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Revisitingtheampthatgave"Solid-State"abadname:the

FenderSolid-StateTwinReverb

Tilmann Zwicker, Helmut Lemme & Manfred Zollner, GITEC (https://gitec-forum.de/wp/en/ & https://www.gitec-forum-eng.de)

Part2Circuitdesign

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5.Electroniccircuitdesignanalysisandmeasurements5.0CircuitdiagramsandsomegeneralobservationsTheFendercustomer-supportdepartmentshowedexemplarycommitmentinfindingcircuitdiagramsfortheSolid-StateTwinReverb(abbreviatedinthefollowingwithSSTR)-thisismuchappreciated.Unfortunately,theresolutionofsomeofthescans(orthequalityoftheoriginaldiagrams)isnotveryhighsoitisdifficulttomakeoutsomeofthedetailsandcomponentvalues.Neverthelesstheoverallstructureisclear.Thediagramswereceivedarefoundintheappendix.Excerptsareshownwiththecorresp.sectionsastheyarediscussed.Ontheprintedcircuitboard,therearesomespacesforcomponentsleftempty.PossiblytheywerereservedforotherversionsoftheSSamplifiers,sincethesamecircuitboardwasusedfortheSSProReverb,SSSuperReverb,andtheSSTwinReverb,withonlytheSSTwin-Reverbhavinga"StyleControl"inbothChannels.WehavenotverifiedindetailwhethereverycomponentandeveryconnectioninourSSTRwasamatchtotheavailablecircuitdiagrams.ItisinanycasehighlylikelythattherewereseveralcloselyrelatedbutnotidenticalversionsofthisseriesoftheFendersolid-stateamplifiers-aswasalwaysthecaseforFendertubeamps,aswell.Anydifferencesbetweentheversionswillbe,however,sosmallthatthediscussionsandconclusionselaboratedbelowholdingeneralforalltheampsoftheseries.5.1Preamp(1st&2ndstage,volumecontrol&"Bright"-switch)inbothchannelsThefirstthingthesignalfromtheguitarsignal"sees"enteringtheamplifierisveryfamiliar:thepassivecircuitwithinputs"1"(highsensitivity&impedance)and"2"(lowersensitivity&impedance),2decouplingresistorsof68kΩeach,and1MΩtoAC-ground(Fig.10,left-handsection).Thereisanadditionalcouplingcapacitorthatmosttubeampswouldnothave,buttheresultinghigh-passhasacutofffrequencyof15Hz–lowenoughnottobeofconsequenceforaguitarsignal.

Fig.10:LefttorightInputcircuit,FETimpedanceconverter,firstgainstageand"Volume"-controlwith"Bright"switch

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ThefirstactivecomponentintheSSTRisamostpleasantsurprise:afieldeffecttransistor(FET,Fig.10,center)performsanimpedanceconversion.ItspresenceindicatesthattheengineersatFender–evenunderCBSreign–knewwellwhatanelectricguitarwithmagneticpickupsrequires.Toretainitscharacterandsignalquality,themagneticpickupneedstoseeahighimpedance/5/–whichisjustwhattheFETdelivers.Withit,theinputimpedanceoftheampisdeterminedmainlybytheresistorconnectedbetweentheinput-lineandground.Thisresistoramountsto1MΩ–exactlywhattheFendertubeampshadattheirinputs.Wehavemeasuredaninputimpedanceof800kΩ...closeenough.So:"Welldone,designteam!"-thisisbetterthanwhatmanyothertransistorampsoftheerafeatured...lowimpedanceinputswereunfortunatelydesignedintotransistorampsalltoooften.ItshouldalsobenotedthatatthetimewhentheFendersolid-stateserieswasdesigned,FETwereabrand-newdevelopment.Toincludesuchatruestate-of-the-artcomponentisquitearemarkablefeatforaguitaramplifiercompany.ThiskindofapproachwasapositiveaspectofthefactthatCBSbroughtinnewengineers(someofthemwithexperienceinaerospacedevelopment,/3/)tobuildupanR&D-team.Incidentally,some(nonlinear)characteristicsofFET'saremuchmoretube-likethanthecorrespondingcharacteristicsinbipolartransistors.Whetherthisaspectwasknownorplayedanyrolewhentheampwasdesignedisunknown.TheFETdoesnotprovideanygain,itssolepurposeisthehighimpedanceattheamplifierinput.AsideeffectofthisisthattheFETwillnotbeeasilyoverdrivenandanytransientphenomenaduetore-chargingoftheinputcapacitorwillbeminimal–averycleaninputstageindeed.Thesecondstage(Fig.10,,centerright)consistsofabi-polartransistorandhasconsiderablegain.ItfeedstheVolumecontrollocatedquiteearlyintheoverallcircuit.Thisapproachcancreatesomenoiseissues(andindeedtheampisrelativelynoisy)butperformsverywellinkeepingdistortionlow.Becausethevolumecontrolinfluencesalldrivelevelsofthesubsequentstages,anyuncontrollableoverdrivecanonlyhappeninthefirstgainstage-andhereonlywithverystrongpickups.

