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Guide to the

6L6 / 6550

KT66 / KT88

by

Pasquale Russo

 

-The Audio Beam Tetrode Review-

 

 

 

 

1.th Edition : March 2009

 

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. Besides 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 treatment about 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 best 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 1940s 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

 

  • Generality

 

History of the beam power tetrode

7

The tetrode

9

The internal structure of a true screen grid tetrode

9

The beam tetrode

12

The pentode

15

The harmonic and inter-modulation distortion

17

Valve characteristic curves

 

18

  • The 6L6 Family

 

6L6 by RCA

24

6L6s by Others

42

VT-115

45

1614

46

1622

52

6L6G

55

6L6G branded by European makers

60

6L6GX

67

6L6GT

68

6L6GTY

69

VT-115A

70

CV1286

72

CV1947

73

Notes about the CV codes

75

6L6GA

79

6L6GAY

86

6L6WGA / 5932

87

6L6GB

95

6L6WGB / 5881

113

5881, a new beam power tube by C.E. Atkins

124

7581

133

6L6GC

148

7581A

163

350A

175

350B

176

6L6 Family: Ratings comparison

187

Summary of the 6L6 Family

190

Suffixes used in 6L6 Family

193

Notes about US makers codes

194

RCA date codes

195

Tung-Sol date codes

199

Sylvania date codes

199

GE date codes

201

Ken-Rad date codes

 

205

  • KT66

 

GEC story

207

The early KT66

208

MOV/GEC KT66 1940-1950s

209

MOV/GEC KT66 1950s

210

Marconi KT66

211

MOV/GEC KT66 1960s

211

MOV/GEC KT66 1970-1980s

212

MOV / GEC factory codes

238

MOV / GEC date codes

239

CV1075 by GEC

241

CV1075 by Cossor

243

VT-75

 

244

  • 6550

 

6550 by Tung-Sol

245

  - Black plate period

246

  • 1.th  version

247

  • 2.nd version

247

  • 3.th version

248

  -  Solid grey plate period

248

  -  Three hole-grey plate period

249

6550 by Sylvania

251

Notes about Tung-Sol 6550s

 

253

  • KT88

 

KT88 by GEC / MOV

263

  • Early version

264

  • 2.nd version

265

  • 3.th version

266

  • 4.th version

267

  • 5.th version

268

Notes about GEC codes

269

Notes about GEC/MOV made in China

270

CV5220

288

The getter mirror question in GEC/MOV KT88s

 

289

  • 6550 A

 

6550A by GE

291

  • GE 6550A welded plates

292

  • GE 6550A stapled plates

293

  • JAN GE 6550A

294

  • MPD GE 6550A

 

295

  • 6550A / 6550 by Sylvania

 

Sylvania 6550A

305

Sylvania 6550 (Coke Bottle)

 

306

  • 6550 KT88 6550A ratings comparison

 

307

  • Other KT88s

 

KT88 by Tesla

309

  • Early version

310

  • Late version (KT88S)

311

  • RE 40 AK

312

KR AVVT New Emission Labs KT88s

313

KT88 by EAT

313

KT88 by JJ

314

KT88 by Sovtek

315

KT88 by Svetlana

316

KT88 by Svetlana winged C logo (SED)

317

KT88 by Svetlana S logo (New Sensor)

318

KT88 by Electro-Harmonics

319

KT88s made in China

320

KT88SC by Penta Labs

 

320

Alphabetical Index

322

References

327

 

******************************************************************************

OMITTED

******************************************************************************

 

******************************************************************************

OMITTED

******************************************************************************

 

6L6G

 

Introduced by RCA  in 1936, few months later the introduction of the metal 6L6, this tube has the same ratings but higher direct inter-electrode capacitances (see RCAs official datasheets) and different shape.

It has glass bulb, named ST-16  (ST = shouldered or spherical tubular, 16 = the size, which is the widest dimension in eights of an inch).

Base IO (G-7AC), 5-5/16 maximum overall length and 2-1/16 maximum diameter. Contrarily to the metal  version, the pin 1 dont have connection. The valve was made until 1960s.

 

 ******************************************************************************

OMITTED

******************************************************************************

 

 

******************************************************************************

OMITTED

******************************************************************************

 

Notes about the CV codes

Jeremy M. Harmer in his web-site Virtual Valve Museum * published very interesting notes about the CV codes.

* http://www.tubecollector.org/ 

CV stands for Common Valves produced by various tube makers for the British Government Services under his specifications and approval.

Ordinarily on the glass of the valve are printing some symbols, like an arrow, letters and numbers, and it dont carry the manufacturers name or logo.

The CV code is printed within a rectangular shape divided in two sections and it is formed by :

   Upper level on left:  Broad arrow  (Government mark)

   Upper level on right: CV number (which correspond to the number of CV registration)

   Lower level on left:  Date code  (ordinarily two letter code)

   Lower level on right: K (manufactured under British Government specification) or J (made to the JAN or Military specification), followed by B (approved by a UK authority), U (approved by UK and US authorities), D (approved by an Australian authority), or X (approved for MDAP* equipment but that valve dont has received qualification approval). This two-letter was followed by a slash and the code of the manufacturer (one or two-letter).    

 

******************************************************************************

OMITTED

******************************************************************************

SUMMARY OF THE GEC/MOV KT66s

Years

Bulb

Glass

Base

Top / Bottom

1940s

ST-shaped

Grey coated

Black

 

Late 1940s -1950s

ST-shaped  or

 Tubular dome

Grey coated

Black

One L-shaped on top

Late 1950s

Tubular dome

Grey coated

Brown

Two L-shaped on top

Early 1960s

Tubular dome

Clear

Brown

Two L-shaped on top

1964 - 1968

Tubular dome

Clear

Brown

Two bottom ring getter

1969 - 1983

  Tubular dome *

Clear

Brown

Two bottom ring getter

******************************************************************************

OMITTED

******************************************************************************

******************************************************************************

OMITTED

******************************************************************************

 

 

 

 

 

 

 

 

The Guide is available periodically on e-Bay  or soon emailing me.

 

 

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