The State of Hollow State Audio (Extract)

The State of Hollow State Audio in the Second Decade of the 21st Century

Richard A. Honeycutt LEARN DESIGN SHARE

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The State of Hollow State Audio in the Second Decade of the 21st Century

â—?

Richard A. Honeycutt

an Elektor Publication

LEARN DESIGN SHARE

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This is an Elektor Publication. Elektor is the media brand of

Elektor International Media B.V. 78 York Street London W1H 1DP, UK Phone: (+44) (0)20 7692 8344 © Elektor International Media BV 2020 First published in the United Kingdom 2020

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All rights reserved. No part of this book may be reproduced in any material form, including

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British Library Cataloguing in Publication Data

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ISBN 978-1-907920-79-0

Catalogue record for this book is available from the British Library

Prepress production: Jack Jamar Graphic Design | Maastricht Printed in the Netherlands

Elektor is part of EIM, the world’s leading source of essential technical information and electronics products for pro engineers, electronics designers, and the companies seeking to engage them. Each day, our international team develops and delivers high-quality content - via a variety of media channels (e.g., magazines, video, digital media, and social media) in several languages - relating to electronics design and DIY electronics. www.elektor.com

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Table of Contents

About the Author

Richard A. Honeycutt has been fascinated by electronics ever since he bought an electronics magazine at a bus station and read it while riding the bus. He began repairing radios and building projects using parts salvaged from old TV’s given to him by an uncle who owned a furniture store. By high school, Richard had appropriated a room in his house for a lab.

Building his own stereo amplifier and speaker systems, repairing the school vacuum-tube PA amplifier, and fixing friends’ tube car radios took up much of his time. He worked as student engineer at WFDD, a 50-kW classical-music FM station, while in college, and earned his First Class Commercial FCC license at the age of 21. Graduating with a B.S. degree in Physics, he worked as a writer of engineering manuals for the Bell telephone System, as proprietor of a retail musical instrument and pro-sound shop, and as president of a company that manufactured portable pro-sound systems. Next came a 20-year career teaching Electronics Engineering Technology at the college level, after which he expanded his audio/ acoustics consulting business to full time. He earned his Ph.D. with a specialization in Electroacoustics in 2004.

Honeycutt has been writing magazine articles for publication in The Audio Amateur, Popular Electronics, Electronics World, Speaker Builder, and audioXpress since he was 23.

He has been vacuum-tube columnist for audioXpress since 2012, and acoustics columnist since 2013. He has published two textbooks: Electromechanical Devices (Prentice-Hall) and Op Amps and Linear Integrated Circuits (Delmar). The recent Elektor book Acoustics in Performance was released in January, 2018.

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The State of Hollow State Audio in the Second Decade of the 21st Century

Table of Contents

Chapter 1 ● O rigins and Destination of Hollow-State Audio . . . . . . . . . . . . . . . . . . . . 12 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Radio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Phonographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Television . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Other Markets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 The Birth of Hollow-State Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Early Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Triodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Tetrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Pentodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Modern Tube Filaments and Envelopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 New Life for Vacuum Tube Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Chapter 2 ● B asics of Hollow-State Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Hollow-State Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Vacuum Tube Biasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Voltage Gain of Amplifier Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 The Frequency Response of Hollow-State Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . 33 Frequency Response Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Low-Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 High-Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Hollow-State Power Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Single-Ended Class-A Amps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Push-Pull Triode Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Choices: Single-ended or Push-Pull; Triodes or Pentodes? . . . . . . . . . . . . . . . . . . 43 Achieving Higher Efficiency with Class AB and B Amplifiers . . . . . . . . . . . . . . . . . . . . 44 Class B Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Transformer-Coupled Push-Pull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Phase Splitters—Paraphase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Differential Amplifier (Long-Tailed Pair) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Single Triode or Split-Load Phase Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Biasing and the AB Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Complete Push-Pull Power Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Chapter 3 ● Commercial Hollow-State Power Amplifiers . . . . . . . . . . . . . . . . . . . . . . 52 Classic Tube Power Amplifier Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Knight 10-W Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Heathkit W5M . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Fisher Model 70 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Dynaco Mark II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

