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A fascinating deconstruction of the classic Fender and Marshall circuits. Spice simulations, modification suggestions and some really great insight we’ve not seen anywhere else.

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“Inside Fender and Marshall Tube Amps”

by JC Maillet

ISBN 0-9684849-0-5

 

Modern Analysis of Vacuum Tube Amplifier Circuitry

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2001 Revised Edition

– TABLE OF CONTENTS –

Mathematical Modeling of Vacuum Tube Triode Devices 1

Qualitative Description of Electronic Transconductance Devices 2

Quantitative Description of Electronic Transconductance Devices 4

Classic Exponential Description of Vacuum Tube Triodes 5

Small-Signal Description of Triodes 7

Small-Signal Interpretation of Classic Plate Equation 11

Application of Computer Techniques to Triode Gain Stages 13

Pspice Transient Analysis Output 15

Pspice DC-Transfer Analysis Output 16

Pspice AC Analysis Output 17

Pspice Analysis of Common-Cathode Triode Gain Stages 18

Gain Stage Transfer, Distortion and Definition of Signal Gain 22

Small-Signal Analysis of the Common-Cathode Gain Stage 31

AC Analysis of the Cathode Bypassed Common-Cathode Gain Stages 39

Pspice Analysis of Common-Plate Triode Gain Stages 41

Small-Signal Analysis of the Common-Plate Follower Stage 45

Parametric Modeling of Triode Gain Stages 49

Common-Cathode DC Transfer Characteristics 52

Parametric Variations of the 12ax7 Common-Cathode Transfer Specs 54

Variational Common-Cathode Test Conclusions 61

Common-Plate DC Transfer Characteristics –

Parametric Variations of the 12ax7 Common-Plate Transfer Specs 62

Differential Triode Circuits 65

Differential-to-Single-Ended Circuit Conversion 67

Phase Driver Circuits 69

Transfer Simulation of Late-Period Fender Phase Driver Circuitry 72

Parametric Variation Simulation of Late-Period Fender Phase Drivers 73

Hi-and-Lo Supply Voltage Simulation of Four Phase Driver Circuits 75

General Structure of Vacuum Tube Amplifiers 79

Variations Amongst Circuit Blocks 80

The Fender Power Supply 81

Audio Circuits as power Supply Loads 83

The RC-Ladder Filter Network – defining the TSR Index 84

Rectifier Turn-On Characteristics 87

The CBS power Supply: Fender design Alterations 88

Fender Push-Pull Output Stages 89

Fender Vibrato Circuits 93

Bias-Modulating Vibrato Circuits 94

Photocell Based Vibrato Circuits 98

Phase-Cancellation Vibrato Circuits –

Bandwidth Control 100

Signal Coupling and Cathode Bypass Caps –

Tone Stack Component Values 104

Transient Fidelity in Guitar Tube Amps 106

The Non-linear Dynamics of Triode Gain Stages 107

Variable Compression from Non-linear Systems with Memory 108

The Role of Grid-Blocking Resistance 110

Interstage Resistance: tweed and Blackface Designs 111

Shadowing: Dielectric Adsorption in Signal Caps 114

Class-AB Biasing 116

Making The Notch Go Away 117

Maintaining Power Tube Matching Over Time 120

Recommended Tranny-Shunting Safety Precautions 121

Fine-Tuned Bias by Ear –

Neutral Fender Mods 122

Single-Ended Tweed Filament Hum Cancellation –

Silverface Filament and Bias Circuits 124

Late 60s and Later Silverface Bias Circuits 125

Improved Fender Bias Circuit 126

Alternative Bias Circuit Sourcing –

Lead Dress for Improved Hum Cancellation –

Examples of Modded Fender Silverface Amplifiers 128

Silverface Power Supply Variations 129

Silverface Signal Path Variations 130

Balance of Overall Overdrive Characteristics 131

Balance of Overall Amplifier Power and Gain 132

MOD (A) Hot 6L6 Pentode Performance –

Hot Blackface-style 6L6 Output Stage 135

MOD (B) Extreme Single-Channel Blackface Performance 137

MOD (C) Tweed Signal Path with Direct Coupled Reverb Recovery 139

MOD (D) British Super Tweed Reverb 142

MOD (E) Deluxe Reverb: The Stranglehold Variation 143

Low-End Deluxe bandwidth Extension –

Deluxe Power Supply Revamp 145

MOD (F) Sampo Kolkki Reverb Re-route 161

Master Volume Control in Fender Amp Mods 147

Marshall Circuit Overview 150

Bridgeable Volume Controls 151

The Master Volume Control –

Marshall Power Supply and Filter Distribution Network 152

Marshall Amp Mods 153

Hot-Rodding / Restoring the JCM800 Master Volume Preamp 154

Dual-Mode Marshall Switching Circuit 155

Adapting the 2-channel Switching Circuit to JCM800s 157

Adapting the 2-channel Switching Circuit to JCM900s 160

MOD (G) to Model 2550 JCM900s 161

MOD (G) to Model 4100 JCM900s 163

Annex

The Small-Signal Algebra of Triode Gain Stages 166

AC-Circuit Algebra of common Gain Stages 170

Vacuum Tube Modeling for Pspice Simulation 174

Basic network properties –

One Last Look at the Power Function 185

Fender Schematic Chronology 187

Fender Amplifier Bias Data by Model Name 188

Fender Amplifiers – Partial Listings 191

Tweed A, B, C, D, D, F, … –

Brownface 5G, 6G, … 205

Blackface aa763, AB763, … 212

Silverface •68, •69, 70, … 220

My Lonely Triode Pspice Models 223

The Triode pspice Models of Reynolds and Koren 224

Phase Driver Pspice Simulation Files 225

The Naked Champ 227

Crude Power Supply Simulation –

Some Ideas from Ken Fischer on Plexi-ing Metalface Marshall Amps 230

Observations and Recommendations –

Cleaning Up the Metal-face model 1959 SL100 231

Adjusting or Disabling the Marshall Feedback Loop 232

Tubes and Transistors 233

Transistor DC Terminal Characteristics –

Tube Amps and Effects 235

Playing with Effects Live –

About Digital Signal Processing (DSP) Circuits –

Controlling Distortion by curbing Low End response 238

Canadian Made Garnet and Traynor Amps 239

Miscellaneous Layout 243