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Table of Contents
Cover image
Front-matter
Copyright
Preface
Dedication
Acknowledgements
Chapter 1. Circuit Analysis
Chapter 2. Basic Building Blocks
Chapter 3. Dynamic Range
Chapter 4. Component Technology
Chapter 5. Power Supplies
Chapter 6. The Power Amplifier
Chapter 7. The Pre-Amplifier
Appendix
Index


Front-matter
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
£3, 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
present thermionic theory in an accessible form without getting too bogged
down in maths. Primarily, it is a book for practical people armed with a
calculator or computer, a power drill and a (temperature-controlled)
soldering iron.
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 excellent
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
class contact – not by committees and paper chases.
Early on, he became a member of the Audio Engineering Society, and has

designed and constructed many valve pre-amplifiers and power amplifiers,
loudspeakers, pick-up arms and a pair of electrostatic headphones.
It is now 18 years since work began on the 1st edition of Valve Amplifiers,
yet much has changed in this obsolete technology 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 – enabling spectrum analysis via a £100 sound card that was the
province of world class companies only 20 years ago. As a happy
consequence, this edition benefits from detailed measurements limited


primarily by the author’s time. Computer modelling is now freely available
– exemplified by Duncan Munro’s PSUD2 linear power supply freeware.
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 Luddite with the patience to access the Internet – 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, other analogue components are dying. Capacitors are the worst
affected 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 have contracted and potentiometers face a similar Darwinian

fate. It is particularly galling to discover a use for Zeners just as major
semiconductor manufacturers stop making them.
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 products on sound quality ( and convenience, because although
nobody mentions 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 – which is perhaps a step forward from the 1980s
when it was FM radio and vinyl.
Note for the MP3 generation: That shiny 120 mm disc was invented for
storing music (such as Beethoven’s 9th Symphony) at far higher quality
than a compressed download. Try it some time – 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, of which he has many colourful memories.


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
appropriately 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 and
trouble to breach the publishing citadel and give the author hugely useful
feedback.


Chapter 1. Circuit Analysis
In order to look at the interesting business of designing and building valve
amplifiers, we need some knowledge of electronics funmentals.
Unfortunately, fundamentals are not terribly interesting, and to cover them
fully would consume the entire book. Ruthless pruning is, therefore,
necessary to condense what is needed in one chapter.
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
particularly poignant loss. All that is left is ordinary algebra, and although
there are lots of equations, they are timid, miserable creatures and quite
defenceless.
If you are comfortable with basic electronic terms and techniques, then
please feel free to go directly to Chapter 2, where valves appear.

Mathematical Symbols
Unavoidably, a number of mathematical symbols are used, some of which
you may have forgotten, or perhaps not previously met:
a≡ b
a is totally equivalent to b
a= b

a equals b
a≈ b
a is approximately equal to b
a∝ b
a is proportional to b
a≠ b
a is not equal to b
a> b
a is greater than b


a< b
a is less than b
a≥ b
a is greater than, or equal to, b
a≤ b
a is less than, or equal to, b
As with the = and ≠ symbols, the four preceding symbols can have a slash
through them to negate their meaning ( a ∋ b, a is not less than b).
√a
the number which when multiplied by itself is equal to a (square root)
an
a multiplied by itself n times. a4= a× a× a× a ( a to the power n)
±
plus or minus
∞
infinity
°
degree, either of temperature (°C), or of an angle (360° in a circle)

∣

parallel, either parallel lines, or an electrical parallel connection
Δ
a small change in the associated value, such as Δ Vgk.

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 induced a charge.)
Charged objects come in two forms: negative and positive. Unlike charges
attract, and like charges repel. Electrons are negative and positrons are
positive, but whilst electrons are stable in our universe, positrons encounter
an electron almost immediately after production, resulting in mutual
annihilation and a pair of 511 keV 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 a matter of which way you look 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
same number of electrons and is positively 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 neutralising the charge.
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 1 C 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, but much handier.
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 per hour through that section.
Similarly in electronics, we are not concerned with the total flow of
electrons since the dawn of time, but we do want to know about electron
flow at any given instant. Thus, we could define the flow as the number of
coulombs 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
usually stated in the following form.
One coulomb is the charge moved by one ampere flowing for one
second.

This is still rather unwieldy, so symbols are assigned to the various units:
charge has symbol Q, current I and time t.

