Valve Amplifiers

690 Pages·2012·13.66 MB·English

by  Morgan Jones (Auth.)

Valve Amplifiers
Valve Amplifiers
Fourth Edition
Morgan Jones
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Preface
Almost 40 years ago the author bought his first valve amplifier; it cost him
d3, and represented many weeks’ pocket money. Whilst his pocket money
has increased,so have his aspirations, and the DIY need was born.
Although there were many sources of information on circuit design, the
electronics works gave scant regard to audio design, whilst the Hi-Fi books
barely scratched the surface of the theory. The author, therefore, spent much
time in libraries trying to link this information together to form a basis for
audio design. This book is the result of those years of effort and aims to pres-
ent thermionic theory in an accessible form without getting too bogged down
inmaths. Primarily, itis abookfor practicalpeoplearmed witha calculatoror
computer,apowerdrillanda(temperature-controlled)solderingiron.
The author started a B.Sc. in Acoustical Engineering, but left after a year
to join BBC Engineering as a Technical Assistant, where he received excel-
lent tuition in electronics and rose to the giddy heights of a Senior Engineer
before being made redundant by BBC cuts. He has also served time in
Higher Education, and although developing the UK’s first B.Sc. (Hons.)
Media Technology course and watching students blossom into graduates
with successful careers was immensely satisfying, education is achieved by
classcontact(cid:1) notby committees andpaper chases.
Earlyon,hebecameamemberoftheAudioEngineeringSociety,andhas
designed and constructed many valve pre-amplifiers and power amplifiers,
loudspeakers,pick-uparmsandapairofelectrostaticheadphones.
It is now 18 years since work began on the 1st edition of Valve
Amplifiers, yet much haschanged inthis obsoletetechnology since then.
The relentless infestation of homes by computers has meant that test and
measurement has become both cheaper and more easily integrated, either
because it directly uses the processing power of a computer, or because it
borrows from the technology needed to make them. Thus, the Fast Fourier
Transform has become a tool for all to use, from industrial designer to keen
amateur(cid:1)enablingspectrumanalysisviaad100soundcardthatwastheprov-
inceofworldclasscompaniesonly20yearsago.Asahappyconsequence,this
edition benefits from detailed measurements limited primarily by the author’s
time. Computer modelling is now freely available (cid:1) exemplified by Duncan
Munro’sPSUD2linearpowersupplyfreeware.
The spread of Internet trading has made the market for valves truly
global. Exotica such as Loctals, European ‘Special Quality’ valves, and final
generation Soviet bloc valves are now all readily available worldwide to any
ix
x Preface
Luddite with the patience to access the Internet (cid:1) we no longer need to be
constrained to conservative (but expensive) choices of traditional audio
valves. Even better, many of the 1950s engineering books that you thought
had gone forever are available from the second-hand book sellers on the
Internet.
Paradoxically, although digital electronics has improved the supply of
valves,otheranaloguecomponentsaredying.Capacitorsaretheworstaffected
by the lack of raw materials; polycarbonate disappeared in 2001, and silvered-
mica capacitors and polystyrene are both endangered species. Controls have
succumbed to the ubiquitous digital encoder, so mechanical switch ranges
havecontracted and potentiometersfacea similar Darwinian fate. It is particu-
larlygallingtodiscoverauseforZenersjustasmajorsemiconductormanufac-
turersstopmakingthem.
Despite, or perhaps because of, these problems, valves and vinyl have
become design icons, both in television adverts and the bits in between. The
relentless hype from manufacturers of audio servers that favour convenience
over sound quality has forced manufacturers of CD players to justify their
productsonsoundquality(andconvenience,becausealthoughnobodymentions
it, a CD player is unable to wipe your entire music library at the drop of an
operating system). CD and vinyl are now the only reliable sources of quality
audio(cid:1)whichisperhapsastepforwardfromthe1980swhenitwasFMradio
andvinyl.
Note for the MP3 generation: That shiny 120mm disc was invented for
storing music (such as Beethoven’s 9th Symphony) at far higher quality than
a compressed download.Try it some time (cid:1)you might even like it.
Dedication
The author would like to dedicate this book to the dwindling band of BBC
engineers, particularly at BBC Southampton, and also to those at BBC Wood
Norton,ofwhichhe hasmanycolourful memories.
xi
Acknowledgements
Special thanks are due to Euan McKenzie who undertook the onerous task of
proofreading at long distance on short notice and in record time to an appro-
priately low uncertainty.
Thermionic design cannot proceed in a vacuum, so the author is grateful
for the perceptive insights and insults freely offered by Stuart Yaniger over
the recent years.
An annual celebration of awe and wonder has been the European Triode
Festival. This delightfully civilised bacchanalia has humbled the author with
splendid works of art and engineering whilst at the same time reassuring him
that he was not alone. Thank you, Christian, for first inviting me, and even
more thanks to subsequent organisers for successfully maintaining the
momentum.
Finally, the author would like to thank those readers who took the time
andtroubletobreachthepublishingcitadelandgivetheauthorhugelyuseful
feedback.
xiii
Chapter 1
Circuit Analysis
In order to look at the interesting business of designing and building valve
amplifiers, weneedsomeknowledge ofelectronicsfunmentals. Unfortunately,
fundamentals are not terribly interesting, and to cover them fully would con-
sume the entire book. Ruthless pruning is, therefore, necessary to condense
