Building Valve Amplifiers PDF
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Building Valve Amplifiers Building Valve Amplifiers Second Edition Morgan Jones AMSTERDAM(cid:129)BOSTON(cid:129)HEIDELBERG(cid:129)LONDON NEWYORK(cid:129)OXFORD(cid:129)PARIS(cid:129)SANDIEGO SANFRANCISCO(cid:129)SINGAPORE(cid:129)SYDNEY(cid:129)TOKYO NewnesisanimprintofElsevier NewnesisanimprintofElsevier 32JamestownRoad,LondonNW17BY,UK 225WymanStreet,Waltham,MA02451,USA 525BStreet,Suite1800,SanDiego,CA92101-4495,USA FirstEdition2004 SecondEdition2014 Copyright©2014ElsevierLtd.Allrightsreserved Nopartofthispublicationmaybereproduced,storedinaretrievalsystemortransmittedinanyform orbyanymeanselectronic,mechanical,photocopying,recordingorotherwisewithoutthepriorwritten permissionofthepublisher PermissionsmaybesoughtdirectlyfromElsevier’sScience&TechnologyRights DepartmentinOxford,UK:phone(144)(0)1865843830;fax(144)(0)1865853333;email: permissions@elsevier.com.AlternativelyyoucansubmityourrequestonlinebyvisitingtheElsevier websiteathttp://elsevier.com/locate/permissions,andselectingObtainingpermissiontouseElseviermaterial Notice Noresponsibilityisassumedbythepublisherforanyinjuryand/ordamagetopersonsorpropertyasa matterofproductsliability,negligenceorotherwise,orfromanyuseoroperationofanymethods,products, instructionsorideascontainedinthematerialherein. Becauseofrapidadvancesinthemedicalsciences,inparticular,independentverificationofdiagnosesand drugdosagesshouldbemade BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress ISBN:978-0-08-096638-0 ForinformationonallNewnespublications visitourwebsiteatstore.elsevier.com PrintedandboundintheUK 14 15 16 17 18 10 9 8 7 6 5 4 3 2 1 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
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