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 higher
potential to a lower potential.
If two points are at the same potential, no current can flow between them.
What is important is the potential difference ( pd).
A potential difference causes a current to flow between two points. As this
is a new property, we need a unit, a symbol and a definition to describe it.
We mentioned work being done in charging the balloon, and in its very


precise and physical sense, this is how we can define potential difference,
but first, we must define work.
One joule of work is done if a force of one newton moves one metre
from its point of application.
This very physical interpretation of work can be understood easily once we
realise that it means that one joule of work would be done by moving one

kilogramme a distance of one metre in one second. Since charge is directly
related to the mass of electrons moved, the physical definition of work can
be modified to define the force that causes the movement of charge.
Unsurprisingly, because it causes the motion of electrons, the force is called
the Electro-Motive Force, and it is measured in volts.
If one joule of work is done moving one coulomb of charge, then the
system is said to have a potential difference of one volt (V).

The concept of work is important because work can be done only by the
expenditure of energy, which is, therefore, also expressed in joules.

In our specialised sense, doing work means moving charge (electrons) to
make currents flow.

Batteries and Lamps
If we want to make a current flow, we need a circuit. A circuit is exactly
that a loop or path through which a current can flow, from its starting point
all the way round the circuit, to return to its starting point. Break the circuit,
and the current ceases to flow.
The simplest circuit that we might imagine is a battery connected to an
incandescent lamp via a switch. We open the switch to stop the current flow
(open circuit) and close it to light the lamp. Meanwhile, our helpful friend


(who has been watching all this) leans over and drops a thick piece of
copper across the battery terminals, causing a short circuit.
The lamp goes out. Why?

Ohm’s Law
To answer the last question, we need some property that defines how much

current flows. That property is resistance, so we need another definition,
units and a symbol.
If a potential difference of one volt is applied across a resistance,
resulting in a current of one ampere, then the resistance has a value
of one ohm (Ω).

This is actually a simplified statement of Ohm’s law, rather than a strict
definition of resistance, but we don't need to worry too much about that.
We can rearrange the previous equation to make I or R the subject.

These are incredibly powerful equations and should be committed to
memory.
The circuit shown in Figure 1.1 is switched on, and a current of 0.25 A
flows. What is the resistance of the lamp?


Figure 1.1 Use of Ohm’s law to determine the resistance of a hot lamp.

Now this might seem like a trivial example, since we could easily have
measured the resistance of the lamp to 3½ significant figures using our
shiny, new, digital multimeter. But could we? The hot resistance of an
incandescent lamp is very different from its cold resistance; in the example
above, the cold resistance was 80 Ω.
We could now work the other way and ask how much current would flow
through an 80 Ω resistor connected to 240 V.

Incidentally, this is why incandescent lamps are most likely to fail at
switch-on. The high initial current that flows before the filament has
warmed up and increased its resistance stresses the weakest parts of the
filament, they become so hot that they vaporise, and the lamp blows.

Power
In the previous example, we looked at an incandescent lamp and rated it by
the current that flowed through it when connected to a 240 V battery. But
we all know that lamps are rated in watts, so there must be some connection
between the two.
One watt (W) of power is expended if one joule of work is done in
one second.

This may not seem to be the most useful of definitions, and, indeed, it is
not, but by combining it with some earlier equations:


So:

But:
So:

We obtain:

This is a fundamental equation of equal importance to Ohm’s law.
Substituting the Ohm’s law equations into this yields:

We can now use these equations to calculate the power rating of our lamp.
Since it drew 0.25 A when fed from 240 V, and had a hot resistance of 960
Ω, we can use any of the three equations.
Using:

It will probably not have escaped your notice that this lamp looks
suspiciously like an AC mains lamp, and that the battery was rather large.

We will return to this later.


Kirchhoff’s Laws
There are two of these: a current law and a voltage law. They are both very
simple and, at the same time, very powerful.
The current law states:
The algebraic sum of the currents flowing into, and out of, a node is
equal to zero.

What it says in a more relaxed form is that what goes in, comes out. If we
have 10 A going into a node, or junction, then that much current must also
leave that junction – it might not all come out on one wire, but it must all
come out. A conservation of current, if you like (see Figure 1.2).

Figure 1.2 Currents at a node (Kirchhoff’s current law).

