Guide to the 6L6 / 6550 / KT66 / KT88 / 6550A

FOREWORD

The critical and commercial success of my previous book: Guide to the EL34/6CA7, encouraged me to write this new Guide concerning the Beam Power Tetrodes.

Nobody certainly can deny the enormous importance of the beam tetrodes for the evolution and progress of transmitting and audio electronics devices. Therefore is clearly evident how much is useful an arrangement of this very huge material.

However, description and listing of all beam tetrode would been too long and difficult, so I limited my study to the audio beam tetrodes, omitting any exposition of the beam tetrodes used exclusively in transmitting equipments.

The beam tetrode, though theoretical invention of the English MOV, under the joint ownership of EMI and GEC, may be considered true achievement of American RCA which realized in 1936 its first practical utilisation, introducing the 6L6. This one tube obtained colossal success worldwide and stimulated the US and European makers to realize new better versions. So the English GEC/MOV, only a year later, introduced the KT66.

The American reply to the KT66 was very quick. RCA made new glass versions of the 6L6 and transmitting tubes like the 807, 6V6, 813. Western Electric in 1940’s introduced the 350A and later the 350B.

In 1955 the American Tung-Sol made the 6550, more powerful and stable than the KT66 and in 1957 GEC/MOV produced the KT88, with higher ratings than the Tung-Sol 6550.

The glorious history of the audio beam power tetrode reached the top in 1971, when the American GE introduced in the market the 6550A, having the same output power of the KT88 but lower plate and screen current.

Late versions of the 6550, KT88, 6550A, made in Europe, Russia and China, are generally considered  technically and sonically inferior, although not always it is true.

I spent my efforts collecting and ordering the enormous material about these tubes.   For each tube I reported physical characteristics, chronological evolution, versions, ratings and, when it was possible, the official datasheets. Over 300 color pictures enrich and elucidate the text. Besides many tables can help the reader to memorize information  and main characteristics of every one.

Though I am Italian, I chosen English to reach as many people. I hope that the reader will want excuse my grammatical mistakes, considering my Guide an useful support for the knowledge and identification of this great tube family, without any literal claim. Any advise and criticism will be welcome.   

March 2009         

Pasquale Russo

CONTENTS