Building Valve Amplifiers, Second Edition

Book Preface

As its title suggests, this book is concerned with the practical problems of building a valve amplifier that has already been designed, making it the companion book to Valve Amplifiers (fourth edition), which contains far more equations and is concerned with circuit design.

This new edition’s word count is more than twice that of the first edition. So where is all the new material? All chapters have been updated, but noting the most commonly asked questions, the metalworking and test principles chapters have doubled in size, and the entirely new practical projects chapter is the second largest. The wiring and performance testing chapters have grown significantly, and an appendix appeared.

Although construction techniques have changed little in the 10 years since the first edition, measurement techniques and data analysis have been transformed by computers. Digital oscilloscopes have ousted analogue, and audio measurement systems based on a recording quality sound card offer an unprecedented performance to price ratio. Analogue circuit modelling via LTspice has spread from the professional world to the amateur, and the author took to it like a duck to juggling. However, perseverance paid off, and he now finds it invaluable when dealing with semiconductors and passive components whose characteristics are tied firmly to fundamental physics. Sadly, valve characteristics are determined by production engineering, making valve models less accurate, and distortion predictions distinctly dubious.

One possible way of building an amplifier is to choose the most expensive components on offer, have a chassis CNC-machined from a single billet of aerospace aluminium alloy (very popular commercial ploy at the moment), then use whichever designer wire is currently fashionable. However, the author assumes that you are clutching this book because you want to know how to build a valve amplifier that is better than you could afford for the same cost 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 usually both.

It is undoubtedly easier to do metalwork in a fully equipped machine shop, but even if domestic harmony precludes machine tools requiring an engine hoist to move them, surprisingly good work can be done by a power drill in a stand plus a few carefully selected hand tools. Nevertheless, drill presses, lathes, and bandsaws are all now available at amateur prices (and perhaps more significantly, sizes) making them well worth investigating. In addition to the standard techniques, a number of “cheats” will be shown that allow you to produce work of a standard that appears to have come from 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, good layout comes naturally. Even the most carefully considered designs need a little fettling once built. Electronic test equipment ranges from 31/2 digit DVMs through gigahertz oscilloscopes, megahertz impedance analysers and signal generators, back to PC-based virtual instruments. They all cost money, but once 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 happen, 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 good test equipment.