AVO 160A Instructions for use

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AVOCT160AOperating Instructionsand ServiceManual
ThisAVOCT160Amanualistheresult ofcombining(and editing)twodifferentsetsofphotocopiesof
AVOCT160AmanualsandalsofrominformationfromtheoriginalAVOCT160manuals.ThisAVO
CT160Amanualhasbeen kept asoriginalaspossible,eventhoughsomephotocopiesof the AVO
CT160A pageshavebeen replacedwithAVOCT160 pageswherethetextdidnot differinanypoint, but
wherethe qualityoftheCT160 scannedpageswasmuchbetterthan theCT160A scannedpages. All
figuresandphotoshavebeenkept intheiroriginalshape althoughmost havehadtobeeditedduetothe
poorqualityofthe originalphotocopies.
The schematicsat the endofthe manualhavebeenextendedwiththreenewsetswherethefirst isan
unalteredoriginalredrawnschematic,thesecond schematicisaredrawnand corrected schematicand
thethirdisaredrawn,correctedandmodifiedschematic.Thesecondschematichasbeencorrectedin
thefollowingareas:
D2: Changedfrom66V RMS winding to99VRMS winding,calculatingthecurrentinthe circuit
showsthatthe 99V RMS winding isnecessaryforthecircuittoworkcorrectly.
R4& R41:Changedplacesinschematic,the componentliston page17 ofthe ServiceManual
andalsothecomponentliston thepagebeforetheoriginalschematicshowsthisasthe correct
order,aswell asan internalcomponent placementcomparisonbetween aCT160A andaCT160.
R37&R37 shownasR37A and R37B aspercomponentliston page18 ofthe ServiceManual,
consistingofone 13 "selected"resistoreach.
SH6: Ground connection fortags2-5, wasmissingfromoriginalCT160A schematicbut canbe
foundinlaterCT160schematicsand alsointheoriginalCT160A & CT160 testers.
WIRES & COMPONENTS:Movedforclarity.
The thirdschematichasbeenmodifiedinthefollowing areas:
All ofthecorrectionsabovefromthesecond schematicplus:-
D5, R42& R38: Anodevoltagerectification added,componentsusedD5=D1, R42=R38=100
k±10%1W–thismodificationhasbeentested andproved toworkinanoriginalAVOCT160
andisalsousedbyAVOinthe AVOMkIV.
D6, D7& C4: Meterprotectioncircuitadded,D6=D7=D1,C4=8µF/ 63VDCNonpolarized
(preferablyPolypropylene)–thismodification hasbeentestedand proved toworkinanoriginal
AVOCT160andisalsousedbyAVOintheAVOMkIV.
Ifyou wanttoread moreabout the innerworkingsoftheAVOValveTestersincluding theAVOCT160
and AVOCT160AI recommendthatyouvisitthe“UKVintageRadioRepairandRestoration
Discussion Forum“here http://www.vintage-radio.net/forum/index.php and readthethreads:
“Everythingyou wantedtoknowaboutthe innerworkingsofan AVOMkIV Valvetester!”
“Somemorethat youwanted toknowabouttheinnerworkingsofanAVOValvetester!”
“Allthatyou wantedtoknowabout the innerworkingsofan AVOCT160Valvetester!”
“AVOCT160-anodecurrentmeasurements, calibrationresistorsand themeterworkings!”
“AVOCT160A –Specialeditionof theAVOCT160 -comparison!”
I haveincludedadaptedversionsoftheCT160A andCT160textsaboveinthisdocument.
The workofeditingandtestingthecorrectionsandmodificationsinthe AVOCT160Amanual, andwriting
of thearticlesabove, hasbeen performedbyMartinForsberg,Sweden, and Euan MacKenzie, Australia.
Sweden2010-11-18

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The followingtextsareadaptationsdone tothetextspublishedbyme,MartinForsbergandEuan
MacKenzie,at the “UKVintageRadioRepairandRestoration Discussion Forum“and
containimportantinformationabouttheAVOCT160A ValveTesteraswellasimportant informationonthe
AVOCT160 ValveTesterwhichtheAVOCT160Aisbased on.