Fig.11:Frequencyresponsesgeneratedbyactivatedthe"Bright"-switchinvariouspositionsofthevolumecontrol.Thehigherthevolumeisset,thelesseffectivethe"Bright"-functionbecomes-justlikeinallFendertube-amps.Actualmeasurementoftheoverallfrequencyresponseoftheampfrominputtooutput(16Ωload).

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Thevolumecontrol(Fig.10,left-handsection)canbebridgedbythe"Bright"capacitorviathe"BrightSwitch"-thiscircuitcorrespondsexactlytotheoneusedinFendertubeamps-itismerelymappedtothelowerimpedancelevelsintheSSTR.AsisthecaseinFendertubeamps,theeffectofthis"BrightSwitch"isverystrongatlowvolumesettingsbutdecreasesasthevolumeisturnedupandvanishesatfullvolume(Fig.11).Theelectronicswherelowsignallevelsareprocessedhaveanextrashieldtoreduceinterference(seeFig.7ontheright-handsideofthepreampmodule).5.2Intermediateamps(includingStyleSwitch)inbothchannelsThenexttransistorstageprovidesmoregain,andalsodrivesthe"StyleSwitch"thatauthorTZadmiredsomuch(atleastonpaper)backintheday.ItisnoteworthythatneverbeforeorafterdidFenderamplifiersincludesuchamysteriousandseeminglycool(butalsodisappointing-seebelow)feature."Style"isanimportantissueinmusic.Ifanamplifieroffersachoiceof"styles",wecanassumeitwilladaptitssoundtothespecificneedsofamusicianplayinginthegivenstyle.Ofcourse,whatisattributedtoa"style"sound-wisewilldependmuchonthebackground,experienceandresultingexpectationsofanygivenuser.Today,wewouldprobablyexpecteitherasoftsound,orabright,clearbutnotveryaggressivesoundfor"Pop",araunchy,distortedsoundfor"Rock'n'Roll"andabright,cuttingsoundfor"Country".Thesearequitedifferentsoundsthatrequiredifferentlinearandnon-linearprocessing-i.e.intermsofcircuitry,wewouldsurmisethat"style"isseriousbusinesswithquiteabitofchangeinthecharacteroftheamplification.Asthefirstinthelineofitscontrolelementsineachchannel,theSSTRprominentlyoffersa"Style"-switch,offeringthreepositions:"Pop","Normal"and"CW/RR".Fromamodernperspective,thislabelingtriggersthreethoughtsimmediately:-a)whatonearthmight"Normal"indicate?Whatisa"Normal"style?Wouldanymusicianwanttobelabeled"Normal"?Wouldwealwaysseektobespecialratherthannormal?-b)whyisCountryandRock'n'Rollgivenoneandthesamesetting?Aren'ttheseREALLYdifferentstyles?Well...todaytheyare–butbackintheday,maybebothwerejust"bright"?-c)whataboutotherstyles?Ifthereis"Pop"and"CW",shouldn't"Jazz"besomehowalsobeinthere?Could"Normal"indicate"Jazz"?No,thatcan'tbe...whenwasJazzever"normal"?The1968and69Fendercatalogs/9//10/relateinthesection"specialamplifierfeatures"thatthestyleswitchregulatesthemiddleregister...inanycase,weareleftabitpuzzledbythisstyleswitch,andwebettertakealookattheassociatedschematics(and–later–listen,aswell).

