Building Valve Amplifiers

647 Pages·2014·67.99 MB·English

by  Morgan Jones (Auth.)

Building Valve Amplifiers
Building Valve Amplifiers
Second Edition
Morgan Jones
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P
REFACE
Asitstitlesuggests,thisbookisconcernedwiththepracticalproblemsofbuildingavalve
amplifier that has already been designed, making it the companion book to Valve
Amplifiers (fourth edition), which contains far more equations and is concerned with
circuitdesign.
This new edition’s word count is more than twice that of the first edition. So where is all
the new material? All chapters have been updated, but noting the most commonly asked
questions, the metalworking and test principles chapters have doubled in size, and the
entirely new practical projects chapter is the second largest. The wiring and performance
testingchaptershavegrownsignificantly,andanappendixappeared.
Althoughconstructiontechniqueshavechangedlittleinthe10yearssincethefirstedition,
measurement techniques and data analysis have been transformed by computers. Digital
oscilloscopes have ousted analogue, and audio measurement systems based on a recording
quality sound card offer an unprecedented performance to price ratio. Analogue circuit
modelling via LTspice has spread from the professional world to the amateur, and the
authortooktoitlikeaducktojuggling.However,perseverancepaidoff,andhenowfinds
it invaluable when dealing with semiconductors and passive components whose character-
istics are tied firmly to fundamental physics. Sadly, valve characteristics are determined
by production engineering, making valve models less accurate, and distortion predictions
distinctlydubious.
Onepossiblewayofbuildinganamplifieristochoosethemostexpensivecomponentson
offer, have a chassis CNC-machined from a single billet of aerospace aluminium alloy
(very popular commercial ploy at the moment), then use whichever designer wire is cur-
rently fashionable. However, the author assumes that you are clutching this book because
you want to know how to build a valve amplifier that is better than you could afford for
the same cost ready-made. For that reason, some of the physics that you slept through at
vii
PREFACE
school will reappear, but with the valuable bonus that it will allow you to make reasoned
choicesthatimprovequalityorsavemoney(cid:1)usuallyboth.
It is undoubtedly easier to do metalwork in a fully equipped machine shop, but even if
domestic harmony precludes machine tools requiring an engine hoist to move them, sur-
prisingly good work can be done by a power drill in a stand plus a few carefully selected
hand tools. Nevertheless, drill presses, lathes, and bandsaws are all now available at ama-
teur prices (and perhaps more significantly, sizes) making them well worth investigating.
In addition to the standard techniques, a number of “cheats” will be shown that allow you
to produce work of a standard that appears to have come from a precision machine shop.
Thiswillenableyourcreationtobeathingofbeautythatcanbeproudlydisplayed.
The rules for good audio construction are not complex. It’s just that there are rather a lot
ofthem.Oncethelogicisunderstood,goodlayoutcomesnaturally.
Even the most carefully considered designs need a little fettling once built. Electronic test
equipmentrangesfrom31/ digitDVMsthroughgigahertzoscilloscopes,megahertzimped-
2
ance analysers and signal generators, back to PC-based virtual instruments. They all cost
money, but once you understand the operating principles, you can choose which features
areworth payingfor, which can besafely ignored,and how touse what you can affordto
itsbestadvantage.
Startlingly, years of experience don’t make the author any less frightened at the instant of
first switch-on. Accidents do happen, but there are ways of minimising the quantity of
smoke. Sometimes, an amplifier is stubborn, and just doesn’t quite work properly, requir-
inggenuinefaultfinding.
This book is distilled from years of bludgeoning recalcitrant electronics, thumping metal,
andsuckingteethatthepriceofgoodtestequipment.
viii
A
CKNOWLEDGEMENTS
The author would like to thank Euan MacKenzie for proofreading (once again), although
heshouldemphasisethatanyremainingerrorsareentirelytheauthor’sresponsibility.