AVOCT160A–Special edition oftheAVOCT160-comparison!
Thistextwillendeavourtoexplainhowthe AVOCT160A worksincertainareasincomparison tothewell
knownAVOCT160.Someofthe informationcomesfromMr.YutakaMatsuzakawhokindlysentmea
copyoftheAVOCT160AOperating InstructionsandcalibrationInstructions.Mr.YutakaMatsuzakahave
publishedsomeinformationon hiswebsite, http://www6.wind.ne.jp/yutak/avo_ct160/index.htm,about
calibrationoftheCT160AandalsoacomparisonoftheCT160 and CT160A. Ihavealsobeen given
photocopiesofanotherversionoftheCT160AmanualbyFrankPhilipsewhichhereceived fromWimDe
Grotte,thesecopieshaverecentlybeenpublishedon FrankPhilipse’swebsite:
http://frank.pocnet.net/instruments/AVO/index.html .
Someofthemodificationsthat AVOdidtotheCT160A havealreadybeenmentionedinthe article:”AVO
CT160-anodecurrentmeasurements, calibrationresistorsandthe meterworkings!”on the“UK
VintageRadioRepairandRestorationDiscussionForum“,whichalsofollowsthistext.But
thedifferenceswillbe describedinmoredetailinthisarticle.
The AVOCT160AhasbeenreferencedtoastheDutchMilitaryversionsinafewplacesandIhavefound
no othermentionsofit.
PleasealsounderstandthatI havenot had access toanAVOCT160Amyselfforthisarticle,onlyto
secondhandpicturesandinformationplusphotocopiesofAVOmanuals.Howeverthemodificationsare
notcomplicatedonceyouunderstandhowanAVOCT160 and anAVOMkIVworksunderthehood.
Anyerrorsaremyown,but Ihavemademybest tocheckeverything beforepublishingithere.
Part1:ModificationsdonebyAVO
AVOhaveupdatedtheAVOCT160A toworkmorelikethe laterAVOMkIVinmanyareas,butnotall.
Apartfromthe looksonthefrontpanelsandtheextravalveholderpanelconnected viathenewcontact
on theoriginalvalveholderpanelmostmodificationsareeasy todescribe andunderstand if you
understandhowthe originalAVOCT160 works.ModificationsthatAVOhavedonetothe AVOCT160A
are:
A. All valveshavebeenreplaced withSilicon diodes.(D1–D4),making it necessaryforfurther
modificationsdescribedbelowtocompensatefordifferent voltage dropthan valves, and alsofordifferent
behaviourcompared tovalves.
B. Therangeforcurrentcapabilitiesforrectifiervalveshavebeenextendedupto180mA,thissimple
modificationwillbedescribedbelowinthe partdescribing theover-currentrelaymodificationastheyare
closelyconnected. (R40 +modificationsforovercurrentrelayRL3)
C. Sincethevalveshavebeen disposedofthe 6.3V(markedas5.8V inthe AVOCT160 schematic)
winding fortheheatershavebeen removedfromtheschematicfortheHeatervoltage transformer.