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Table of Contents McIntosh MC75 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Pro Tube Power Amplifier Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Western Electric’s Early Days . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 RCA’s Early Days . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 DuKane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Altec Lansing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Chapter 4 ● Designing Hollow-State Preamplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Controlled-Source Theory in Hollow-State Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Ideal Current Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Controlled Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Active Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Designing Hollow-State Triode Audio Preamplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Tube Choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Tube Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Load Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Capacitor Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Linearity in Audio Preamplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Grid-Current Limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Determining Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Phonograph Preamplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Dealing with Low Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Standardized Equalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Phonograph Pickups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Power Supply Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Tube-Type Microphone Preamplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Mixers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Portable Racks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Modern Tube Microphone Preamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Incorporating Triodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Reflected Plate Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Starved Amplifier Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Triode Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Low Control-Grid Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Adding a Dual Op-Amp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Applying Starved Design to Pentodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Space-Charge Tubes and Their Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Types of Space-Charge Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 A Project to Familiarize Ourselves with Design Using Space-Charge Tubes . . . . . . . . 116 The Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Control-Grid Bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Grid-Leak Biasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Changing Triodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Input Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Increasing the Input Resistance by Going Hybrid . . . . . . . . . . . . . . . . . . . . . . . 122 Bias Adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 Modifying the Circuit to Accommodate Parts-Supply Issues . . . . . . . . . . . . . . . . 125

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The State of Hollow State Audio in the Second Decade of the 21st Century Chapter 5 ● Vacuum Tube Characteristics and Load Lines . . . . . . . . . . . . . . . . . . . . 128 Finding Information on Vacuum Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Data Sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Typical Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Plate and Transfer Characteristic Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Dependence of Parameters on Plate Current . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Selecting the Operating Point for a Hollow-State Amplifier . . . . . . . . . . . . . . . . . . . . 136 First Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 Optimizing the Q-Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 AC Load Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Predicting Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Pentodes and Screen-Grid Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Chapter 6 ● The “Tube Sound” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Understanding Nonlinear Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Nonlinearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Modulation Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Timbre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 Perception of Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 Frequency Response Graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Solid-State Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 The Great Debate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Overdriving Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 How do different Amps distort differently? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 Distortion signatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 Perceptibility of Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Other Possible Determinants of “Tube Sound”—Damping Factor . . . . . . . . . . . . . 158 A Magical “X” Factor”? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 Differences in Amp Sound: how do we find the Truth? (Investigations) . . . . . . . . . . . 159 Accuracy or Pleasantness? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 Subjective Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 Program Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Test Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Differences in Amp Sound—what is the Truth? (Results) . . . . . . . . . . . . . . . . . . . . . 166 The Great Debate: Origin and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 The Gold Standard of Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Differences in Distortion Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 A/B/X Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Audio Pro’s Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 The Search Continues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Choosing a Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

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Table of Contents Chapter 7 ● Power Supplies for Hollow-State Audio Equipment . . . . . . . . . . . . . . . . 172 Overview, Transformers, Rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Sections of the Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Power Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 Voltage Doublers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Half-Wave Doubler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Full-Wave Doubler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 Output Waveforms and Voltage Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Power Supply Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Filament Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Sample Power Supply Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Setting Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Rectifier Type and Transformer Voltages and Currents . . . . . . . . . . . . . . . . . . . . 190 Filter Capacitor, Resistor, and Inductor Values . . . . . . . . . . . . . . . . . . . . . . . . . 190 Bias Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Troubleshooting Power Supplies for Hollow-State Equipment . . . . . . . . . . . . . . . . . . 194 Blown Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Low or Distorted Audio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Hum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Complete Failure to Operate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 Chapter 8 ● Selecting Tube Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Which Tube for Which Purpose? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Pre-WWII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Miniature Tube Types Introduced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Octal Triodes that are Still Available . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 Pentodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 Which Power Tube for Which Purpose? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 Triodes, Tetrodes, Pentodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 Beam Power Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 Power Triodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 Comparing Hollow-State Rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Specifications and Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Swapping Rectifier Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 Chapter 9 ● Output Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 Why Transformer-Couple the Output? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 Description and Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 Specifying Output Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Ideal and Real Output Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Design Parameters: the Physics and Construction of Transformers . . . . . . . . . . . . . . 228

