Fender Bassman 5F6A book
Circuit Analysis of a Legendary Tube Amplifier

The Fender Bassman 5F6A

Richard Kuehnel
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New in the Third Edition

The third edition extends the heritage of the Fender Bassman 5F6-A by examining the modifications Marshall incorporated into its famous "Bluesbreaker Combo" and "Plexi" amplifiers. The differences are quite significant. They give each of these amps a completely different sound, unique dynamics, and a separate place in history. Their commonalities with the 5F6-A, however, are also quite striking. Given a universe of possible modifications to these circuits, it is enlightening to know where Marshall chose to focus its innovation.

The third edition also adds a chapter on the dynamics of the power amp when severely overdriven. Intentionally overdriving a Class AB1 design was unthinkable in the age of Frederick Terman and the Radiotron Designer's Handbook, so there hasn't been a lot written on the subject. Guitar players are known for pushing their amps beyond their high-fidelity limits, however, so the effects are important. The treatment here isn't as exhaustive as in Richard Kuehnel's Power Amps book, but it is specific to the 5F6-A and its Marshall-inspired offspring.

From the Author

Marshall circuit modifications did not progress along a straight time line and some parts in later years actually reverted back to their original values. So in generalizing these circuits I remove chronology altogether and examine, from a contemporaneous perspective, how the variations affect amplifier performance. In taking a structural approach to these amps, I fix the parts that never changed and use the parts that were modified as free variables without constraints. If, for example, Marshall decreases a capacitor value from 0.1uF to 0.022uF, I examine the significance, from a design perspective, of using 0.047uF or 0.01uF. This creates a palette of choices depicting the selections that were made and the ones that could have been made.

From Dave Hunter

While the industry was doing its best to move the format forward or to convince the amp-buying public that that was what it was doing, at least one great artist after another was discovering th e unmitigated glory of a cranked 5F6-A Bassman, and the way that one of these rough-edged old bass amps from the late 50s could sing like sweet hellfire when you injected a good six-string guit ar and found its sweet spot. Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A knows this, and pays homage to this template of tone like no other book on the market. The sound that emanates from any guitar amp is more than the sum of its parts, and only a detailed analysis of exactly how your signal is shaped along the way can reveal just how that magic is made between input and output. This is exactly what this book gives us, and why its an important addition to the knowledge. And man, how I wish I'd had that particular knowledge a little sooner in life, when an original tweed Bassman still cost a lot less than a new high-gain amp with a four-month waiting list.

From the Back Cover

The Bassman was introduced in 1952 as a bass guitar amplifier. It had a huge 15-inch speaker with a closed back and used a pair of beam power tubes for lots of volume. Throughout the decade it would be modified and improved until culminating in the model 5F6-A. A legend was born.

Ironically it didn't quite make it as a bass amp, but the musical tones it created from a guitar made history. The Bassman's long-tailed-pair phase splitter with negative feedback became the foundation for many great rock and blues sounds. Many other amplifier manufacturers created new Bassman-derived designs that themselves became the legendary instruments of a rock generation. For more than four decades the 5F6-A circuit has been tweaked, enhanced, and re-introduced under many different logos and the late-1950's original has become one of the most collected guitar amplifiers of all time.

This book examines this famous amplifier by studying its circuit design in great detail. It starts by breaking the amplifier into its major components: the 12AY7 preamp, the 12AX7 voltage amp, the 12AX7 cathode follower, the tone stack, the long-tailed-pair phase splitter, the push-pull power amplifier, and the power supply. Each component is analyzed to determine how it works and to derive the design formulas needed to predict its performance. The results are then compared to bench tests of the actual circuit. Finally all of the components are put together to analyze total system behavior and to discover how and where the amp transitions into distortion.

From the Publisher

Why would a member of the IEEE Circuits and Systems Society, an organization dedicated to advancing the state of the art in electronics, write an entire book about the design of a vacuum tube amplifier manufactured more than four decades ago? Even back in 1959, the Bassman's innovation must have paled in comparison with the technological leaps being made by space-age transistor circuits.

To understand the motivation behind the book you need to understand what the tweed Bassman means to guitar players. To generations of professional musicians the Bassman's mix of harmonic overtones is the holy grail of tone, the perfect balance between clean reproduction and distortion-driven tonal ecstasy.

Its place in history is ironic, because the 5F6-A wasn't even designed for the guitar - as its name implies it was intended to be a bass amplifier. Yet today more than ever the amp's sound hits the mark with guitar players the world over. More than forty years after the original 5F6-A ceased production many of the best guitar amps, Marshalls in particular, are still based on its long-tailed-pair, negative feedback circuit design. It is to this lasting legacy that the book pays tribute.

The book includes a Z-folded insert with the complete 5F6-A schematic and the original manufacturer's voltage measurements, plus an additional Z-folded insert with the Generalized Fender-Marshall Circuit: the original 5F6-A with superimposed modifications Marshall incorporated into the JTM45, Bluesbreaker, and Plexi.