D. The500 resistorintheD/Rcircuithavebeenreplaced withtwo1K resistortohandlethe higher
currentrange.(R12& R13)

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E. Theovercurrent relaywindingRL3havebeenshunted witharesistorforthe 120mAand180mA
ranges,anextraswitchwaferfortheSETIa& DRswitchhavebeenadded forthismodification. (R37&
SJ1, unfortunately,inmyopinion,re-numbered byAVOtoSJ1asit wouldlogicallyhavebeen SJ5asSJ1
–SJ4arealreadypresentinthe AVOCT160)
F. The gridvoltscontrolhavebeen modifiedwithaswitchforthe capabilityof doubled voltage upto80V,
thecircuit havealsobeenmodified,moredetailsofthisinthetextbelow.(R4, R39,RV5andSL1&2have
beenintroduced plusanewwinding on theAnode&Screenvoltstransformer)
G. Thecalibration resistorcircuit havebeenmodifieddue tothemodificationdone tothegridVoltscontrol,
thismodificationisalsodescribedbelow. (D2,RV6, R41andR3plus99VRMS windingfrom
Anode/Screen voltstransformer–oldR4removedfromcircuit)
H. TheAnodeCurrentcontrolshavehad theseriesresistorchangedtocompensateforthecurrent
throughthecircuit whenaSilicondiodehavebeeninserted.(R6)
I.The SilicondiodeintheScreenvoltscircuithavemadeitnecessarytointroduceanextraresistorto
ground.(R38)
Inall otheraspectsthemodificationstatusof the AVOCT160Acorrespondstothelastmodificationstatus
of theAVOCT160 uptoresistorR36 fortheGridtoCathodeconnection.
ModificationA&C:
Replacing the valveswithSilicon diodesmeant that theextraheaterwindingisnotnecessaryanymore
and ithasbeenremovedfromtheschematics,butsinceno actualAVOCT160Ahavebeen availablefor
check itisnotknownbymeifthewindingisstill thereornot.TheSilicondiodesalsomeansthat some
circuitswill havetobecompensatedsincetheyhavealowervoltagedrop thanthevalves.
ModificationB, D&E:Currentmeasuringrangeforrectifiervalves increasedup to180mA.
Tomeasurerectifiervalvesonthe180mArangeAVOhavemodifiedtheCT160A inthe samewaythat the
AVOMkIVisdesigned,withoneextraresistorontheSETIa& DR switchforthemeasuringrangeand
alsoaresistor, R37,whichisconnectedinparallell withtheovercurrent relaycoilRL3.Thisextraresistor,
R37, isworkingasashuntmaking lesscurrent runthroughtherelaycoilsoit willnotactuateasbeforeon
theold120mA rangeandon thenew180mA range.Thisisnecessarysorectifierswithahighercurrent
capabilitycanbe measured,the relaywouldotherwiseactuatetoearly.The 500 resistorR12onthe
D/Rcircuit intheAVOCT160havebeen replacedwithtworesistors,R12 & R13at1K inparallell
making500 , tobeabletohandlethecurrentflowingthroughthematthehighercurrentrangesfor
rectifiers.

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ModificationF:
The GridVoltscontrolhavehaditsrange extended byaswitchwhichmakesitpossibletodoublethe
voltageon thescale, intheX1positionyouhave0-40Vand intheX2positionyouinstead have0-80V.
Thismodificationhasbeendone byintroducinganewwindingon theAnode/Screenvoltstransformer.
Thiswindingnowhavethree taps,ground,55V RMS and 120V RMS. TheswitchSL1&2forthe X1and
X2rangeswitchesbetweenthe55VRMSandthe120VRMS windingsplusitalsoinsertsorshorts
resistorR39whichcompensatesthe gmmeasuringvoltagecircuit.Whenthe120VRMSwindingis
selected resistorR4and potentiometerRV5isconnected forcalibrationpurposesofthe0-80V rangein
thesamemannerasRV3isusedforthe 0-40V range. Thebiggestchangetothe circuit isthat the
negativevoltage whichearlierwasusedfromanoutof phasewinding of66V RMShasbeenremoved all
together!ThisisthencontradictingAVOstext inthepatentthatthisvoltageisnecessaryforthevalvenot
toformadiodebetween thegrid(orscreen)tothe cathodewhen thevalveundertestisnotconducting.
Maybethisisatrade-off forsomereason,maybe it wasmoreexpensivetohaveitthereand haveit
changed instep withthegridvoltsswitch,ormaybeAVOfound that it wasnot necessarytohavethat
voltagethere.