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The State of Hollow State Audio in the Second Decade of the 21st Century Chapter 10 ● Tone Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 The Introduction and Development of Tone Controls . . . . . . . . . . . . . . . . . . . . . . . . 233 Tone Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Electrical Filter Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 “One-Knob” Continuous Tone Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 “Two-Knob” (Bass/Treble) Tone Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 Tone Controls: Yes or No? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Midrange Controls in Instrument Amps, but Not in Home-Stereo Amplifiers . . . . . . . . 238 Guitar-Amp Tone Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 More on Loudness Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Chapter 11 ● Dynamics Processing with Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 What is “Dynamics Processing?” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Operating Parameters of Dynamics Processors . . . . . . . . . . . . . . . . . . . . . . . . . 247 Effects of Dynamics Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 Classic Hollow-State Dynamics Processing Approaches . . . . . . . . . . . . . . . . . . . . . . 251 Voltage-Controlled Attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Voltage-Controlled Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Compressor Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Chapter 12 ● Hollow-State Guitar Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 The Development of Tube Guitar Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 Circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 The Tone Character of Tube Guitar Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 Fundamentals and Harmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 The Effect of the Speaker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 Tone Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 In Which Stage Does Clipping First Occur? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 Distortion in Tube Guitar Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 When the Public Developed a Taste for Distortion . . . . . . . . . . . . . . . . . . . . . . . 275 The Basics of Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Effects of Guitar-Amp Design on Distortion Sound . . . . . . . . . . . . . . . . . . . . . . . . . 281 Preamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 Tone Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Phase Splitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Push-Pull Output Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 Output Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 Overdriving the input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 A Solid-State Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

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Table of Contents Chapter 13 ● Hollow-State Capacitor Microphones . . . . . . . . . . . . . . . . . . . . . . . . . 292 Design and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 Tube Impedance Translators for Capacitor Mics . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 Chapter 14 ● Troubleshooting Hollow-State Equipment . . . . . . . . . . . . . . . . . . . . . . 312 Elementary: What are the Symptoms? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 Sherlock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 Looking at it . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 Deeper and deeper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 Troubleshooting the Tough Ones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Divide and Conquer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Testing Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 Continuity Testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 Emission Testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 Mutual Conductance Testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 Dynamic Conductance Tester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 DC and AC Parametric Testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Appendix A ● Resources (in Order Referenced in Text) . . . . . . . . . . . . . . . . . . . . . . 335 Appendix B ● Tube Data Sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

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The State of Hollow State Audio ● in the Second Decade of the 21st Century

Chapter 1 ● Origins and Destination of Hollow-State Audio Introduction

The electronics revolution that began in the 20th century actually grew from roots starting at least as early as 1878 when William Crookes invented and demonstrated the cathode ray tube. Then in 1883, Edison discovered the “Edison Effect”, in which a current flowed in an evacuated glass bulb with a heated filament and an unconnected metal support structure. Although electrical technology was in place—with practical applications—long before this time, the study and use of electronics, which involves the motion of electrons in a vacuum or semiconductor, began with Crookes.

Radio

In 1900, Reginald Fessenden first demonstrated wireless transmission of a voice by radio.

Marconi’s first successful transatlantic radio communication followed in 1901. Neither Fessenden’s nor Marconi’s experiments involved electronic devices.

In 1904, John Ambrose Fleming created the first useful electronic device: the Fleming Valve, a vacuum diode that outperformed the Branley coherer as a sensitive detector for radio signals. In 1906, Lee de Forest patented the vacuum triode, which was first envisioned as a still-more-sensitive radio detector, but was soon recognized as being capable of amplification of weak signals. In that same year, Fessenden and de Forest transmitted the song “O Holy Night” and a Bible reading on Christmas Eve, using a spark gap transmitter. The Electronics Era had begun! Figure 1-1 shows a DeForest Radiohome receiver.

Figure 1-1: DeForest Radiohome Vacuum Tube Receiver

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Chapter 1 ● Origins and Destination of Hollow-State Audio

Charles Harrold started the first broadcasting station, in San Jose, California, in 1909, also using a modulated spark gap transmitter. This station later became KCBS. Although vacuum tube detectors (rather than crystal diodes) were becoming common about this time, it was about 1920 when RCA began producing the first commercially available radio receivers and transmitters using tubes for detection and amplification.

