Mesa/Boogie Mini Rectifier Twenty-Five User Manual

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TUBE NOISE & MICROPHONICS
Youmayoccasionallyexperiencesomeformoftubenoiseormicrophonics.Cer-
tainlynocauseforalarm,thisquirkybehaviorcomeswiththeterritoryandtheTone.Muchlikechangingalightbulb,youdon’t
needatechniciantocurethesetypesofminoruserserviceableannoyancesandinfact,you’llbeamazedathoweasyitisto
curetubeproblems...bysimplyswappingoutapre-amporpowertube!
Firstmaywesuggestthatyousettheamplifieruponsomethingsothatyoucangettothetubescomfortablywithouthavingto
benddown.Italsohelpstohaveadequatelightingasyouwillneedtoseethetubesocketsclearlytoswaptubes.Usecaution
andcommonsensewhentouchingthetubesaftertheamplifierhasbeenonastheymaybeextremelyhot!Iftheyarehotand
youdon’twanttowaitforthemtocooloff,trygraspingthemwitharagandalsonotethattheglassdownaroundthebulbous
silverytipisconsiderablylesshotwhichmakesiteasiertohandle.Gentlyrockthetubebackandforthasyoupullitawayfrom
itssocket.
DIAGNOSING POWER TUBE FAILURE
Therearetwomaintypesoftubefaults:shortsandnoise.Bothlargeand
smalltubesmayfallpreytoeitheroftheseproblemsbutdiagnosisandremedyisusuallysimple.
Ifafuseblows,theproblemismostlikelyashortedpowertubeandshortscaneitherbemildorsevere.Inamildlyshortedtube
theelectronflowhasovercomethecontrolgridandexcesscurrentflowstotheplate.Youwillusuallyheartheampbecome
distortedandbegintohumslightly.Ifthisoccurs,quicklylookatthepowertubesasyouswitchtheamptoSTANDBYandtry
toidentifyoneasglowingredhot.Itislikelythattwoofapairwillbeglowingsincethe“shorted”tubewillpulldownthebias
foritsadjacentmates,butonetubemaybeglowinghotter—andthatoneistheculprit.Theothertwoareoftenfine—unless
they’vebeenglowingbrightredforseveralminutes.
Becausethereisnophysicalshortinsidethetube(justelectronsriotingoutofcontrol)merelyswitchingtoSTANDBYforafew
momentsthenbacktoONwillusuallycuretheproblem...atleasttemporarily.Watchthetubescarefullynow.Shouldtheproblem
recur,theintermittenttubewillvisiblystarttooverheatbeforetheothersandthusitcanbeidentified.Itshouldbereplacedwith
onefromthesamecolorbatch,shownonitslabel.Callusandwewillsendoneouttoyou.
Thesevereshortisnotnearlysobenign.Intheworstcases,amajorarcingshortoccursbetweentheplateandthecathode
withvisiblelightninginsidetheglassandamajornoisethroughthespeaker.Ifthisisseentohappen,IMMEDIATELYturnthe
amptoSTANDBY.Bythistimethefuseprobablywillhaveblown.Suchashortisusuallycausedbyaphysicalbreakdowninside
thetubeincludingcontaminatecominglooseorphysicalcontact(ornearcontact)betweentheelements.Replaceitandthe
fusewiththeproperslo-blotypeandpoweruptheampusingthepowerupprocedureaswedescribedearlierinthismanual.
TUBE NOISE
Oftencausedbycontaminationwithininatube,theculpritcanusuallybeidentified,andbylightlytapping
ontheglass,youwillprobablyhearthenoisechange.Hearingsomenoisethroughthespeakerswhiletappingonthe12AX7’s
isnormalhowever.AndtheonenearertheINPUTwillalwayssoundlouderbecauseitsoutputisbeingfurtheramplifiedbythe
second12AX7.
Thepowertubesshouldbeallbutquietwhentheyaretapped.Ifcracklingorhissingchangeswiththetapping,youhaveprob-
ablyfoundtheproblem.Toconfirmanoisypowertube,merelyputtheRectoonStandby,removeitfromitssocketandturn
itbackon.ItwillcausenodamagetoruntheRectobrieflywithonepowertubemissing.Youmaynoticeaslightbackground
hum,however,asthepush-pullbecomesunbalanced.Wheneveryouaretryingtodiagnoseasuspecttube,keepyourother
handonthePOWERandSTANDBYswitchesreadytoshutthemoffinstantlyintheunlikelycaseyouprovokeamajorshort.
Ifyouthinkyou’velocatedaproblemtubebutaren’tsure,werecommendsubstitutingthesuspectwithanewonejusttobesure
ofyourdiagnoses.Youwillbedoingyourselfandusabigfavorbyjustfollowingthesimpleguidelinespreviouslymentioned
regardingtubereplacement.You’llprobablybesuccessfulwithmuchlesseffortthanisrequiredtodisconnecteverythingand
haultheunittoatechnicianwhowillbasicallyperformthesamesimpletests.Ifthetubesarestillwithintheirsix-monthwar-
rantyperiod,wewillhappilysendyouareplacement.Justnotethecolordesignationonthetubelabelsothatwecansendyou
theappropriatematch.

Mesa/Boogie Mini Rectifier Twenty-Five Specifications

General

Brand	Mesa/Boogie
Model	Mini Rectifier Twenty-Five
Category	Musical Instrument Amplifier
Language	English