ModificationG:
HereAVOhavechangedthecalibrationresistorcircuitintroducingapotentiometer,RV6,forcalibration
purposesandalsochangedresistorR3to1.22 M .AVOalsochanged thecircuit byintroducingaresistor
(R4, wronglynamed R41intheoriginalschematics)toground,thishasearlierbeen mentionedas
necessaryforshortingthechargebuildupinsidetheSilicon diode when itisnotconducting.Atthesame
timethe windingwhichsuppliesthiscircuit hasbeenchanged tothe99VRMSwinding,thisisnecessary
toensurethatthe samecurrentasintheAVOCT160 isdriven throughthecircuitforthemeasuring
purposes.
The calculationforthe99VRMS windingbecomes:
99V RMS equals89.13V Mean,theconversion factorbeing1.1107.
99V RMS / 1.1107 =89.13V Mean.
Thisvoltageishalf-waverectifiedwhichresultsinhalfthe voltage aftertherectification, minusthe voltage
drop ofthediode,whichcan beassumedtobe 0.7V.
89.13VMean/ 2–0.7V =43.87V
When thisvoltageisdividedbythecurrentnecessaryinthemeasuring circuit, whichhasearlierbeen
showntobe35.775µA theresultingcalibration resistancewillbe:
43.87V/ 35.775µA=1.226M ,witha+/-1%1.22 M resistorwhichrangesfrom1.207M upto1.232
Myou mightend upjustoutsidetherangepossibletoadjustwiththe20 K potentiometerRV6soitis
best tousearesistorcloseto1.22M .

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ModificationH:
Thismodification,of seriesresistorR6, hasalreadybeen describedintheearlierarticleonAVOCT160
modificationsandAnode Current control“inthe UKVintageRadioRepairandRestoration
Discussion Forum“andonlyconfirmsourcalculationsthere.Thecurrentthrough theAnode Current
controlsneedstobe exactly12.5mAandwhen thevalveisreplacedwithaSilicon diodewithamuch
lowervoltagedrop thedifferenceinvoltage dropwill havetobecompensated forwithanincreasein
resistorR6.
ModificationI:
AVOalsochanged thecircuitbyintroducingaresistorR38inthe ScreenVoltscontroltoground, thishas
earlierbeenmentionedasnecessaryforshortingthecharge buildupinside theSilicon diode when itisnot
conducting.
Ashasearlierbeenmentioned itispossibletoinsertaSilicon diode intheAnodeVoltscircuit justafterthe
Anode VoltsswitchbeforetheElectrodeSelectorswitch,plusan extraresistorof100K +/-10%at1
Watt –thisresistorneedstobe thereforthesamereasonasR38 above.
NewValveSockets:
The extraValveHolderpanelthatisconnected viaacabletotheValveHolderpanelon theCT160A box
havebeenequippedwiththefollowingvalvesockets:
PencilTubes,5WiresinLine,FlyingLead, 7WiresinLine, B7A,B9D,SpecialBasefor2C39, Acorn,
Nuvistor,F8andB8D
Thisfinishesthe currentcomparisonbetweentheAVOCT160andtheAVOCT160A.
Part2:ModificationsdonebyAVOtotheover-currentrelayprotection
HereissomemorenewinformationasI havestudied the,soon finished,AVOCT160Amanualinsome
moredepth!Byfinished Imeanthat Ihavesoonfinishedcleaningupallthepages.
WhileAVOwaschangingthecurrent rangeforDiodesand Rectifiers, byaddingthe 180mA range,they
alsochangedwhichvoltagesthatareused fortestingDiodesand Rectifiers. IntheAVOCT160theyused
thewindingsfor40V, 75V and125V tocoverthe rangesfrom1mA up to120mA but intheAVOCT160A
thesewindingshavebeenchangedto40V, 75V,150Vand200V. Thetwohighervoltagesareused in
conjunctionwiththe added SwitchSJ1whichconnectsresistorR37inparallelwithrelaycoilRL3forover
currentprotection. Fromreading the componentlistIcouldalsosee that AVOusedtworesistorstomake
up theresistanceofR37,whichisstatedas13 each. Thiswill thenmakeR37 tohavearesistance
value of 6.5 .
