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Building Valve Amplifiers //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH000-PRE- LIMS.3D – 1 – [1–8/8] 5.5.2004 11:48AM Building Valve Amplifiers This Page is Intentionally Left Blank //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH000-PRE- LIMS.3D – 3 – [1–8/8] 5.5.2004 11:48AM Building Valve Amplifiers Morgan Jones AMSTERDAM · BOSTON · HEIDELBERG · LONDON NEW YORK · OXFORD · PARIS · SAN DIEGO SAN FRANCISCO · SINGAPORE · SYDNEY · TOKYO Newnes is an imprint of Elsevier //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH000-PRE- LIMS.3D – 4 – [1–8/8] 5.5.2004 11:48AM Newnes An imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 200 Wheeler Road, Burlington, MA 01803 First published 2004 Copyright Ó 2004, Morgan Jones. All rights reserved The right of Morgan Jones to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 All photographs (including front cover) by Morgan Jones No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publisher. Permissions may be sought directly from Elsevier’s Science and Technology Rights Department in Oxford, UK: phone: (þ44) (0) 1865 843830; fax:(þ44)(0)1865853333;e-mail:[email protected] completeyourrequeston-lineviatheElsevierhomepage(http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’. British Library Cataloguing in Publication Data Jones, Morgan Building valve amplifiers 1. Audio amplifiers – Design and construction 2. Audio amplifiers – Maintenance and repair I. Title 621.3081535 ISBN 0750656956 Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress For information on all Newnes publications visitourwebsiteathttp://books.elsevier.com Typeset by Integra Software Services Pvt. Ltd, Pondicherry, India. www.integra-india.com Printed and bound in Great Britain Working together to grow libraries in developing countries www.elsevier.com | www.bookaid.org | www.sabre.org //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH000-PRE- LIMS.3D – 5 – [1–8/8] 5.5.2004 11:48AM CONTENTS Preface vi Acknowledgements viii Part 1 Construction 1 1 Planning 3 2 Metalwork for poets 57 3 Wiring 94 Part 2 Testing 169 4 Test equipment principles 171 5 Faultfinding to fettling 258 6 Performance testing 306 Index 349 v //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH000-PRE- LIMS.3D – 6 – [1–8/8] 5.5.2004 11:48AM PREFACE This practical book is intended to bridge the gap between a theoretical circuit diagram and relaxing to the sound of a reliable, well-built amplifier. It is the complement to ‘‘Valve Amplifiers’’ (which has equations in it). One possible way of building an amplifier is to choose the most expensive components on offer, then assemble them on a cus- tom-built chassis using that year’s choice of designer wire. However, the author assumes that you are holding this book because you want to know how to build a valve amplifier that is significantly better than you could afford for the same price ready-made. For that reason, some of the physics that you slept through at school will reappear, but with the valuable bonus that it will allow you to make reasoned choices that improve quality or save money – possibly both. It is undoubtedly easier to do metalwork in a fully equipped machine shop, complete with folding machine, drill press, lathe, mill, and copious hand tools. Nevertheless, it is perfectly possible to do good work with a power drill in a stand and a few carefully selected hand tools. In addition to the standard tech- niques, a number of ‘‘cheats’’ will be shown that allow you to produce work of a standard that appears to have been done by a precision machine shop. This will enable your creation to be a thing of beauty that can be proudly displayed. The rules for good audio construction are not complex. It’s just that there are rather a lot of them. Once the logic is understood, a good layout comes naturally. vi //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH000-PRE- LIMS.3D – 7 – [1–8/8] 5.5.2004 11:48AM PREFACE Even the most carefully considered designs need a little fettling once built. Test equipment ranges from cheap DVMs to oscil- loscopes and spectrum analysers with Gigahertz of bandwidth, to PC-based virtual instruments. They all cost money, but if you understand the operating principles, you can choose which features are worth paying for, which can be safely ignored, and how to use what you can afford to its best advantage. Startlingly, years of experience don’t make the author any less frightened at the instant of first switch-on. Accidents do hap- pen, but there are ways of minimising the quantity of smoke. Sometimes, an amplifier is stubborn and just doesn’t quite work properly, requiring genuine faultfinding. This book is distilled from years of bludgeoning recalcitrant electronics, thumping metal, and sucking teeth at the price of test equipment . vii //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH000-PRE- LIMS.3D – 8 – [1–8/8] 5.5.2004 11:48AM ACKNOWLEDGEMENTS Many individuals and organizations have assisted in the prep- aration of this book. Some even knew that they were doing so. Inspiration came from a variety of sources. Some, simply because they asked significant questions, others because they displayed good technique, others because they generously made equipment available for photography, and yet others because they demonstrated techniques to be avoided . In particular, the author would like to thank Paul Leclerq for proofreading (once again), and for the varied and generous assistance given by Brian Terrell. viii //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH001.3D – 1 – [1–56/56] 5.5.2004 11:45AM PART I CONSTRUCTION This Page is Intentionally Left Blank //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH001.3D – 3 – [1–56/56] 5.5.2004 11:45AM CHAPTER 1 PLANNING In this first chapter, we will investigate how to plan the mech- anical layout of a valve amplifier. At this stage, freedom of choice is unlimited, whereas later it will be restricted. So it is important that the choices and compromises made now are the best ones. Whilst good planning will not save a poor design, poor planning can certainly ruin a good one. Chassis layout Valve amplifiers use a number of large components that must be positioned relative to one another such that the connecting wires between each component are as short as possible, but that the components and their associated wires do not interfere with each other. Chassis layout breaks down into the following considerations: . Electromagnetic induction: Minimising hum induction from chokes and transformers into each other and into valves. Heat: Output valves, etc. are hot and must be cooled. Con- versely, capacitors run cool, and must be kept that way. 3 //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH001.3D – 4 – [1–56/56] 5.5.2004 11:45AM Building Valve Amplifiers . Unwanted voltage drops: All wires have resistance, so the wiring must be arranged to minimise any adverse effects of these voltage drops. Electrostatic induction: Minimising hum from AC power wiring is not often a problem, because even thin conductive foil provides perfect electrostatic screening. Mechanical/safety: Achieving an efficient chassis arrange- ment that is easily made, maintained, and used. Acoustical: Almost all components are microphonic, but valves are the worst. We should consider which components are most sensitive to vibration, and minimise their exposure to it. Aesthetic: The highest expression of engineering is indistin- guishable from art. If you have a superb chassis layout, it will probably look good. Conversely, if it looks poor, it is prob- ably a poor layout . We have a seven-dimensional problem. A poor transistor amp- lifier might be able to hide behind the fence of negative feed- back, but valve amplifiers using an output transformer cannot usually tolerate more than 25 dB of feedback before their sta- bility becomes distinctly questionable. Consequently, layout is critical to performance. The large components are generally the mains transformers, output transformers, power supply chokes, power supply cap- acitors, and valves. The traditional way of deciding how to position them is to cut out pieces of paper of the same size as the components and shuffle them around on a piece of graph paper. Alternatively, the lumps themselves can be arranged and glanced at for a few days until the best layout presents itself. Even better, components can be shuffled around and a chassis designed using an engineering drawing package on a computer, with the enormous bonus that a template of the layout can be printed with all the fixing holes precisely positioned, saving errors in marking out. Although it takes time to draw a valve holder or a transformer precisely, it has only to be done once 4 //SYS21///INTEGRAS/ELS/PAGINATION/ELSEVIER UK/BVA/3B2/REVISES_23-04-04/0750656956-CH001.3D – 5 – [1–56/56] 5.5.2004 11:45AM Chassis layout and you will quickly build up a library of mechanical parts.
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