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Fig.12:Secondgainstage(right)with"Style-Switch"(center)andcommon-collector-driverforthenextstage(right)

Itturnsoutthat,circuit-wise(Fig.12),thestyleswitchismerelyalinearfilterofthesimplestsort.Theswitchinteractswiththeprecedingtransistorandmakesaconnectionofeitherthecollectorortheemitterwitharespectivecapacitor(toground).Thishastheeffectofatreblecutfortheformerconnection("Pop")oratrebleboostforthelatter("CW/RR").Inamiddleposition,neithercapacitorisconnectedi.e.thetransmissioncharacteristicfor"Normal"isflat.Therearesomeresistorsinthecircuit,aswell,buttheseonlyhavethejobtoavoidanybuildupofDCinthecapacitorsandtosuppressany"popping"thatcouldappearwhentheswitchpositionischanged.Fig.13showsthemeasuredfrequencyresponsesforthethreeswitchpositions.

Fig.13:Effectofthe3settingsofthe"Style"-switchonthefrequencyresponse."Normal"correspondstothegreenlinei.e.noeffect.Actualmeasurementoftheoverallfrequencyresponseoftheampfrominputtooutput(16Ωload).

Therefore,the"Style"-Switchisabitofalet-down...andaswewillhear,therespectivedifferencesinsoundmatchourdisappointment.Theyareuseful,butthelabels"Pop"and"CW/RR"raisedtheexpectationstoohigh–andtheexplanationinthe1968and1969Fendercatalogsissimplywrong:it'satreble-filter,notoneformiddle.

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5.3Tonecontrolstagesinbothchannels(withalittlebitofhistorythrownin)The"Style"-switchwasaFender"first"(and"last",sinceitdisappearedwiththisseriesofsolid-stateamps),indicatingthatFenderreallytriedtodosomethingdifferentwiththeseamps.Thisapproachfullycontinueswiththetone-controlstage.Thatthebass-andtreble-controlsarelabeled"cut","flat"and"boost"isafirstindicator–butlet'sbranchoutabitandlookattraditionalFender–andother–tonecontrolsfirst.Duringthe1950's,Fenderexperimentedalotwithtonecontrols.The(tube)ampsprogressedfromasingle"Tone"controlofferingmerelycutting(andsometimesalsoboostingtosomeextent)thetreblerangeviaseparatebassandtreblecontrolstotheclassictone-stackappearingforthefirsttimeinthe1958Bassmanamp.ThiscircuitwassosuccessfulthatallsubsequentFendertubeampsbasedtheirtonecontrolcircuitsonit.Theinterestingaspectofthe"tonestack"(ase.g.foundontheclassictubeTwinReverb)isthatitdoesnotallowforcuttingeitherbassortreblefrequenciesrelativetothemiddlefrequencyregion,seee.g./6/.Evensetat"0",thebass-andtreble-rangesarenotattenuatedbutmerelyleftattheminimumoverallattenuationthatthecircuitinherentlyhasdependingonthesettingofthe"Middle"control.Turningup"Bass"or"Treble"willemphasizetherespectivefrequencyrange–i.e.evensetting"Bass"or"Treble"tolowvaluessuchas"3"willalreadyemphasizebassandtreblerelativetothemiddlefrequencies.Thetone-stackwillthusgenerateanotchatthemiddlefrequenciesassoonasbassandtrebleareturnedup.Thedepthofthenotchdependsonthesettingofthe"Middle"-control.Thelattercanhoweverneveractuallyemphasizethemiddlefrequencyrangebutonlycutit;itthushasitsimpact"theotherwayround"comparedtothebassandtreblecontrols.Thetonestackworkssowellforelectricguitarsbecauseitnevergeneratesa"thin"or"nasal"soundbutalwaysleavesahealthydoseofbassandtreblewhileenablingtheguitaristtotuneoutsome"muddy"midrangeviatheMiddlecontrolifnecessary.Itisdifficulttoreallydial-inabadsoundwiththetone-stack-typicallyonceyouplugin,youhaveadecentsoundirrespectiveofthesettingsandcanoptimizeitfurtherwiththecontrols.NowonderFenderkeptthiscircuitonceitwasdiscovered,andnowonderthatalmostallamplifiersanywherehaveaversionofthistonestackattheheartoftheirtonecontrolcircuitry.Itisagreatmatchforelectricguitarandgives-withits"automatic"helpfulcoloringoftheguitarsound–immediategratificationwithtendencytoalwayscutthroughtheauditory"clutter"thatoftenoccursinsituationsofpeopleplayingtogether.InHiFi-audio,ontheotherhand,theFendertone-stackwasneverevenconsidered–exactlyforthesamereason:itcolorsthesound!Forguitar,wheretheampispartofthemusicalinstrumentproducingthesound,youwantthat,butinHiFi,whereyouwantre-producerecordedsoundneutrally,thisisdeath.InHiFi,youwanttostartwithanunaffectedsoundandpossiblemodifythattocompensatefordeficienciesintheloudspeakersorinthelisteningenvironment.Therefore,inthisarea,adifferentkindoftonecontrolhasbeenuniversallyadopted:theso-calledBaxandall-circuitnamedafterPeterBaxandallwhodesignedandpublisheditin1952.Thiscircuithasanoverallattenuationthatcreatesaflatfrequencyresponsewiththebassandtreblecontrolssetmidwayto"5".Asbassandtrebleareturnedupordowntherespectivefrequencyrangeisactuallycutorboostedrelativetothemiddlefrequency.The