From the point of view of the node, the currents leaving the node are
flowing in the opposite direction to the current flowing into the node, so we
must give them a minus sign before plugging them into the equation.


This may have seemed pedantic, since it was obvious from the diagram that
the incoming currents equalled the outgoing currents, but you may need to
find a current when you do not even know the direction in which it is
flowing. Using this convention forces the correct answer!
It is vital to make sure that your signs are correct.
The voltage law states:
The algebraic sum of the EMFs and potential differences acting
around any loop is equal to zero.

This law draws a very definite distinction between EMFs and potential
differences. EMFs are the sources of electrical energy (such as batteries),
whereas potential differences are the voltages dropped across components.
Another way of stating the law is to say that the algebraic sum of the EMFs
must equal the algebraic sum of the potential drops around the loop. Again,
you could consider this to be a conservation of voltage (see Figure 1.3).

Figure 1.3 Summation of potentials within a loop (Kirchhoff’s voltage law).

Resistors in Series and Parallel
If we had a network of resistors, we might want to know what the total
resistance was between terminals A and B (see Figure 1.4).


Figure 1.4 Series/parallel resistor network.

We have three resistors: R1 is in parallel with R2, and this combination is in
series with R3.
As with all problems, the thing to do is to break it down into its simplest
parts. If we had some means of determining the value of resistors in series,
we could use it to calculate the value of R3 in series with the combination of
R1 and R2, but as we do not yet know the value of the parallel combination,
we must find this first. This question of order is most important, and we
will return to it later.
If the two resistors (or any other component, for that matter) are in parallel,
then they must have the same voltage drop across them. Ohm’s law might
therefore, be a useful starting point.

Using Kirchhoff’s current law, we can state that:
So:

Dividing by V:


The reciprocal of the total parallel resistance is equal to the sum of
the reciprocals of the individual resistors.
For the special case of only two resistors, we can derive the equation:

This is often known as ‘product over sum’, and whilst it is useful for mental
arithmetic, it is slow to use on a calculator (more keystrokes).
Now that we have cracked the parallel problem, we need to crack the series
problem.
First, we will simplify the circuit. We can now calculate the total resistance
of the parallel combination and replace it with one resistor of that value –
an equivalent resistor (see Figure 1.5).

Figure 1.5 Simplification of Fig. 1.4 using an equivalent resistor.

Using the voltage law, the sum of the potentials across the resistors must be
equal to the driving EMF:
Using Ohm’s law:


But if we are trying to create an equivalent resistor, whose value is equal to
the combination, we could say:
Hence:

The total resistance of a combination of series resistors is equal to
the sum of their individual resistances.
Using the parallel and series equations, we are now able to calculate the

total resistance of any network (see Figure 1.6).

Figure 1.6

Now this may look horrendous, but it is not a problem if we attack it
logically. The hardest part of the problem is not wielding the equations or
numbers, but where to start.
We want to know the resistance looking into the terminals A and B, but we
do not have any rules for finding this directly, so we must look for a point
where we can apply our rules. We can apply only one rule at a time, so we
look for a combination of components made up only of series or parallel
components.
In this example, we find that between node A and node D there are only
parallel components. We can calculate the value of an equivalent resistor
and substitute it back into the circuit:


We redraw the circuit (see Figure 1.7).

Figure 1.7

Looking again, we find that now the only combinations made up of series
or parallel components are between node A and node C, but we have a
choice – either the series combination of the 2 Ω and 4 Ω, or the parallel
combination of the 3 Ω and 6 Ω. The one to go for is the series
combination. This is because it will result in a single resistor that will then
be in parallel with the 3 Ω and 6 Ω resistors. We can cope with the three
parallel resistors later:

We redraw the circuit (see Figure 1.8).

Figure 1.8

We now see that we have three resistors in parallel:

Hence:

We have reduced the circuit to two 1.5 Ω resistors in series, and so the total
resistance is 3 Ω.
This took a little time, but it demonstrated some useful points that will
enable you to analyse networks much faster the second time around:
• The critical stage is choosing the starting point.
• The starting point is generally as far away from the terminals as it is
possible to be.
• The starting point is made up of a combination of only series or parallel
components.
• Analysis tends to proceed outwards from the starting point towards the
terminals.
• Redrawing the circuit helps. You may even need to redraw the original
circuit if it does not make sense to you. Redrawing as analysis progresses