Mesa/Boogie Mini Rectifier Twenty-Five - Bias Adjustment Principles

29PAGE
AFeatureArticlebyRandallSmith

Here’saquestionweoftenhear:
“Why doesn’t MESA put bias adjustments in their amplifiers?”
Well,there’sashortanswerandalonganswertothisquestion.
Theshortansweristhatduringmy12yearsofrepairingFenders,
oneofthemostfrequentproblemsIsawwasbiascontrolsthat
wereeithersetwrongorthathadwanderedoutofadjustmentdue
tovibration.Asanyhonesttechwilltellyou,there’slot’sofeasymoneytobemadebysprinkling“holywater”onamplifiers
...uh,whatImeanttosayis“Yourampneededbiasing.”SeewhatImean?Whatcustomerisgoingtoarguewiththat?
Itonlytakesamomentandavoltmeter:TheFenderdiagramshowshow:“Adjustthistrimpotfor-52volts.”That’sit.Noth-
ingmore.
Nowdon’tbefooledintothinkingthattubes“draw”moreorlessbias,theydon’t.Thewayabiassupplyisconnectedtoatube
isakintoadeadendroad,itjusttrailsofftonowherewithoutreallycompletingacircuit.It’sastaticvoltageandregardlessof
whattubeisinthesocket—orevenifthetubesaren’tpluggedinatall,itdoesn’tchangethebiasvoltageabit.
Sotheendoftheshortansweristhis:Sinceabiassupplyneedstoputouttherightvoltageandnevervary,Iwantedtobuild
amplifiersthatwereindividuallyhardwiredtothecorrectvaluesandNEVERneededadjustment.Andfor25years,that’show
MESA/Boogieshavebeenbuilt.
Timetochangetubes?Justplugourtubesintoanyoneofourampsandyou’reDONE.Notechneeded.NObillsandnoBS
aboutbiasing.Andmostimportant:ThebiasisRIGHTbecauseitcan’tchange!
Now,youwantthelonganswer?Here’smoreinformationonhowourhard-wiredbiasavoidstrouble.Pleasereadon.
Butfirst,let’smakeanimportantdistinction.Ourbusinessisdesigningandbuildinghighperformanceamplifiers.Andforthiswe
needtubeswhosevarianceiswithinanarrowrange.Ourwarehouseisfullofrejects...oh,theywork—theyjustdon’tperform
withinourtolerancerange.Wehaveaverysophisticatedcomputer-basedtubetestingsystem(nicknamed“Robotube”)that
matchesandmeasurestubesoversevenimportantparameters.Itcanevenpredictwhichtubesarelikelytohaveashortened
lifetime—eventhoughtheyworkperfectlyduringthetest.
Becauseourbusinessisbuildingqualityamps,wecanaffordtorejectalotofwaywardtubes.Theguysyouhearcomplaining
becauseBoogiesdon’thavebiasadjustersareprimarilyinthebusinessofsellingtubes-notamps.Theydon’twanttothrow
away30percentoftheirinventory,sotheypromotetheideathattubesoutsideourparameterscanbeusedto“customize”
amplifiersandtheycriticizeusbecauseourampscan’tbeadjustedtoaccommodatetheirout-of-MESAtolerancetubes.
Nowyoumightbethinking,“ButIthoughtyoujustsaidthattubesdon’t“draw”bias,thereforetheydon’teffectthebiassupply
andthusitdoesn’tneedtobeadjustable.”
Whenyousetthebias(whetherit’sbyselectingtherightresistors,aswedo,oradjustingatrimmer—whichisquicker)what
youaredoingisestablishingthecorrectamountofidleCURRENTthatflowsthroughthepowertubes.Butyoucan’tadjust
thecurrentdirectly,youcanonlychangeitbyadjustingtheamountofbiasVOLTAGEthatgoesontothetubes’controlgrids.
VoltageandcurrentareNOTthesame.CurrentistheAMOUNTofelectricity,the“quantity”—andismeasuredinamperes.
Voltageisthedegreeofelectriccharge—likethe“pressure”tousetheoldwateranalogy.Letmeillustratehowdifferent
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PLATE
Structure of a 6L6 / 5881
Beam Power Pentode.
BEAM-
CONFINING
ELECTRODE
SCREEN
GRID
GRID
CATHODE
BIAS ADJUSTMENT