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overallattenuationistypicallycompensatedsothatat"zero"-positionofbass-andtreble-controls,allfrequenciescanpassunaffectedinthesamewaywithoutanycoloringofthesound.As"Bass"and"Treble"areturnedup,abroad"notch"appearsforthemiddleofthespectrum,andastheyareturneddown,themiddleprominentlyremainsineffectcreatingamid-boost.TheoneamplifiercompanythatworkedwiththeBaxandallcircuitalmostfromtheget-goandneverreallydeviatedfromit(atleastduringtheir"classic"years)wastheAmpegcompany.TheoriginalgoaloffounderEverettHullwastoamplifytheacousticbasswithoutchangingthetonalcharacter.Hethereforewashappytohavethe"neutral"BaxandallcircuitandincludeditinthelegendaryAmpegB-15ampandallitsderivatives.ItalsofounditswayintotheclassicAmpegguitarampsreachingtheirgloriouspeakintheAmpedSVTandV4(againwithalltheirderivatives).Inthelatteramps,theBaxandallcircuitwascomplementedbyanactivemidrangecontrolallowingforboostandcutat3selectablefrequencies,andbyhigh-andlow-frequencyboost-switches.AnybodynotconvincedthatthistonecontrolworkedextremelywellonlyneedstolistentotheRollingStones'1970"GetYerYa-Ya'sOut"liverecording.ItdoesnotgetmuchbetterthanthesoundMickTaylorachieveswiththeAmpegs.Butlet'sgetbacktooursolid-stateFender.Itwouldappearthatthedesignteam,lookingforsomethingdifferentcomparedtothetraditionalFenderamp(andsound),andpossiblyhavinginmindtoarriveatamoreflexibleamp,optedtouseatonecontrolbasedontheBaxandall-circuit.TheresultisshowninFig.14:amodificationonthebass-sideofthenetworkandtheuseofafeedbackpathfromthenexttwotransistors(thefirstprovidinggainandthesecondbeingacommon-collector-type-buffer)insteadofgroundasthereference.Thisisreallyquiteelaborate,moderndesignworkagainemphasizingthattheengineersworkingonthesolid-stateampsatFenderwerenoslouches.

Fig.14:Tonecontrolnetwork:avariationonthewell-knownBaxandall-circuit

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Intermsoffrequencyresponse(Fig.15),thisisaninterestingcircuit:turningupbothbass-andtreble-controlsyieldsafrequencyresponsenotentirelyunlikethatofthetypicaltone-stack.Turningdownbothcontrolgivesanewresponsewithbothanactualbass-andtreble-cutnotpossiblewithatraditionaltone-stack.Withbothcontrolsinthecenter,wegetaratherflatfrequencyresponse.

Fig.15:FrequencyresponsesachievablewiththeSSTRtonecircuit.Blue:treble-controlatmax.andmin.withbass-controlsetto"flat";black:bass-controlatmax.andmin.withtreble-controlsetto"flat";green:bothcontrolssetto"flat".Actualmeasurementoftheoverallfrequencyresponseoftheampfrominputtooutput(16Ωload).