The sales of radio sets boomed in the 1920s, and although sales declined during the Great Depression, by 1935 there was a radio receiver in two of five US homes. Figure 1-2 shows a 1924-vintage Atwater Kent Radio receiver. As the economy improved, market penetration climbed to four of five by 1938. These early radios typically used seven or more vacuum tubes each. In 1935, the “All-American Five” superheterodyne was introduced, and ultimately most radios used this design, incorporating only five tubes. Figure 1-3 shows a late-1930s Philco radio receiver. Figure 1-4 shows an Arvin All-American Five of slightly newer vintage. After WWII, FM broadcasting gradually caught on, and an AM/FM set usually used six or seven tubes. The ever-increasing number of radios created a large market for vacuum tubes, not only for the manufacture of new sets, but also for replacement. In the 1940s and 1950s, local hardware stores, drug stores, and even barber shops often offered free tube testing to support their sales of replacement tubes.

Figure 1-2: 1924 Atwater Kent Radio receiver

The beginning of the end for the tube market for radio receivers came in 1954 when Regency introduced the first all-transistor radio receiver. Sony followed with a vest-pocket receiver in 1960.

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The State of Hollow State Audio ● in the Second Decade of the 21st Century

Figure 1-3: 1939 Philco Tabletop Radio Receiver

Figure 1-4: Arvin All-American Five Table Radio

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Chapter 1 â—? Origins and Destination of Hollow-State Audio

A smaller market for vacuum-tube radios was AM and FM tuners used in home high-fidelity systems and in radio stations as on-the-air monitors. An H. H. Scott FM tuner is shown in Figure 1-5. Particularly in home hi-fi, new tube units were sold well into the late 1960s.

Figure 1-5: H. H. Scott FM Tuner (Built from a Kit) Phonographs

Phonographs provided another market for vacuum tubes. The first sound recording/reproducing mechanism was the Edison cylinder introduced in 1877. Emile Berliner improved the state of the art with the first flat record in 1887. Both of these machines were mechanical or electromechanical. Even as late as 1925, arguably the best-quality record player was an acoustical model dubbed the Orthophonic, designed by Western Electric and manufactured by the Victor Talking Machine Company. Figure 1-6 depicts this triumph in early-20th-century engineering.

However, in 1925, Brunswick had introduced the Panatrope, and RCA debuted the Victrola, both being vacuum tube players using cone speakers to drive the acoustical horns.

Soon thereafter, engineers recognized the ability of cone speakers to produce good audio (for the time) without the acoustic horn. The resulting smaller electronic phonographs became a great commercial success until the Great Depression all but halted sales. By the mid-1930s, a developing market had sprung up for combination radio/phonographs.

Figure 1-7 shows a portable radio-phonograph made by Admiral.

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The State of Hollow State Audio ● in the Second Decade of the 21st Century

Figure 1-6: Western Electric Orthophonic Record Player

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Chapter 1 ● Origins and Destination of Hollow-State Audio

Figure 1-7: An Admiral Radio-Phonograph

Beginning in 1948, there were several significant improvements affecting the quality of reproduced sound: • Reel-to-reel audio tape recording, based on technology developed by Magnetophon in Germany during and after WWII, helped record companies make and distribute recordings with better fidelity.

• The introduction of the 33 1/3 rpm Long Play (LP) microgroove vinyl record brought lower surface noise and industry-standard (RIAA) equalization to the playback of recorded discs. Classical music fans quickly adopted LPs because, unlike with older records, most classical works would fit on a single LP. • Growth in the number of FM radio stations, with wider audio bandwidth and less susceptibility to signal interference and fading than AM radio, improved the broadcast listening experience.

• Better amplifier designs, such as the classic Williamson push-pull output stage, could provide better frequency response and higher power output capability, reproducing audio without perceptible distortion.