Withacoilresistancecloseto5 , whichEuan MacKenziehashelpedmetomeasure,forRL3this
meansthatyouwill haveroughlya57%decreaseinsensitivityinRL3asthecurrent isdividedbetween
thetworesistorsmaking upR37and the relaycoil.Themathsforthisissimply:
13 inparallelwith13 givesyou6.5 .Thisthengivesthecurrent through therelaycoilRL3 tobe:
6.5 / (6.5 +5 )=6.5/ 11.5=56.5%
Thisfigureof57%correspondsfantasticallywell withwhatAVOdidintheirAVOMkIVtesteronthe
currentrangesforDiode and Rectifiertestingontheranges60mA,120mAand180mA.HereAVOused
two3.9 resistorsinparallelwiththerelaycoilRL3,whichhasaresistancecloseto1.5 inanAVOMk
IV, thiswill alsomeana57% decreaseincurrent sensitivity.Themathsforthisissimply:

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3.9 inparallelwith3.9 givesyou1.95 . Thisthen givesthecurrent throughtherelaycoilRL3tobe:
1.95 /(1.95 +1.5 )=1.95/ 3.45 =56.5%
Ifyou check thecalculationsaboveandtakealldecimalsintoaccount youwillactuallyseethatthesetwo
calculationsgiveexactlythe sameresult, thefactorbetweenthenumbersisexactlythreeand one third
(3.333.....).ThatwaswhatI meantwiththefantasticallywellcorrespondancebetween the AVOMkIV and
theAVOCT160Ahereinthiscircuit!
Isthisjustacoincidenceorsomething thatAVOplanned?Myguessisthatitisplanned butthe resistance
forthe resistorsandRL3 willofcoursevarybetweeneachtester,butinthe AVOCT160A servicemanual
AVOsays that theseresistorswill be "selected",whichprobablymeansthat theyareselectedtoworkin
eachAVOCT160Atesterbeingbuilt.AVOstatedthe tolerancesfortheseresistorstobe+/-10% inthe
AVOMkIVand+/-20%intheAVOCT160A.

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AVOCT160 -anodecurrentmeasurements,calibration resistorsandthemeterworkings!
Thistextwillendeavourtoexplainhowthe AVOCT160worksincertainareas,especiallytheAnode
Currentcontrols, thecalibrationresistorsand the metercircuit.It will explainhowtoreplacethe rectifier
valveswithSilicon diodes,asAVOdidintheirlatermodel,theCT160A; andalsoexplainwhythereisa
slightmodification tothecircuitasaconsequence.It will alsoexplainhowyoucanimprovethecalibration,
bymakingafurtherslightmodification tothe calibrationcircuit.It will alsoexplainhowyou can placea
SilicondiodeintheAnodevoltage circuit and asmallmodificationthat thisentails. Allofthese
modificationshaveactuallybeen triedouton anAVOCT160(SerialNo. 4087YF)andthemodificationdo
notchangetheresultsinanyway;you getexactlythesameresultswhen comparisonstoastandardised
valve(aCV455)aremade, bothbeforeandafterthe modificationswereperformed.Thereisalsoashort
descriptionofthefunction of R14 andSW3,whichareused intheSETZEROpositionon themA/Vdial.
The protectionofthe meterisalsodiscussed and acomparison withAVO’sotherValveCharacteristic
Metersismade.
Thistext isacollaboration betweenme,MartinForsberg, and myfriend EuanMacKenzie, whereI have
writtenmost ofthe textandEuan havemade mostofthemeasurementsandallofthe modificationsinhis
AVOCT160. Euan hashelped mewithproof readingof thetextandalsosuppliedindepthinformation
aboutcomponentsandmathematics.
Itisquitelong, but I reallyhopethatyou find itworthwhiletoread through totheend!