Especial thanks are due to Susan for sustaining the author during the many hours he was
closetedinthelaboratory.
ix
C 1
HAPTER
P
LANNING
Inthisfirstchapter,wewillplanthemechanicallayoutofavalveamplifierandfindthatitis
determinedbyfundamentalphysics.Atthisstage,freedomofchoiceisunlimited,whereasit
willlaterberestricted,soitisimportantthatthechoicesandcompromisesmadenowarethe
bestones.Whilstgoodplanningwillnotsaveapoordesign,poorplanningcancertainlyruina
goodone.Anycomponentthatrequiresholestobedrilledorcutinthechassisshouldundergo
thorough electrical testing before a layout is planned (cid:1) especially if the part is not current
production.Itisgallingtodiscoverthatacomponentisfaulty(andworse,irreplaceable)after
custom metalwork has been done and wiring completed (cid:1) details on component testing are
giveninChapter4.
Chassis layout
A valve amplifier uses a number of large components needing relative positioning that
minimisesthelengthofconnectingwires,yetpreventsthemandtheirwiringfrominterfer-
ingwitheachother.Chassislayoutbreaksdownintothefollowingconsiderations:
1. Electromagnetic induction: Minimising hum induction from chokes and transformers
intoeachotherandintovalves.
2. Heat: Output valves etc. are hot and must be cooled. Conversely, capacitors run cool
andshouldbekeptthatway.
BuildingValveAmplifiers.DOI:http://dx.doi.org/10.1016/B978-0-08-096638-0.00001-1
Copyright©2014ElsevierLtd.Allrightsreserved. 3
BuildingValveAmplifiers
3. Unwanted voltage drops: All wires have resistance, so the wiring must be arranged to
minimiseanyadverseeffectsofthesevoltagedrops.
4. Electrostaticinduction:MinimisinghumfromACpowerwiringisnotoftenaproblem,
because even thin conductive foil provides perfect electrostatic screening, but paths
shouldbekeptasshortaspossible.
5. Mechanical/safety: Achieving an efficient chassis arrangement that is easily made,
maintained,andused.
6. Acoustical: Almost all components are microphonic, but valves are the worst. We
should consider which components are most sensitive to vibration, and minimise their
exposure.
7. Aesthetic:Thehighestexpressionofengineeringisindistinguishablefromart.Ifyouhave
a superb chassis layout, it will probably look good. Conversely, if it looks horrible, it is
probablyapoorlayout...
We have a seven-dimensional problem. A poor transistor amplifier might be able to hide
behind the fence of negative feedback, but amplifiers having an output transformer rarely
tolerate more than 25dB of feedback before their stability becomes distinctly question-
able.Consequently,chassislayoutiscriticaltoperformance.
The large components are generally the mains transformers, output transformers, power
supply chokes, power supply capacitors, and valves. The traditional way of deciding their
positioning was to cut out pieces of paper of the same size and shuffle them around on a
piece of graph paper. Alternatively, the lumps themselves can be arranged and glanced at
overafewdaysuntilthebestlayoutpresentsitselftotheviewer’ssubconscious.
Finally, components can be shuffled around and a chassis designed using an engineering
drawingpackageonacomputer,withtheenormousbonusthatatemplateofthelayoutcanbe
printedwithallthefixingholespreciselypositioned,savingerrorsinmarkingout.Althoughit
takestimetodrawavalveholderoratransformerprecisely,itonlyhastobedoneonce,and
youwillquicklybuildupalibraryofmechanicalparts.Inconsequence,theauthorhasalmost
forgottenhowtoperformtraditionalmarkingoutusingascriber,ruler,andsquare.
It is vital to make the chassis large enough!
This point cannot be emphasised too strongly. Achieving neat construction on a cramped
chassis requires a great deal more skill and patience than on a spacious chassis. There are
many considerations that must be taken into account, so it is vital that this stage is not
4
Chassislayout
rushed. Each of the following design considerations might not make a great difference in
itself,butthesumoftheireffectsisthedifferencebetweenawinnerandan“alsoran”.
