielectric Constants and Dissipation Factors

Appendix C: Philips EF86 Data Sheet

• References....................................................217
• Index.........................................................219

Publisher: Pentode Press
800 5th Ave. #101-147
Seattle, WA 98104-3191
Book Format: 7-inch by 9-inch paperback
Pages: 223
ISBN: 978-0976982227
Publication Date: February 6, 2007