Thereismoretocomeinthefuturewithalookat theDutchMilitaryspecialeditionoftheAVOCT160
namedAVOCT160A.The AVOCT160Amight havebeenused byotherstoobutithasbeenreferenced
toastheDutchMilitaryversionsinafewplacesandI havefound noothermentionsofit.
Part1:AnodeCurrentControls;and replacing theCV140 valveswithSilicondiodes:-
Inthe AVOCT160 theanodecurrent ismeasuredbymeansofaPotentiometer, whichisalaboratory
instrument fortheprecisionmeasurementof anunknownvoltage.If youdidPhysicsinthesixthformat
school,youwouldhaveencountered the Potentiometer; initssimplestformitiscomprisedofa1mlength
ofNichromewire,alongside a1mwoodenruler;driven bya2V leadacidcellandcalibrated using a
galvanometerinserieswithWestonStandardcell,todetect thenull. It isacommon misconceptionthat
theCT160 isaBridge; itisnot, becauseaBridge hasfourarms,whereasthe Potentiometeronlyhas
three.IntheCT160,the anode currentflowsthrougha200 sensing resistor,thevoltagedropproduced
acrossthisresistoristhen compared toaknownvoltage; whichisdevelopedinaseparatecircuit,
comprised ofaconstantcurrentflowingthroughnine switchedresistors, R15to23,andone variable
resistor,RV4;whicharelabelled the ‘ANODE CURRENT’controls.
The anodecurrent controlsperformthe sameactionasthe backing-off controlsdointheAVOMkIII and
MkIV,theyeffectivelybalanceoutthe largestandinganodecurrent, whichisflowingthroughthe200
resistorinthe anode circuit.Thismakesitpossibleforthe metertoshowthesmallcurrent change
producedbythemA/V control, whenthemA/V measurementisbeingperformed.Inthe CT160,theanode
currentcontrolsaredesignedinsuchawaythatyou havetoread theanodecurrent, whichisflowing
throughthevalve,onthescalesprovidedon thecontrols, not onthemeter. Whereasinthe AVOMkIII
and MkIV,thebackingoffcontrolsarenotprovidedwithanyscales,andtheanode currentisindicatedon
themeter.
The anodecurrent controls(orbacking-offcircuit)act asapowersupply,whichisdesigned toproduce
exactlyonevoltforeachstepfrom0mA(0V)to90mA(9V)onthe rotaryswitch, orcoarseanodecurrent
control;plusthevariable0mA (0V)-10mA (1V)fromthefine anode current control; (infact,the maximum
on thefineanodecurrentcontrolisactually11.25mA, or1.125V). Thusmakesit possibleto‘balanceout’
amaximumof10.125V;whichcorrespondstoan anodecurrent of101.25mA. Thesmallextravoltage on
thefine anode currentcontrolisintended toproduceasmalloverlapon eachrange; sinceitispossibleto

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turnthefine controlpastthe 10onthe scale. Thisisprovidedsoyoucansee if thecurrentisslightly
higherthanthemaximumon that current range,orifyoushouldswitchtoahigherrange.When the
maximum(indicated)anodecurrentof100mAisbeing measured,inrealityonly50mA isflowingthrough
the200 anode current resistor; becausethe valveworksasahalfwaverectifierfortheappliedAC
voltagetoit. Now50mA flowingthrough200 producesavoltagedrop of10Vacrossthe anodecurrent
senseresistor; whichisthen compared,usingthePotentiometer,tothevoltageproducedbytheanode
currentcontrolcircuit.
Inthe coarseanodecurrentcontrol,thevoltagedropineachstep ofthe rotaryswitchmustequalexactly
1V;sinceeachstep hasaresistanceof80 , then theremust be aconstant current of12.5mAflowing
throughthecircuit. Nowthisvalueexceedsthe manufacturer’smaximumvalueof 9mAforthe
CV140/EB91/6AL5valves, whichAVOusedforrectificationintheCT160, andislikelytolead toa
shortened servicelife. Theseparticularvalveswereactuallydesigned asFMdetectordiodes,notas
currentrectifiers!