Electromagneticinduction
Almost all of the largercomponents either radiate a magnetic field or are sensitive to one.
Not all of a transformer’s primary flux reaches the secondary, so leakage flux might
inducecurrentsintogridwiring,therebydevelopingvoltagesacrossassociatedresistances.
Whether or not these currents and voltages are significant depends on the signal level and
sourceimpedanceatthatpoint,sooutputvalvesarelessofaproblemthantheinputstage.
Couplingbetweenwoundcomponents
Wound components such as transformers and chokes can easily couple into one another,
so hum can be produced by a mains transformer inducing current directly into an output
transformer. Fortunately, the cure isreasonably simple, and may be summarised bya sim-
pleratiowhosevaluemustbeminimised:
cosθ
Induction~
d3
Theangleθanddistancedareshowninthediagram.SeeFigure1.1.
d
θ
Figure1.1
Orientingtransformersforminimumcoupling.
Rotatingtransformercoresby90(cid:3)(cos90(cid:3)50),sothatthecoilofonetransformer(orchoke)
isnotalignedwiththeotherisveryeffective,andtypicallyresultsinanimmediate25dBof
5
BuildingValveAmplifiers
practicalimprovement.Evenbetter,ifonecoilisdrivenfromanoscillatorwhilsttheinterfer-
ence developed in the other is monitored (oscilloscope or amplifier/loudspeaker), careful
adjustmentofrelativeanglescanoftengainafurther25dB.
Because coupling decays with the cube of distance [1], as the distance between offending
itemsisincreased,theinterferencefallsawayrapidly.However,simplyincreasingthegap
between two adjacent transformers from 6 to 25mm does not materially reduce the inter-
ference, because the transformers are typically 75mm cubes, and the spacing that applies
is the distance between centres, which has only changed from 81 to 100mm, resulting in
only 5.5dB of theoretical improvement. Unfortunately, when large transformers are this
close, coupling no longer obeys the inverse cube law because the dipole equation upon
which it is based carries the implicit assumption that the separation is much greater than
dipolelength,soa3dBreduction,orless,ismorelikely.
Anotherconsequenceofsizeisthatifotherlayoutconsiderationsforceanoutputtransformer
andmainstransformertoalmosttouch,weshouldnotonlyensurethattheircoilsareatright
anglestooneanother,butalsoaligntheircentres.Whenthecentresarealigned,eachedgeof
onecoilinducessignificantcurrentintothereceivingcoil,butbecausetheedgedistancesare
thesame, their inducedcurrents are equal and opposite, socancel. Butifone transformer is
slidtooneside(whilstremainingatrightangles),theedgedistancesdiffer,completecancel-
lationnolongeroccurs,andincreasedinductionresults.
Beawarethatthepreviousargumentofedgedistanceequalityandconsequentcancellation
assumes that the transformer manufacturer made each winding fill complete layers. They
usually do, because it avoids difficult winding, but a dual chamber bobbin having the
mains primary in one chamber and secondaries in the other destroys this fundamental
assumptionandonlytestingcandetermineitsoptimumorientation.
Although smoothing chokes are gapped, and therefore inevitably leaky, they don’t gener-
allyhavemuchalternatingvoltageacrossthem,sotheirleakageislow,andtheycanoften
be used to shield output transformers from the mains transformer. The exception to this
rule is the chokeinput power supply, which hasa substantial alternating voltage across its
choke,soitsleakagefieldcanbesignificant.
A poorly designed mains transformer’s core can easily be saturated by the large current
pulses drawn by a large reservoir capacitor in combination with a semiconductor rectifier,
producing a particularly ragged leakage flux, and this can be quickly identified using a
searchcoil(seeChapter4fordetailsonsearchcoilconstruction).SeeFigure1.2.
6