Mesa/Boogie Mini Rectifier Twenty-Five User Manual

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voltageandcurrentare:Whenyouscrapeyourfeetacrossacarpetedfloorindry,winteryconditions,yourbodycanbecome
chargedwith50,000to100,000voltsofstaticelectricity.Andwhenyoureachforthedoorknob,asparkjumpsandyoufeel
it!Thevoltageissuperhighbutthecurrent(measuredinmicro-amps)istiny-otherwiseyouwoulddiefromelectrocution.
Contrastthiswithyourcarbattery,whichputsoutamere12volts.Youcanlayyourhandsrightacrosstheterminalsandnot
feelathing.Yettheamountofcurrentavailablecanruntoseveralhundredamperes..enoughtoturnoveracoldengineand
getitstarted.
Socurrentandvoltagearetwototallyseparateelectricalparameters—thoughwhenyoumultiplythemtogether,youget
POWER,whichismeasuredinwatts.
Whenyousetthebiasofanamplifier,youareadjustingthestaticVOLTAGEatthecontrolgridofthetubeinordertoproduce
adesiredamountofidleCURRENTflowingtothetube’splate.Asmallchangeingridvoltage,producesalargechangeinthe
amountofcurrentflowing—andthat’sbasicallyhowatubeworks.Saythatagainbecauseit’ssuperimportant:Asmall
changeinvoltageatthegridcausesalargechangeincurrentflowingtotheplate.See,that’stheessenceofamplification:A
smallchangecausingalargechange.Andhereit’sasmallvoltagechangecausingalargecurrentchange.
Thebiasconditionsarewhatdetermineshowmuchcurrentflowsthroughthebigpowertubeswhenyou’renotplaying.And
whatdrivesyourspeakersisfluctuationsinthatcurrentflowwhenyouAREplaying.Iftheamountofcurrentincreasesand
decreases440timespersecond,thenyou’llhearanAnote.Ifthefluctuationsincurrentflowarelargeandstillat440per
second,you’llhearanAthatisLOUD!
Butforpurposesofbiasing,it’stheamountof“platecurrent”flowingwithnosignalappliedthat’simportant.Unfortunately
currentishardtomeasurebecausethecircuitmustbeinterrupted—asin“cutthewire”—andthemeterspliced“inseries”
withthebrokencircuit.ButmeasuringVOLTAGEiseasy.Itisnotnecessarytointerruptthecircuitbecauseavoltagereading
canbetakeninPARALLELwiththecircuitintact.
Thus,asamatterofconvenience,mostbiassettingsaregiveninvoltsatthegrid...eventhoughcurrentthroughtheplateis
theimportantfactor.Infactplatecurrentissoinconvenient(anddangerous)tomeasurethatFenderdoesn’tevenstatewhat
thecorrectvalueshouldbe.Theyonlygivethegridvoltagethatwillproducethatcurrent.(That’stheminus52.)Butthatonly
happensifthetubesbeingusedare“inspec.”
AslongasthetubesARE“inspec”,therightbiasvoltagewillalwaysgivethecorrectplate“CURRENT”—butthenthere’sno
needforthebiasvoltagetobeadjustable!
IfthetubesareNOTinspec,thentheonlyproperwaytore-setthebiasistocutthecircuitandmeasurethecurrentwhile
adjustingthebias...butnomanufacturerIknowevenSTATESthedesiredcurrentvalue!Bethatasitmay,whentheoriginal
biasvoltageisalteredfarenough,itwillcompensateforthetube’sabnormalperformanceandthecorrectamountofidlecur-
rentflowmaythenberestored.Clearlythisissomethingmostrepairtechsshouldnotattempt.
SomenewerampshaveLEDindicatorsconnectedtothecircuitwhichwillturnonwhentherightthresholdofcurrentflow
hasbeenreached.Thisisanimprovement,andalmostworthyifyou’rewillingtoacceptresistorsandlightsaddedintoyour
amplifier’saudiopath—whichwearen’t.
Theother“advantage”ofthissystemisthatitallowssomeampmanufacturerstoavoidmatchingtheirpowertubes.The
thinkingisthatadjustingthebiastoeachtubeseparatelyeradicatestheinherentdifferencesbetweenthetubesbyinsuringthat
thesamecurrentflowsthrougheachone.
Again,thishassomemerit..butit’sstillnotasgoodasusingtubesthatarematchedinthefirstplacebecausecompensating
forthemis-matchcausesthepush-pullcircuititselftobecomeunbalanced.Twowrongsdon’treallymakearight.
BIAS ADJUSTMENT (Continued)


Mesa/Boogie Mini Rectifier Twenty-Five Specifications

General

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