Mesa/Boogie Mini Rectifier Twenty-Five - Page 33

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Butbacktotheelectronflow.Iftheelectronsarestronglyattractedtoapositivelychargedplate,thenitfollowsthattheyare
stronglyrepelledbyanegativelychargedplateandtheyare.Thus,ifanalternatingcurrent-suchascomesfromatransformer
-isappliedtotheplate,electronswillflowonlyduringthetimeswhentheplateispositivelycharged.Duringperiodsofnegative
platecharge,electronflowisstoppedandthespacechargeofelectronsremainscompressedintheareaaroundthecathode.
Thusadiodetube-onewithacathodeandananode-ismostlyusedtorectifyalternatingcurrentintodirectcurrentbypass-
ingitwithoutrestriction,butinonedirectiononly.ThisalsoexplainswhyclosingtimeisstrictlyenforcedatIrishpubs:During
normaloperation,thetrafficflowissimilarlyunimpededanduni-directionaltowardthebarandthisprocessrectifiesthework-
daynegativity.Itgoeswithoutsayingthatnooneleavesaslongastheatmospherearoundthebarremainspositivelycharged.
TRIODES
ThissectionisacontinuingtechnicaltreatiseontheworkingsofIrishPubsbuttomakeiteasierforthelayman
tounderstand,itisexplainedintermsofvacuumtubetechnology.Entertheoriginalbar-freebeerandnodoors.Well,itturns
outthatsomecontrolovertheflowcanbeanecessaryandusefuladvantage.Thisledtotheinventionofthoseswinginglou-
veredsaloondoorswhichareopenatthetopandbottom.Theyarepatternedafterthecontrolgridofthevacuumtube,which
isalooselywoundcoilofthinwirelocatedbetweenthecathodeandtheplate.
InaTriodetheplateisalwayspositivelychargedwithhighvoltageD.C.andeventhoughthegridisblockingthepath,those
negativeelectronscanstillFEELthestrongattraction-justastheIrishmencanseeinthroughthelouversofthebardoors.
Theyknowwhatpleasuresliebeyond,buttogetthererequiresovercomingthenegativeinfluencescontrollingtheaccess.This
negativeinfluenceistypicallycalledaBias.Inelectronictermsthatmeansthegridissuppliedwithavoltagewhichisslightly
MORENEGATIVEthanthealreadynegativeelectrons.ThemorenegativetheBias,themoreittendstoneutralizetheattraction
oftheplateandrepeltheelectronsbacktowardthecathode.
TheIrishcanbesimilarlychargedwithBias,butunlessyouareIrishyourself,thistypeofBiasingmaybemoredifficultto
understand.Theeffectissimilarthough:ThemorenegativetheBias,themoreitimpedesforwardprogress.Generallyspeak-
ingthough,theelectronicBiasofthegridiseasiertoovercome,andfortwomainreasons:First,theBiasisset-likethebar
doors-toallowsomepassage.Second,thegridismostlyNOTTHERE,likethelouvereddoorswhicharemostlyopenspaces.
Unliketheplatewhichissolid,thegridislikeacoiledbedspring.Itcancreatearepellingfieldbutmostlyit’semptyspacein
betweenwidelyseparatedwindingsofwire.It’sveryeasytocontroltheelectronsastheypassthroughthegrid’sforcefield:
Changingthegridvoltageonlyslightlywillhaveanenormouseffectonhowmuchcurrentflowsthrough...andthat’swhat
AMPLIFICATIONis:asmallchangeinvoltageatthegridcausingalargechangeincurrentflowingtotheplate.
Thepurposeofthelouveredbardoorsissimilartothatofthegrid,namely,togivemomentarypausewhilestillrevealingthe
promisewithin.Hesitationmostlygiveswaytotemptation,buttherearethosefewstalwartIrishmenwhothinktwiceandde-
cidetocomebacklater.Mostjustpauseslightlythengoonthrough.Thatisthepurposeofthebardoors:topreventeveryone
fromcrowdinginallatonce-andasthedoorismadelessofabarrier,widerspacesbetweenthelouvers,moreofthebar’s
attractiveinfluenceisfeltoutsidethusamplifyingthecustomerflowandincreasingthecrowdatthebar.
PENTODES
Occasionallythough,bardoors-eventhelouveredtype-werefoundtobetooeffective,andtoomanycus-
tomersturnedaway.Somethingfurtherwasneededtoincreasetheattractionofthebarandovercometheresistancecreated
bythedoor.Thusthecocktailwaitresswasinvented.
Onceagaintheideawasinspiredbythevacuumtube.Ithadbeendiscoveredinsometubes,oftenlargepowertypes,thatthe
distancetotheplatewastoogreattoattractenoughelectronspastthenegativeinfluenceofthecontrolgrid.Soanothergrid
coiloffinewirewasinsertedbetweenthefirstgridandtheplate.Thiswascalledthescreengridandcarryingahighlypositive
charge,itfunctionedasa“bait”fortheplate.
InaproperlydesignedpowertubesuchasanEL84ora6V6,thewindingsofthescreengridarepreciselyalignedtofallinthe
shadowofthecontrolgrid.Thiswaytheelectronsrespondingtothepullofthescreengridarelinedupinsheetsastheypass
betweenwindingsoftheinnercontrolgrid...onlytofindthattheyhavebeenfooled!Oncepastthecontrolgridanddrawntoward

ON TRIODES, PENTODES & IRISHMEN: (Continued)