whatisneededinonechapter.
It is thus with deep sorrow that the author has had to forsaken complex
numbers and vectors, whilst the omission of differential calculus is a particu-
larly poignant loss. All that is left is ordinary algebra, and although there are
lots ofequations, they are timid, miserable creatures andquite defenceless.
If you are comfortable with basic electronic terms and techniques, then
pleasefeel free togo directly to Chapter 2, where valvesappear.
MATHEMATICAL SYMBOLS
Unavoidably, a number of mathematical symbols are used, some of which
you may have forgotten, orperhaps notpreviouslymet:
a(cid:1)b a istotally equivalent to b
a5b a equals b
a(cid:3)b a isapproximately equal tob
a~b a isproportionalto b
a6¼b a isnotequal tob
a.b a isgreater than b
a,b a islessthan b
a$b a isgreater than, or equal to,b
a#b a islessthan, orequal to,b
As with the 5 and 6¼ symbols, the four preceding symbols can have a
slashthroughthem tonegatetheir meaning (a3b, aisnot lessthan b).
ValveAmplifiers.DOI:10.1016/B978-0-08-096640-3.00001-0
©2012ElsevierLtd.Allrightsreserved. 1
2 ValveAmplifiers
Oa the number which when multiplied by itself is equal to a (square
root)
an a multiplied byitselfn times. a45a3a3a3a (a tothe powern)
6 plus or minus
N infinity
(cid:4) degree,either of temperature ((cid:4)C), orof anangle(360(cid:4) in acircle)
j parallel, either parallel lines, oran electrical parallel connection
Δ a small change inthe associated value, such asΔV .
gk
ELECTRONS AND DEFINITIONS
Electrons are charged particles. Charged objects are attracted to other
charged particles or objects. A practical demonstration of this is to take a
balloon, rub it briskly against a jumper and then place the rubbed face
against a wall. Let it go. The balloon remains stuck to the wall. This is
because we have charged the balloon, and so there is an attractive force
between it and the wall. (Although the wall was initially uncharged, placing
the balloon on the wall induceda charge.)
Charged objects come in two forms: negative and positive. Unlike
charges attract, and like charges repel. Electrons are negative and positrons
are positive, butwhilst electrons are stable in ouruniverse, positronsencoun-
ter an electron almost immediately after production, resulting in mutual anni-
hilationand apair of511keV gamma rays.
If we don’t have ready access to positrons, how can we have a positively
charged object? Suppose we had an object that was negatively charged,
because it had 2,000 electrons clustered on its surface. If we had another,
similar, object that only had 1,000 electrons on its surface, then we would
say that the first object was more negatively charged than the second, but as
we can’t count how many electrons we have, we might just as easily have
said that the second object was more positively charged than the first. It’s
just amatterof which way youlook at it.
To charge our balloon, we had to do some work and use energy. We had
to overcome friction when rubbing the balloon against the woollen jumper.
In the process, electrons were moved from one surface to the other.
Therefore, one object (the balloon) has acquired an excess of electrons and
is negatively charged, whilst the other object (woollen jumper) has lost the
samenumberofelectrons and ispositively charged.
The balloon would, therefore, stick to the jumper. Or would it? Certainly
it will be attracted to the jumper, but what happens when we place the two
in contact? The balloon does not stick. This is because the fibres of the
jumper were able to touch the whole of the charged area on the balloon, and
the electrons were so attracted to the jumper that they moved back onto the
jumper, thus neutralisingthe charge.
Chapter | 1 CircuitAnalysis 3
At this point, we can discard vague talk of balloons and jumpers because
we have just observed electron flow.
An electron is very small, and doesn’t have much of a charge, so
we need a more practical unit for defining charge. That practical unit is the
coulomb (C). We could now say that 1C of charge had flowed between one
point and another, which would be equivalent to saying that approximately
6,240,000,000,000,000,000 electrons had passed, butmuchhandier.
Simply being able to say that a large number of electrons had flowed
past a given point is not in itself very helpful. We might say that a billion
cars have travelled down a particular section of motorway since it was built,
but if you were planning a journey down that motorway, you would want to
know the flow of cars perhour through that section.
Similarly in electronics, we are not concerned with the total flow of elec-
trons since the dawn of time, but we do want to know about electron flow at
any giveninstant.Thus, wecoulddefine the flow asthe numberofcoulombs
of charge that flowed past a point in one second. This is still rather long-
winded,and we will abbreviate yet further.
We will call the flow of electrons current, and as the coulomb/second is
unwieldy, it will be redefined as a new unit, the ampere (A). Because the
ampere is such a useful unit, the definition linking current and charge is usu-
ally stated in the followingform.
Onecoulombisthechargemovedbyoneampereflowingforonesecond.
chargeðcoulombsÞ5currentðamperesÞ3timeðsecondsÞ
This is still rather unwieldy, so symbols are assigned to the various units:
chargehas symbol Q, current I andtime t.
Q5It
This is a very useful equation, and we will meet it again when we look at
capacitors (which store charge).
Meanwhile, current has been flowing, but why did it flow? If we are
going to move electrons from one place to another, we need a force to cause
this movement. This force is known as the electro motive force (EMF).
Current continues to flow whilst this force is applied, and it flows from a
higherpotential toa lower potential.
If two points are at the same potential, no current can flow between
them. What isimportant isthe potentialdifference(pd).