Fundamentals of Valve Amplifiers

By Stephen Edwards

This document is a knowledge summary of my one and a half years of study and practical research into the operation, testing and repair of valve amplifiers.

• Language: English
• Publisher: Stephen Edwards
• Release date: Nov 3, 2023
• ISBN: 9798223587040

Book preview

Table of Contents

Fundamentals of Valve Amplifiers

Introduction

Contents

Introduction           2

Tommy Flowers/Alan Turing         2

Invention and Application         3

Operation           3

Fleming/Edison           4

Basic Diode           5

Diode Operation           6

Grid Addition           7

12AX7 Valve           8

Valve Constants - Gain, Anode Resistance and Transconductance     9

Root Mean Square          10

Load Lines           11

Cathode Bias           17

Negative Feedback          19

Harmonic Distortion          24

Tetrodes and Pentodes - Screen and Suppressor Grids      25

Valve and Transistor Differences         26

Frequency Dependent Power Feeding        28

Valve and Transistor Distortion Differences       31

The Tremolo Circuit          32

Amplifier Classes - A, B, and AB         39

The Future of Vacuum Tube Technology?        41 

Fundamentals of Valve Amplifiers

Introduction

This document is a knowledge summary of my one and a half years of study and practical research into the operation, testing and repair of valve amplifiers.

The vacuum tube or thermionic valve became a topic of interest for me only in the last year and a half, as a guitarist, and ex telecoms/IT technician, because of its technical history (Bletchley Colossus):

http://en.wikipedia.org/wiki/Colossus_computer

in code breaking during WW2, and that a GPO engineer Tommy Flowers helped build it:

http://en.wikipedia.org/wiki/Tommy_Flowers

http://en.wikipedia.org/wiki/Alan_Turing

"The Enigma of Intelligence - Andrew Hodges

HarperCollins Publishers Ltd; New edition edition (28 Jan 1985)

ISBN-10: 0045100608"

Flowers was asked by Alan Turing (Turing Machine inventor, mathematician and Bletchley code breaking genius) to build the hardware for the first computer which utilised valves as switching elements.

As a musician, the valve has an almost mythical reputation as a superior tonal element in audio equipment, most famously in guitar amplifiers made by Fender and Marshall companies from the 1950s onwards, which are directly related to the way social culture in the West evolved, particularly with the emergence of Rock and Roll and its evolution through 1960s hippie culture into heavy rock through the 1970s and beyond partly because of amplifier design development for the requirement of higher power, louder amplifiers.

I will try to keep this document as easy to read and  layperson friendly from a technical point of view as possible, as I explain a little from many aspects of the history, technical function, use and desirability in audio equipment of these magical little glass devices that have had such a massive yet generally underrated impact on our technical and social development over the last century, being THE key component to enable the existence of 1st generation radio and TV sets.

Invention and Application

The Thermionic Valve has proven to be both enduring and enigmatic over the 110 years of its existence since 1904. It has survived a massive decline after the 1970’s in sales, application and popularity with the impact of the tiny and light transistor that emerged to replace it in all