The purpose of this book is not to add another volume to the many excellent ones available on what makes the vacuum tube work. Rather, it is intended to shed light on the almost completely neglected subject of why these versatile devices sometimes do not work.
Informed scientists and engineers have frequently stated that the life of a vacuum tube in normal service should exceed 5,000 or even 10,000 hours. The fact that some of them do not last this long is well known. The question then is, "Why do they so often give less than their predicted or possible potential?"
J. M. Bridges, Director of Electronics, Office of the Assistant Secretary of Defense, speaking before the RETMA (now EIA) "Symposium on Reliable Applications of Vacuum Tubes" at the University of Pennsylvania in May 1956, said: "It has been demonstrated by service tests that the average number of tube failures per operating hour in two equipments of equal complexity, having approximately the same tube complement, can differ by as much as a factor of ten, due entirely to differences in the thoroughness and completeness of engineering design."