Thetreble-controlishighlyeffective,workingacrossanextendedfrequencybandfrom300Hzupwardsandmaxingoutatboostandcutlevelsofabout20dBfrom2kHz(thecutrangeextendsevenfurtherathigherfrequencies).Thebass-control,ontheotherhand,isabitlacklusterwithlessthan10dBboostandcutatverylowfrequencies(andevenlessatguitar-relevantfrequenciesofabove80Hz).However,bringinginthefrequencyresponsesoftheStyle-controlandtheBright-switch(bothofwhichoperateathigherfrequenciesupwardofabout2kHz,overallfrequencyresponsesarepossiblewiththisamp(especiallyinthetreblerange)thatwouldindeedputtheoldtone-stacktoshame.Whetherthesearefrequencyresponsesthatareactuallydesirablefortheguitaristisanentirelydifferentmatter.

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5.4Mixer-stageforbothchannelsThemixerstage(Fig.16)isnothingspecialunlessmultiplecommon-collectordriversarecounted:eachchannelhasoneofthem,drivingapassivemixerwithtwo12-kΩ-resistorssummingthesignalsfromthetwochannels,andfeedtheresultingsignaltoyetanothergain-stage-into-common-collector-drivercircuit.Thelatterprovidestheoutputofthepreamplifiermodule.

Fig.16:Mixerstageandpreamp-outputamplifier.Notemultiplecommon-collectordrivers.5.5Reverbcircuit("Vibrato"channel)Thereisnotmuchtoreportregardingthereverbcircuit(Fig.17a).Againstageisfollowedbyasmallpush-pullpowerampthatdrivesthereverbspring,thelatterhopefullyhavingawellmatchedtoalow-impedanceinput(wedidnotmeasurethis).Atwo-stagerecovery-amp(Fig.17b)takescareofgivingthereverbsignalthenecessarylevels.Ifanything,thefollowingisnoteworthy:-thepush-pullstageofthereverbdriverincludesTHEONEPNP-transistorinthewholeamplifier-allothertransistorsareoftheNPN-kind.-thereverbinputsignalistakenfromaheadofthetone-controlsignal(thelinebranchingoffaheadofC26;left-handsideofFig.12),andthereturnsignalisfedtothecircuitafterthetonecontrol(thelinejoiningresistorsR44andC30inFig.12),i.e.thereverbis-forbetterorworse–notsubjecttothetonecontrols.-atleastinourSSTR,thereverbisoverlystrong–itisdifficulttosetthecontroltoobtainareverblevelthatcouldbecalledsensibleinmostmusicalsettings.Thereverbsignalalsoseemssomehowoverdriven.Weweretemptedtomodifytheampaccordinglybutoptednottodosobecausewedidnotwanttoaltersuchahistoricpiece,afterall.

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Fig.17a:Reverbdrivercircuit.NotetheonePNP-transistorinthewholeamp!

Fig.17b:Reverbrecoverycircuit.Possibly,the3,3-kΩ-resistoratthereverbcontrolistheculpritthatmakesthereverbdifficulttocontrol.Incidentally,thisisoneofthefewcomponentssolderedinbyhandonthereversesideofthecircuitboard...

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5.6Tremolocircuit("Vibrato"channel)Again,thereisnotanythingveryspecialaboutthetremolocircuitry.ItisstructurallyverysimilartothecorrespondingcircuitsintheFendertubeamps.Atransistorwithaphase-shifting-feedback-networkactsaslow-frequency-oscillator(LFO),andasecondtransistorasabufferdrivingalampthatinteractswithanLDR.Thelatterloadsthelinegoingfromthe"Vibrato"-channelintothemixercircuitandimprintsacorrespondingrhythmicattenuationastheLFOrhythmicallydrivesthelamp(Fig18).TheonenoteworthydifferencetothetubeampswithLDR-tremoloisthatthesolid-stateseriesemploysanEdison-typelampwithfilament,whilethetubeampsworkwithaglow-dischargelamp.Thelatterhasamuchfasterreactiontimethat(dependingontheoverallcircuit)mayresultinamuch"harder"tremoloeffect.