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The State of Hollow State Audio ● in the Second Decade of the 21st Century

These developments were in large part responsible for the “high-fidelity” industry, whose beginning was marked by the introduction of Audio Magazine in 1949 and High Fidelity Magazine in 1951. High-Fidelity (“hi-fi”) products were supplied by Fisher Radio (founded 1945), Heath Electronics (1947), H. H. Scott (1947), McIntosh Laboratories (1949), Marantz (1952), Harmon Kardon (1953), and Dynaco (1954). Later, Eico Electronics, which had been founded in 1945, Pioneer Electronics of Japan, which had been founded as a loudspeaker manufacturer in 1938, and Knight-Kit, a brand of Allied Radio, also entered the hi-fi electronics market. Many of these manufacturers sold products in kit form. The modern home-audio receiver was not yet practical, because of the size it would have had to be. Instead, a typical hi-fi system consisted of an AM/FM (or just FM) tuner; a preamplifier that contained an RIAA-equalized phonograph input as well as an NAB-equalized tape head input, and one or more “line inputs” to accept signals from additional equipment such as external reel-to-reel-tape decks with internal preamps, and even TVs; and a power amplifier. With the advent of stereo disk records in 1958, two preamp and power amplifier channels were needed. This brought the tube count in a hi-fi system to about 23. But even at the peak, sales of hi-fi systems were not very numerous, compared to radios and TVs. One of the finest hi-fi power amplifiers—the McIntosh 275—is shown in Figure 1-8.

Figure 1-8: The McIntosh 275: A Functional Work of Art!

In the 1950s, smaller portable record players became popular. Although the combination radio/phonograph used the same number of tubes as did a radio set, the portable record player typically used only two or three tubes.

Because of the predominance of radio/phonograph combinations early on, and the relatively small size of the hi-fi and portable record player markets later, phonograph players were never a very large part of the vacuum-tube market. However, hi-fi was the one segment of consumer electronics in which new tube designs continued until the late 1960s or early 1970s.

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Chapter 1 ● Origins and Destination of Hollow-State Audio

Television

Unknown to many people, John Baird invented an electromechanical television set in 1924. Receivers using the Baird design were available in the US and England in 1928.

Philo Farnsworth patented the first all-electronic TV system in 1927, and CBS began experimenting with TV in 1931; NBC, in 1932. In 1937, the coronation of George VI of England was televised, as was the Wimbledon Tennis Tournament. Nine thousand TV sets were sold in London that year.

In 1928, RCA began operation of the experimental TV transmitting station W2XBS, later to become WNBT, then WRCA, then WNBC. In the 1930s, when the station began broadcasting the first regularly-scheduled TV programs, such as baseball games, it was received by about 4,000 locally-owned sets. At this time, Dumont, RCA, and GE all manufactured TV receivers, and at least the Dumont models were available as kits. Figure 1-9 shows inside and outside views of a Dumont Model 30 television.

Figure 1-9: Very Early (1938) TV Set

In 1941, there were about seven thousand TV sets in the US, but that number grew to over 9 million by 1950. By 1953, half of all US homes had a TV, and color TV broadcasting began. As of 1960, US television ownership topped 67 million units. The inside of a 1960s-vintage TV is shown in Figure 1-10.

TV sets used far more vacuum tubes than did radios, as well as greater variety in types of tubes. The average older TV might contain 20-25 tubes, although with the introduction during the 1960’s of more multifunction tubes such as compactrons, this number could drop into the low teens. The solid-state revolution moved more slowly into TVs than in

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The State of Hollow State Audio ● in the Second Decade of the 21st Century

radios. As of the late 1960s, many tube TVs were still being sold, and even as late as the 1970s, the high-voltage rectifier, high-voltage regulator, and CRT were still vacuum tubes. The introduction of the first plasma-display TV in 1995 signaled the death knell for vacuum tubes in TVs, and although there were over one billion TV sets worldwide by 1996, no vacuum tubes were used in new sets.

Figure 1-10: Rear Inside View of 1960s Tube-Type TV Other Markets

Although electromechanical computing devices have existed since the mid-nineteenth century, the first electronic computer was the ENIAC, built at the University of Pennsylvania in 1946. This computer used 17,000 tubes, and there was a tube failure, on average, every two days. In the late 1950s, television shows tried to impress audiences with answers from “the UNIVAC electronic brain”. But in spite of the huge numbers of tubes used in each computer, that market was not a major factor in overall sales of electronic tubes.

This definitely-not-portable computer is shown in Figure 1-11.

Long-distance and cable telephone service utilized vacuum-tube amplifiers and equalizers spaced at intervals along the lines, and these did provide a small but significant market for tubes. Industrial controllers, test instruments, and musical instrument amplifiers all required vacuum tubes as well. But by the early 1980s, vacuum tubes had almost disappeared except for very high-power or high-frequency applications, and CRT’s.