Chapter 1. The First Preamp Stage

Vacuum Tube Diodes and Perveance 7 Cutoff Amplification Factor for Triodes 8 Triode Small-Signal Model 9 The 5F6-A Dual-Channel Preamp 11 DC Operating Point and Load Line 12 Graphical Estimation of Plate Resistance and Transconductance 15 AC Voltage Gain 17 Cathode Impedance 19 Output Impedance 22 Miller Capacitance 23 Frequency Response (#1 Inputs) 25 The #2 Inputs 27 Headroom and Distortion 28 12AY7 Preamp Test Results 30 Summary of 12AY7 Preamp Formulas 32

Chapter 2. The Second Preamp Stage

DC Operating Point and AC Characteristics 35 AC Voltage Gain 36 Negative Feedback 39 Miller Capacitance 41 Output Impedance 43 Input Impedance and Frequency Response 45 Input Characteristics - Simplified 48 Headroom and Distortion 48 Summary of 12AX7 Voltage Amp Formulas 50

Chapter 3. The 5F6A Third-Stage Cathode Follower Circuit

DC Operating Point and AC Characteristics 53 AC Voltage Gain 57 Output Impedance 58 Headroom and Distortion 60 Summary of 12AX7 Cathode Follower Formulas 61

Chapter 4. The 5F6A Tone Stack

A Convenient Low-Frequency Response Approximation 63 Approximate High-Frequency Response 67 Input and Output Impedances 69 More Accurate Frequency Response Calculations 70 Plots of Tone Stack Frequency Response 72 Voltage Amp/Cathode Follower/Tone Stack Test Results 80 Summary of Tone Stack Formulas 83

Chapter 5. The 5F6A Long-Tailed-Pair Phase Splitter

DC Operating Point and AC Characteristics 88 AC Voltage Gain (Simplified Model) 93 AC Voltage Gain (More Accurate Model) 96 Input Impedance 99 Feedback Input Impedance 102 Feedback Gains 106 Headroom and Distortion 107 Phase Splitter Test Results 113 Interaction with the Power Amplifier Input Circuit 115 Summary of Phase Splitter Formulas 117

Chapter 6. The 5F6A Push-Pull Power Amp

Adjusting Published Curves to Different Screen Voltages 123 Composite Characteristic Curves and Load Line 124 Maximum Power Analysis 131 Stress Limits on the GZ34 Rectifier 138 Nonlinear Distortion Analysis 141 Open-Loop Harmonics 146 The Effects of Negative Feedback (Linear Approximation) 147 The Effects of Negative Feedback (Nonlinear Model) 147 Graphing the Response with Negative Feedback 149 Output Impedance 150 Power Amplifier Test Results 153 Summary of Power Amplifier Formulas 155

Chapter 7. 5F6A Power Supply and System Analysis

The Power Supply 158 DC Voltages 159 Power Supply Sag as a Function of Time 159 A Convenient Approximation for Power Supply Response Time 164 AC Ripple Attenuation 166 Complete 5F6A System Model 170 Closed-Loop Stability Tests 173

Chapter 8. Overdriving the Power Amp

Grid Current 177 Bias Recovery 179 Bias Excursion 180 Steady-State Bias Excursion 181 Clipping 184 Negative Feedback Effect 185 The Net Effects of Overdriving the Power Amp 186

Chapter 9. The Marshall JTM45

Substituting a 12AX7 in the First Preamp 187 Bright-Channel Boost 191 Frequency Response Simplified 194 Effects of Increased Negative Feedback 196 Power Supply Modifications 200 Changes in Power Supply Response 203

Chapter 10. The Marshall Model 1962 and Model 1987

Pentodes 204 Beam Power Tetrodes 205 Generalizing the Amplifier Circuit 206 Preamp Tube Substitution 209 Bypass Capacitor Modification 210 Plate Resistor Value 213 Volume Control and Mixing Circuits 214 Second Stage Frequency Response 219 Marshall Modifications to the 5F6-A Tone Stack 221 Tone Stack Low Frequency Response 222 Tone Stack High Frequency Response 224 Power Amp Coupling Capacitors 225 Bias Excursion and Recovery 227 Power Tubes, Output Transformers, Speakers, and Cabinets 229 Negative Feedback 233 Marshall Power Supply Modifications 234 Fold-Out Schematic of the Generalized Fender-Marshall Circuit 237

Chapter 11. Tone Stack Analysis: 5F6-A versus Model 1987

Zeros and Poles 239 The Fender Bassman 5F6-A Tone Stack Revisited 240 Minimum Bass - Maximum Midrange 241 Maximum Bass - Minimum Midrage 245 Low-Frequency Response - Summarizing the Results 251 High-Frequency Response 252 Summarizing the Results 255

Chapter 12. The Enduring Legacy of the Fender Bassman 5F6-A


12AY7, 12AX7, 6L6GC, and GZ34 Datasheets 259 References 289 Index 291