Fig.18:Tremolocircuit:transistorQ20istheheartofanLFOdrivingalampviabufferQ21.TheLDRimprintsthepulsationofthelampontothemixerlineatC39.

Indeed,thetremolo-effect(oftencalled"vibrato"inFenderlingo)inoursolid-stateFenderisnotverydeepbutrathersoft–quitepleasantandmusicallyusefulbutdifferentfromthetubeamps.Thisdifferenceisinfactmentionedinthe1968Fendercatalog/9/,andtheopto-couplercaneasilybeidentifiedasareason.

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5.7PowersupplyMovingnowovertothepower-module,thereisnotmuchtorelateaboutthepowersupply(Fig.19),either.Itisatypicalunregulatedbipolarpowersupplygenerating+40Vand-40Vforsupplyingthepoweramplifierandtakingthe(lower)supplyvoltage(30V)forthepreampelectronicsfromaseparatetransformerwinding,avoidingcomplicatedorlossyelectronicregulation.

Fig.19:PowerSupply.Noteseparatetransformerwindingsforthesupplyvoltagesforpreampandpower-amp.

5.8PoweramplifierThepowerampdesignisinterestinginthatitdoesholdatransformer–albeitnotalarge,heavyoutputtransformeraswefinditinatubeamp,butasmalltransformerthataidsthesignalprocessingforthepush-pullcircuit(Fig.20)i.e.worksasphasesplitter.Theinputsignaltothepowerampisfedtoa3-transistorgain-stagethatdrivesthissmalltransformer.Theoutputsofthetransformerarecounter-woundi.e.theyprovideperfectout-of-phasesignalstothetwoidenticalbranchesofthepush-pulloutputstage.Suchdesignswithphase-splittertransformerswerenotuncommoninearlytransistor-ampdesign-forexample,thepowerampsoftheLeslie760usedthisapproach.ItmadeusingexclusivelyNPN-powertransistorspossible.

Fig.20:Inputsectionofthepowerampuptothephase-splitter(transformer)

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Twofurtherdriver-transistorsfeedtheirsignal,atlast,totheoutputsection(Fig.21).

Fig.21:Outputstage.Notelackofelectronicshort-circuit-protection,fuse...andnomorethan2powertransistors.

WhilesofarthecircuitsweinvestigatedintheSSTRcouldbeassessedaswelldesigned(inputstage),pretentiousbuto.k.("Style"-switchcircuit),uncommonbuteffective(Baxandall-typetone-controls)andreasonably-well-working(tremoloandreverbsections),butnow,inthefinal(power-amp)stage(sic!),itgetsabsolutelydicey(Fig.21).Touseameretwopowertransistorsinamusicalinstrumentamplifierratedat100Wattsispushingeverylimit–andworkingwithconvectioncoolingonly(i.e.noforcedcoolingbyafan)andarelativelysmallheatsinkonlyaddsinsulttoinjury.Fenderdidbelieveinthewaytheydesignedthecoolingoftheheatsink,though:theyevenobtainedapatent/8/forit,reasoningthattheventilationtheycameupwith(usingtheslitsintheangledfrontpanel)workedmorereliablythanconventionalarrangements.Thatmayindeedbecorrectinaway-butit'sstillonlytwotransistorsonasmallheatsink!Experiencehasshownthatatleast4powertransistorsonagenerousheatsinkwouldbeadvisabletohavethejobdonewithconviction.Whileconsequentlyalreadythefirstlookatthepower-ampraisesalotofeyebrows,furtherinvestigationshowsthatthereis–gasp!–noelectronicshort-circuitprotection...Giventheratherhighlikelihoodthatanamplifiersubjectedtooftenabrasivehandlingontheroadwillsoonerorlaterexperienceashortedoutput(duetoadamagedspeakerconnectionorablownspeaker,orsimplyduetoirresponsibletreatmentbytheoperator/guitarist),shortcircuitprotectionisamustfortransistoramps.