Asyouwill havenodoubtnoticed,the maximumavailablescreen voltage intheCT160is300V; thisis
due tothe valvemanufacturer’sPIVlimitof330V, (nowreferred toasRepetitiveReverseMaximum,
RRM). FittingsuitableSilicon diodeswillremovethislimitation;soif youdowishtoutiliseahigherscreen
voltage,youcoulddisconnectthe125V wirefromthe screenvoltageselectorswitch;asthisisthe‘odd
one out’,inthesensethat125Visnotavailableonthe anode voltage selectorswitch. Then movethe
highervoltagesback onetagon thescreen switchwafer, and thenconnectthe lasttagtothe400Vtapon
theHTtransformer.AVOmarkedthetapson theirHTtransformerwiththe‘mean voltage’, soitislabelled
as‘H400’(insteadofusingthe customaryrmsvalue,ie440V).Thescreenselectoroptionswill thenbe
exactlythesameasontheanodevoltageselector.
AsforsuitableSilicondiodes;don’t fitanyolddiodesthat just happen tobelyinginyourscrap box; itis
wellworthfitting soft recoverydiodes,whicharedesignedtominimisecircuitswitchingoscillations;for
examplePhilips’BYW96E, whichisrated at3A and1kVVrrm.Admittedly, wedon’trequireaforward
currentof 3A,howeverthelowercurrentsoftrecoverydiodesdon’tseemtohaveasufficientlyhigh
enoughVrrm;800Vwouldbesufficient.
Ifyou aregoingtoreplacethevalvesintheCT160withSilicondiodes,you will havetoalterthe series
resistanceR6(730 accordingtothelegend,butinreality750 wasused)whichisusedtoobtainthe
correctcurrent flowingthrough theanodecurrent controls,sothat thevoltage acrosseach80 resistor
will still beexactly1V.Ifyou placeastandardSilicondiode inplaceofeachvalvediode (rememberthe
CV140isadoublediode,soone Silicondiodewill be needed foreachdiode)thenyouwill haveto
increasetheseriesresistor,R6,toapproximately935 ,inthe anodecontrolcircuit.Thecalculationfor
thisisasfollows:-theanode currentcontrolcircuit issupplied fromatransformerwindingof50V rms;
whichisequivalent toameanvoltageof45.02V;(theratiobetween rmsandmeanvoltagesbeing 1.1107
forasinusoidalwaveform;whichgivesus50V RMS÷1.1107=45.02VMean).Nowtheapproximate
forwardvoltagedrop foraSilicon diodeis0.7V; howeverforaCV140 itis2.2V(asmeasured atthe
manufacturer’smaximumcurrent of9mA);sothevalueofR6mustbeincreased.
The anodecurrent controlcircuitrequiresanexactcurrentof 12.5mA (derivedfromthe1V volt drop
acrosseach80 resistor,1V ÷80 =0.0125A or12.5mA). Afterrectification bytheSilicondiode, you
then haveavoltage of 45.02V÷2-0.7V =21.81V DCmean. (The45.02Vhastobedividedby2,
becauseitisahalfwaverectifier; themeanvoltage following half-waverectification isobviouslyhalved,
and thefigureof0.7Volt comesfromtheforwardvoltage dropof theSilicon diode).
The totalresistanceinthe circuitwillthen havetobe 21.81V÷12.5mA=1744.8 ;fromthiswemust
subtract the existing resistorsintheanodecontrolcircuit=9x80 +90 =810 ;thusthenewvalueof
R6=1744.8-810 =934.8 . Youcould,ifyouprefer, useasmallvariableresistorinserieswithalarger
resistor,sothatyou canadjust the mean currenttoprecisely12.5mA. Alternatively, you might chooseto

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useacombinationofresistors, eitherinparallel,eg 1.1k// 6.2k; orinseries, eg 910 +24 , toadjustthe
totalcurrent.