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Chapter 1 ● Origins and Destination of Hollow-State Audio

Figure 1-11: The ENIAC (US Army Photo) The Birth of Hollow-State Electronics

The vital place played by vacuum tubes in the development of modern electronics is widely known. Even though they find a restricted range of applications today, the development of radio, TV, movies, home stereo, electronic computers, radar, and many industrial processes absolutely depended upon vacuum tubes. Less widely known is the story of the birth and infancy of the vacuum tube.

Early Tubes

During the 1800s, scientists did much experimentation using glass envelopes that had been partially or completely evacuated of air, and in some cases, partially filled with another gas. Geissler tubes (such as neon lamps) and Crookes tubes were the first ones to become commonly used in laboratories. A Crookes tube, invented by English physicist William Crookes about 1870, consisted of a slightly conical glass enclosure from which as much air as possible had been removed. It had an electrode at each end, and when a high voltage was applied between the electrodes, something traveled from the cathode to the anode. This “something”—at first called cathode rays—was later discovered by J. J. Thompson to be negatively-charged particles. Later, these particles were named electrons.

In 1873, the English professor Frederick Guthrie discovered that a positively-charged, red-hot iron sphere in air would lose its electrical charge over time, although a negatively-charged one would not. This effect was called thermionic emission, but was not understood theoretically at first. In 1883, Thomas Edison was investigating why incandescent lamp filaments first darkened and then burned out at a point very near one of the terminal electrodes. In one experiment, he used a lamp that contained an extra metal support structure that was not connected to the filament. Upon connecting a galvanometer to

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The State of Hollow State Audio ● in the Second Decade of the 21st Century

that electrode, Edison discovered that a small current flowed in the vacuum inside the bulb if the unconnected metal structure was electrically positive relative to the filament.

He patented the Edison Effect, but made no particular effort to understand it, and made little practical use of it.

British scientist William Preece delivered a paper on the Edison Effect bulbs in 1885, and physicist John Ambrose Fleming heard the paper. His interest piqued, he experimented with the Edison Effect and developed the Fleming valve, the first commercial vacuum diode, which he patented in 1904. The valve was first used as a radio detector, being more sensitive and stable than the previously used coherers. In 1906, American engineer Lee de Forest independently invented a vacuum diode, which he patented. Two years later, he found that by adding a third metal structure called the grid, he could make the vacuum detector more sensitive still. Originally called the de Forest valve, or the Audion, we now know this device as the vacuum triode. From the standpoint of today’s understanding of physics, we describe thermionic emission as the neutralization of the positive charge of the hot iron sphere by electrons from gas molecules in the air. The Edison Effect is understood to involve the release of electrons from the heated filament (or cathode) under the influence of an electric field that draws them toward the positive anode (plate, in today’s parlance). Current can flow in only one direction because the anode is not heated to free electrons.

Tubes can have either a directly heated cathode (the filament itself is the cathode) or an indirectly heated cathode (the filament heats a separate metal envelope which emits electrons). Most amplifying tubes have indirectly-heated cathodes.

Triodes

In the triode, the grid may have a positive or negative electrical potential with respect to the cathode. Being much closer to the cathode than the plate, a positive grid exerts considerable pull on the cloud of electrons, accelerating them from the cathode. Since the grid has a small surface area (in modern tubes, it is a spiral of thin wire), most electrons do not strike it, but continue accelerating toward the plate. If the grid has a negative electrical potential with respect to the cathode, the opposite happens, and current from cathode to anode is inhibited. Thus, varying the voltage between the grid and cathode controls the current from the cathode to the plate.

If the grid is made negative enough that current in the tube is cut off, then a small AC voltage applied to it, such as that from a radio antenna, will cause current to flow on positive half-cycles, but no current on negative half-cycles. This “amplified detector” is the application that de Forest saw for the Audion. Today, the triode is most commonly used with the grid biased negatively with respect to the cathode, but not so negatively that current is cut off. Application of a small AC voltage between grid and cathode causes a current of the same waveform to pass through the tube. This current usually flows through a resistor in series between the positive DC supply (called the B+ supply) and the anode, creating an AC voltage drop across that resistor. That voltage can range from a few times to about 100 times the AC voltage applied between grid and cathode. Thus, we have amplification of the AC voltage.