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Tubeampsarerathertolerantanimalsthatdislikeanopenoutput(ifanythingatall),buttransistoramps–dealingwellwithanopenoutput-circuit–hateshortsintheoutput;withoutprotection,notonlywilltheoutputtransistorsdieaquickdeathbuttheyare–duetotheDC-couplingintheoutputstage–verylikelytopullallotherconnectedtransistors(andpossiblyfurthercomponentssuchasapairofpreciousJBLD120-F's,incaseyouhadorderedthemwiththeampasanoptionatconsiderableextracharge)intothedarknesswiththem.Theneedformassive,costlyrepairsisimmediate.CheckingthecircuitdiagramoftheoutputstageSSTR,youcouldpointtothefusebetweenoutputtransistorsandspeakerjack.Indeed,thisismeanttobetheshortcircuitprotection.Thelabelontheampssays"6A"forthefusetype.However,ifthisworksatall,itmakesforaverycumbersomeoperationinthecaseofashortattheoutputbecausetheuserwillhavetocarrysparefusesandmaygothroughquiteanumberoftheminnotimejustbecauseofe.g.afaultyspeakercable.Runningoutoffuseswillrendertheampinoperativeevenifthespeakercableiseventuallyrepaired.Also,youhavetohavetherightkindoffuseathand:fusesratedatasmallerblow-currentwilldietooearly,fusesofahigherblow-valuewillnotdoanyprotection.Incidentally,the1968Fendercatalogmentionsare-settableprotector(circuitbreaker)butshowstheregularfuseinthepictureofthebackoftheamp.The1969Fendercatalogactuallyshowthatitinlaterversionsofthesolid-stateampseries,thisprotectorreplacedtheregularfuse.Theprotectorwouldsensethecurrentanddisconnecttheoutputifthecurrentwastoostrong.Thisindicatesthatthefuse-protectionwasanissueonewayoranother,andincludingtheprotectorwasastepintherightdirection,ifpurelyfromahandlingpointofview:theuserdidnotneedtocarryextrafuses.However,thereisanotherproblemhere–indeedtheaboveformulation"ifthisworksatall"isanintentionalchoice.Oneof"Murphy'slaws"says:"acrucial,expensivecomponentprotectedbyafast-blowingfusewillprotectthefusebyblowingfirst".Indeed,thereissuchadanger:transistoramplifierswithoutshort-protectioncandieincrediblyfast-possiblyfasterthanthefusewillblow.Moreover,giventheratherpunyheat-sinkandameresinglepairofoutputtransistors,therangebetweenwhatcanbeconsideredahealthy,strongoutputcurrentinoperationatfullvolumeononehand,andadangerouslyhighcurrentresultingfromashortontheotherhandwillbequitenarrow.Consideringthetolerancesthatfuses(andre-settableprotectors)havebothintermsofthecurrentatwhichtheydie(orswitchoff),andconsideringtheirreactiontime,itisnotguaranteedthatthiskindofprotectionwillactuallyworkatall.Inparticularthelatteraspectisthescaryone."Fast"fusesarespecifiedtoblowafter20msiftentimesthespecifiedcurrentpasses(i.e.inourcaseafter20msofacurrentof60A).Unfortunately,thereareconditionswherehigh-powertransistors(liketheworkhorse2N3055)candieafteronly1msofacurrentof4A....Bytheway,theoutputtransistorsusedinourSSTRseemnottobeoftherun-of-the-millkind;apparentlytheyareOEMproductsholdingthelabelRCA101568.Ifwehadtoreplacethesecomponents,wewouldn'treallyknowwhattouse.Probablyhigh-power2N3773'swoulddothetrick(relatively)reliably.Butevensuchreallyhigh-powertransistorsmaynotsurvivethekindofmakeshiftprotectionaffordedbyafuse.Electronicprotectionwouldreactfastenoughandshouldhavebeenimplementedinthesolid-stateFenders.

