Pentode Press

The third edition extends the heritage of the Fender Bassman 5F6A 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 5F6A, 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 5F6A and its Marshall-inspired offspring.

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.

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 the 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 guitar 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.

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 5F6A. 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 5F6A 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.

"Full-grown guitarists are known to flop to the ground and convulse in gurgling spasms of lustful froth at the mere mention of the phrase, 'tweed Bassman,' and anyone who has played one in good condition knows exactly why." - Dave Hunter

This book takes an insightful look at the technology that made the magic happen.

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 5F6A 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 5F6A 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 5F6A 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.

Preface to the Second Edition

Preface to the Third Edition

Introduction

Chapter 1. The First Preamp Stage

• Vacuum Tube Diodes and Perveance................................7
• Cutoff Amplification Factor for Triodes.........................8
• Triode Small-Signal Model.......................................9
• The 5F6A 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

Appendices

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

Publisher: Pentode Press
800 5th Ave. #101-147
Seattle, WA 98104-3191
Book Format: 7-inch by 9-inch paperback
Pages: 294
5F6A Schematic Insert: 18-inch by 9-inch, Z-folded
Generalized Fender-Marshall Circuit Insert: 18-inch by 9-inch, Z-folded
ISBN: 9780976982258
Publication Date: September 8, 2009