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Chapter 1 ● Origins and Destination of Hollow-State Audio

The basic circuit is shown in Figure 1-12. In typical audio applications, the plate voltage of a triode is in the range from 100 to 300 V. Thus the voltage at the plate is—let’s say—150 VDC with AC superimposed on it. The AC voltage is the product of the AC voltage at the grid × the gain of the stage. So if the stage has a gain of 80, and a 10 mVPP input signal is applied to the grid, the plate voltage will be 150 VDC with 800 mVAC peak-to-peak superimposed. In other words, the instantaneous plate voltage will vary from 149.6 V to 150.4 V. Since the useful part of this composite voltage is the AC part, a capacitor is used between the plate and the output from the stage to allow the AC voltage to appear at the output terminals, while the DC voltage does not. In most cases, a capacitor is used to block DC from the grid, but this is for slightly different reasons. This simple triode amplifier stage is only one of a number of circuits in which a triode can be connected to provide amplification. Later, we will look at some others, and their particular uses.

Figure 1-12: Basic Triode Stage Tetrodes

Walter Schottky, of the German company Telefunken, analyzed the triode to determine whether the gain could be increased. He decided that some of the energetic electrons striking the plate bounced off, creating a cloud of electrons near the plate. These are called the secondary-emission space charge. They neutralize some of the pull of the plate’s positive potential, causing fewer electrons to flow from the cathode. Schottky suggested a screen grid located between the control grid and the plate. A positive potential on the screen grid overcame the repulsion of the space charge, allowing higher gain. Because of the small surface area of the screen grid, little current flowed to it. Schottky patented a tube using this principle, the tetrode, in 1916-1919 (sources vary on the exact date).

The schematic symbol for a tetrode is shown in Figure 1-13. Tetrodes are seldom used in audio electronics.

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The State of Hollow State Audio ● in the Second Decade of the 21st Century

Figure 1-13: Tetrode Symbol Pentodes

Under certain conditions, the tetrode proved unstable, breaking into spontaneous oscillation and distortion. This problem was overcome by the addition of still another grid, dubbed the suppressor grid. The new, three-grid tube was called a pentode. Bernard Tellegen and Gillis Holst of Philips & Company (the Netherlands), and Round of Marconi (England) created pentodes about the same time: 1926. The schematic symbol for a pentode is shown in Figure 1-14.

Figure 1-14: Pentode Symbol

Usually the screen-grid voltage of a pentode amplifier is somewhat (perhaps as much as 100 V) less than the plate voltage. This voltage is taken from the B+ supply via a screen dropping resistor. The suppressor grid is almost always operated at the same voltage as the cathode, and in many pentodes, is internally connected to the cathode, as indicated in the symbol. Work by J. Owen Harries, Cabot Bull, and Sydney Rodda of England led to a modification of the pentode that could handle higher power. This “beam power tube” replaced the suppressor grid by shaped metal plates called “beam-forming electrodes.”

Since beam power tubes function as pentodes, they are usually represented by the same symbol as pentodes. (However, some technical writers refer to beam power tubes as beam power tetrodes, since they, like tetrodes, have only two grids, not three.) The development of the beam power tube occurred in the late 1930s, concluding the introduction of new varieties of vacuum tubes that are important in audio today.

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Chapter 1 ● Origins and Destination of Hollow-State Audio

Modern Tube Filaments and Envelopes

Today, tube filaments can be made of tungsten, which must be heated white hot to emit sufficient electrons. Tungsten filaments can be impregnated with thorium oxide, which has a higher emissivity, and thus emits enough electrons at a lower temperature. Thoriated tungsten filaments glow bright yellow in use. Filament wire can also be made of a nickel alloy wire with a relatively thick coating of alkaline earth elements (beryllium, magnesium, calcium, strontium, barium), which only have to be heated to a dull red glow to emit sufficient electrons. Naturally, tubes using filaments that operate at lower temperatures require less power and produce less heat during use. Indirectly heated cathodes enclose the filament in a metal sleeve coated with a high-emissivity material such as alkaline earth oxides. During the evolution of vacuum tubes, the envelope of the tubes also evolved. The earliest tubes had no standardization in envelope style, shape, or size.

