Pentode Press

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 bound, 18-inch by 9-inch, Z-folded insert with the complete 5F6A schematic and the original manufacturer's voltage measurements.

I didn't intend to write this book. My goal was to design my own guitar amp. When I discovered that so many amplifiers are based on the 5F6A circuit, I decided that what worked for Jim Marshall and other legendary amp builders might work for me. So armed with the Radiotron Designer's Handbook, the works of Frederick Terman, and many other great references from the tube era, I began to study Leo Fender's famous schematic. I wanted to completely understand how the original circuit worked before attempting to modify it. The results literally filled a book.

This expanded, second edition introduces some new topics. Complete headroom analysis is performed, both for the individual stages and for the system as a whole, for the purpose of determining where distortion is likely to break out first depending on the control settings. The Marshall JTM45, which is almost a direct copy of the 5F6A, is also analyzed to compare its significant differences, most notably the substitution of a 12AX7 in the first preamp, an additional bypass capacitor in the bright channel for the lead model, and triple the negative feedback voltage to the phase splitter.

This edition also takes a completely different approach to estimating the supply voltage sag at maximum power. As was noted in the first edition, the ultimate supply voltages reached are very sensitive to slight changes in signal level and a number of approximations have to be made to keep the problem complexity in check, so even in this edition the supply voltage sag is more or less a ballpark estimate for what is encountered in practice. It is important to address this subject, however, because the sag that is induced by the vacuum tube rectifier and class AB output stage is an essential element of the Bassman's legendary sound. The new methods introduced here are both simpler and more accurate than what was used in the previous edition.

My thanks to the risk-takers who bought the first edition, gave me highly valuable feedback, and encouraged me to continue this work. My thanks also to Fender Musical Instruments Corporation for giving me permission to include their annotated schematic. The Bassman's classic design may not represent new technology, but to generations of performers and music lovers alike it is difficult to find a more magical ensemble of electronic components or a longer-enduring legacy than the 5F6A.

About Richard Kuehnel

Chapter 1. The 5F6A Preamp

• Vacuum Tube Diodes and Perveance................................3
• Cutoff Amplification Factor for Triodes.........................4
• Triode Small-Signal Model.......................................5
• The 5F6A Dual-Channel Preamp....................................7
• DC Operating Point and Load Line................................8
• Graphical Estimation of Plate Resistance and Transconductance..11
• AC Voltage Gain................................................11
• Cathode Impedance..............................................15
• Output Impedance...............................................18
• Miller Capacitance.............................................19
• Frequency Response (#1 Inputs).................................21
• The #2 Inputs..................................................23
• Headroom.......................................................24
• 12AY7 Preamp Test Results......................................26
• Summary of 12AY7 Preamp Formulas...............................27

Chapter 2. The 5F6A Second-Stage Voltage Amp

• DC Operating Point and AC Characteristics......................30
• AC Voltage Gain................................................32
• Negative Feedback..............................................34
• Miller Capacitance.............................................35
• Output Impedance...............................................38
• Input Impedance and Frequency Response.........................39
• Input Characteristics - Simplified.............................41
• Headroom.......................................................43
• Summary of 12AX7 Voltage Amp Formulas..........................43

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

• DC Operating Point and AC Characteristics......................47
• AC Voltage Gain................................................51
• Output Impedance...............................................52
• Headroom.......................................................54
• Summary of 12AX7 Cathode Follower Formulas.....................54

Chapter 4. The 5F6A Tone Stack

• A Convenient Low-Frequency Response Approximation..............59
• Approximate High-Frequency Response............................61
• Input and Output Impedances....................................63
• More Accurate Frequency Response Calculations..................64
• Plots of Tone Stack Frequency Response.........................66
• Voltage Amp/Cathode Follower/Tone Stack Test Results...........74
• Summary of Tone Stack Formulas.................................77

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

• DC Operating Point and AC Characteristics......................82
• AC Voltage Gain (Simplified Model).............................86
• AC Voltage Gain (More Accurate Model)..........................90
• Input Impedance................................................93
• Feedback Gains and Feedback Input Impedance....................96
• Headroom.......................................................99
• Phase Splitter Test Results...................................101
• Interaction with the Power Amplifier Input Circuit............102
• Summary of Phase Splitter Formulas............................105

Chapter 6. The 5F6A Push-Pull Power Amp

• Adjusting Published Curves to Different Screen Voltages.......111
• Composite Characteristic Curves and Load Line.................112
• Maximum Power Analysis........................................119
• Stress Limits on the GZ34 Rectifier...........................126
• Nonlinear Distortion Analysis.................................129
• Open-Loop Harmonics...........................................134
• The Effects of Negative Feedback (Linear Approximation).......135
• The Effects of Negative Feedback (Nonlinear Model)............136
• Graphing the Response with Negative Feedback..................137
• Output Impedance..............................................139
• Power Amplifier Test Results..................................142
• Summary of Power Amplifier Formulas...........................144

Chapter 7. 5F6A Power Supply and System Analysis

• The Power Supply..............................................148
• DC Voltages...................................................149
• Power Supply Sag as a Function of Time........................149
• A Convenient Approximation for Power Supply Response Time.....154
• AC Ripple Attenuation.........................................156
• Complete 5F6A System Model....................................160
• System Headroom...............................................163
• Closed-Loop Stability Tests...................................165

Chapter 8. Modifications Incorporated into the Marshall JTM45

• Substituting a 12AX7 in the First Preamp......................169
• Bright-Channel Boost..........................................173
• Frequency Response Simplified.................................176
• Effects of Increased Negative Feedback........................178
• Power Supply Modifications....................................182
• Changes in Power Supply Response..............................184

Appendices

• 12AY7, 12AX7, 6L6GC, and GZ34 Datasheets......................187
• 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: 220
5F6A Schematic Insert: 18-inch by 9-inch, Z-folded
ISBN: 978-0976982210
Publication Date: November, 2005