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Inallfairness,though,weshouldnotkeepsilentaboutmanyothertransistoramps(evensecond-generationonesliketheRolandJC-120anditsspin-offs)thatdidnothaveanyshort-circuitprotectionattheiroutput,either.Sothelackofsuchcircuitryisnotuncommon(especiallyincomboampswherethereis-incontrasttotheSSTR–afixedspeakerconnection)–butthatdoesnotmakeitforgivable.InparticularacompanysuchasFendershouldhavedonebetter,havingthereputationtobuildthesturdiestandmostdependableampsaround.IncontrasttoFendertubeampswherethetwooutputspeakersareconnectedinparallel,thetwooutputjacks(speakerandextensionspeaker)ofthesolidstateampsareconnectedinseries.Underthecircumstances,thisisasmartmovefortworeasons:-first,withthetwointernalspeakersconnectedinparalleltoalreadygivethenominalloadimpedanceof4Ω,connectingtheexternalspeakerwillnotlowertheimpedancefurther(whichwoulddramaticallyincreasetheriskofoverloading/damagingtheampor,atleast,blowingtheoutputfuse)butincreaseiti.e.makingoperationsafer;and-second,ifthecabletotheexternalspeakerisfaultyinthatitcontainsashort,therewillbenooverloadtotheamp–theexternalspeakerwillsimplynotworkwhiletheinternalspeakersstillwill.Thisseries-connectionofthespeakershasadownside,though:althoughthepower-capacityonthespeaker-sideisincreasedbyanexternalspeaker,thepowerampwill(duetothehigherloadimpedance)providelesspowersointhatsensethearrangementmaybesafebutkindofcounterproductive.Aboutpoweroutput:wemeasured80W(RMS)atthenominalloadof4Ω(i.e.notbadforthe105Wcatalogspecification),60Wat8Ωand40Wat16Ω.Insection4above,wementionedthelowdegreeofserviceability(atleastcomparedtoFendertubeamps).Itisverylikelythatthedesignersofthesolid-stateFenderswouldhavecounteredwiththefollowingreasoning:transistorsamps,withapracticallyendlesslifespanofthemaincomponentsandthemuchlongerlifespanofelectrolyticcapacitors(duetothemuchloweroperatingtemperatures)willneedconsiderablylessservicethanatubeamp,ornoneatall.Thisargumentwould,however,onlyholdsifthebasicdesignofthetransistorampissolid.Whichinthepresentcaseitisnotreally.

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6.Asecondassessment:circuitdesignOuranalysisofthecircuitreveals-awelldesignedinputstage,-ano.k.preamp-designwithaneffectivebutunfamiliartonecontrolsectionthatneedsgettingusedto,-anoverlyhotreverbsection,-apowerampoutputstagethatis–withameretwopowertransistorsonapunyconvection-cooledheat-sink–wayundersizedforanoutputof100Watts,andontopofthisbadlyprotectedagainstashortcircuitattheoutput.Itisthelatteraspectthatfailstoestablishanytrustinthedependabilityoftheamplifieratall.Infact,evenifthisampwouldbethegreatestsoundingampintheworld,onewouldbeilladvisedtotakeitontheroad.Itisawonder(ormereluck)thatourSSTRhassurvivedapparentlywithouttheneedforrepairs.Wewouldthinkthatnotmanyoftheseampswereasfortunate.Thisoutputsectionaloneisalmostadeathsentencefortheamp...Tobecontinued;inthethirdpartofthisarticle,wewill–atlonglast–pluginandplayandlistentotheamp.Appendix1:Literature:-/1/ FenderAmps-Thefirstfiftyyears;JohnTeagleandJohnSprung,HalLeonard,1995-/2/ TheFenderAmpBook;JohnMorrish,Balafon,1995-/3/ TheSoulofTone;TomWheeler,HalLeonard,2007-/4/ Fender-TheinsideStory;ForrestWhite,MillerFreeman,1994-/5/ ElectricGuitar-SoundSecretsandTechnology,HelmutLemme,Elektor,2012-/6/ PhysikderElektrogitarreManfredZollner, https://gitec-forum.de/wp/gitec-community/buch/; Translation: Physics of the Electric Guitar: https://www.gitec-forum-eng.de/the-book/-/7/ JimMarshall-TheFatherofLoud,RichMaloof,UnitedEntertainmentMedia,2004-/8/ SolidStateAmplifier,andControlPanelAssemblyincorporatedtherein,PaulSpranger(Inventor), PatentUS-A-3462553A,1969-/9/ FenderMusicalInstruments...ONTHEGO!,1968Catalog,Fender,1967-/10/ Fenderlovin'care,1969Catalog,Fender,1968

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Appendix2:completecircuitdiagrams

2-ChannelPreamplifierw/Reverb-&Tremolo-Circuits

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