The first standardized envelopes looked much like early light bulbs, with an oval glass envelope and a Bakelite base. Connections were made through four or five metal pins protruding from the base. These were about 4.5” to 5” long. Later variants included the octal (eight-pin base tube), loctal (seven- or eight-pin base), and “miniature” (seven- and nine-pin base versions). There were also a number of tubes, designed for battery-operated equipment, that used cylindrical glass envelopes less than 0.5” in diameter, and had flexible wire leads for soldering to the rest of the circuit.

New Life for Vacuum Tube Technology

There are two areas in which vacuum-tube technology never really died: the former Soviet Union, and the musical electronics market. Tubes continued to be used in new designs for military equipment in the former Soviet Union because of their resistance to destruction by the electromagnetic pulse (EMP) that follows detonation of a nuclear weapon. The EMP would induce voltages large enough to destroy solid-state electronic devices.

Musical instrument amplifiers continued to be built—and even new models introduced— using vacuum tubes, because these amplifiers are often intentionally driven into distortion, and most musicians consider vacuum-tube distortion to be more sonically pleasant than solid-state distortion. Many guitar-amplifier manufacturers include vacuum tube models in their lineups, and there are numerous boutique builders: about 185 in total.

This segment constitutes the primary market for vacuum tubes at present.

There is a third area of prominence for vacuum-tube electronics: high-end home stereo.

Although this market seemed to have vanished in the mid-1970s, the establishment of Conrad-Johnson Design, Inc. in 1977 marked the beginning of a small reversal of the downward trend. Beginning with a preamplifier, Conrad-Johnson introduced a power amplifier in 1978.

They were followed by Quicksilver Audio in Australia in 1981, Cary Audio in 1989, and Vacuum Tube Logic in 1994, with other companies joining in along the way. Although the market for vacuum tube sound reproducing equipment is small—about 1/4 the size of the musical electronics market— there is a dedicated cadre of customers who perceive a difference in the reproduced sound of a tube system compared to a solid-state system.

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The State of Hollow State Audio â—? in the Second Decade of the 21st Century

Today, there are three main manufacturers of vacuum tubes worldwide: Electro Harmonix (US), Shugong (China), and JJ (Slovak republic). Because of private-branding programs, there are literally dozens of tube brands manufactured by these companies. A smaller manufacturer worthy of mention is Western Electric Export Corporation, who now owns the Western Electric trademark, and manufactures some of the proprietary Western Electric vacuum tube models for high-end audio use. They originally used the old Western Electric Kansas City Works factory, but have moved production to Huntsville, AL.

â—? 26

The State of Hollow State Audio in the Second Decade of the 21st Century Richard A. Honeycutt

Richard Honeycutt became interested in electronics when in the sixth grade. By high school, Richard was repairing radios, record players, and PA’s for friends and family.

Richard earned his First Class Commercial FCC license in 1969, at the age of 21.

Working as a technical writer, entrepreneur, sound system designer/installer, electronics repairman, college professor, and consultant, Richard has maintained his interest in both solid- and hollow-state electronics. He has authored two textbooks and two trade books besides The State of Hollow-State Audio.

Vacuum-tube (or valve, depending upon which side of the pond you live on) technology spawned the Age of Electronics early in the 20th Century. Until the advent of solid-state electronics near mid-century, hollow-state devices were the only choice. But following the invention of the transistor (after their process fell to reasonable levels), within a couple of decades, the death of vacuum tubes was widely heralded.

Yet here we are some five decades later, and hollow-state equipment is enjoying something of a comeback, especially in the music and high-end audio industries.

Many issues surround hollow-state audio: • Does it produce—as some claim—better sound? If so, is there science to back up these claims?

• How do hollow-state circuits work?

• How do you design hollow-state audio circuits?

• If hollow-state equipment fails, how do you go about troubleshooting and repairing it?

• Can we recreate some of the classic hollow-state audio devices for modern listening rooms and recording studios?

• How can we intelligently modify hollow-state amplifiers to our taste?

These and other topics are covered in The State of Hollow-State Audio.

ISBN 978-1-907920-79-0

Elektor International Media BV

www.elektor.com

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