Ifyou wantedtomeasurethecurrent, youcouldinsertacurrentmeterinserieswiththecircuit,but you
wouldthenhavetoadjustthecircuitfortheadditionalresistanceintroduced byyourcurrent meter, asit
will alsoproduceavoltagedrop.Itisthereforeeasiertomeasurethevoltagedrop across eachresistor
withadigitalmultimeter,asthatwill haveahighinput resistance,typically10M ,whichwillnotaffectthe
circuitasmuchasthe currentmeasurementwoulddo–quiteapartfromthat,you donot need breakthe
circuitforvoltagemeasurements!
Thisalsomeansthat if youneed toreplacethe90 anode currentpotentiometer, withanothervalue,say
100 (since90 willbe hardtofindnowadays)youcando that, but thenyouwillneedtoreducethe
seriesresistor, R6, bytheadditionalresistanceinthenewpotentiometer,inordertokeepthe currentat
12.5mA through the backing-offcircuit; inthisexample,by10 . Howeveryouwillneedtomakeanew
scalefortheanodecurrent potentiometer,and youwillgetalargeroverlapon eachrange, butitwillstill
givethe correctmeasurementsforanodecurrent. Thepracticalminimumvaluewill be closeto90 , asit
wasoriginally.Ifforanyreason,you needtogolowerthan 80 ,thenyouwillhavetolowereach80
resistor,and increasethecurrent, tomaintainexactly1V. A typicalreason forreplacingitwouldbethat
theoldoneisopencircuit, orperhapshasbecomenon-linear,duetowear,orisotherwisedamaged.
Makinganewscaleisquiteeasy if youusea360°protractor, togetherwithamultimetertomeasureout
eachstep ofeither0.1V, or8 ,andmarkthemontheprotractor,thentransferringthemtoapaperscale.
Or,alternatively, youcandrill aholethroughthecentreofafairlylargeprotractor, orapieceofPCB, then
fasten the potentiometerintheholeandusingalarge knobonthepotentiometer,and asyouturnit,mark
eachpoint on theprotractor/pcb, whichyoucanthentransfertoapaperscale.
NB:-Thereisonecasethathasnotbeencheckedthoroughlysofar,andthat iswhetherthe gm
measurementswill beaffected, if thepotentiometerischangedtoanyvalue otherthan90 !Thereisonly
aslightriskof that, sinceeachofthethree240 resistors,R24 toR26,areusedtocompensateforthe
anode current controlresistances-thisshouldbeinvestigatedfurther, beforeI canrecommendchanging
thepotentiometerforanothervalue;butmyguessisthatthe changedoesn'tmatter, asitisthevoltage
deliveredbetween the twopointsthat formthebacking-offcircuit, iswhatthemeasurementiscompared
with, andsincethatisunchanged,sothenisthe currentandresistanceinthat circuitpath.
However,therealsothe possibilitytoputapotentiometerwithahighervaluein, andthenshortingoutthe
lastpart ofthe trackabove90 ,andthenmakeanewscaletofitthe newpotentiometer;thenthiswill
workjustaswellastheoldone,except thatthenewscalewill be morecramped!
Fortheanode currentcontrolmeasurementstobe accurate,you mustensurethat theremainderof the
componentsintheAVOCT160arewithintolerance,andalsothat thetesteriscalibrated;howeverforthe
anode current controlsinthemselvestobe accurate,you mustensurethatavoltageof1V MeanDCis
developed across eachof the80 resistors.
The otherdiodesintheCV140 valves,V1&V2,canalsobereplaced withSilicon diodes,and fortunately
thevalueused forthe SETVgpotentiometer, RV3,islargeenough sothatnothingneedstobechanged
oraddedthere.EachrectifierintheCV140 hasaforwardvoltage dropof approximately2.2-2.9V,atthe
currentsinvolved(whichrange fromcloseto11mA, toup to14mA,through eachdiode), whereasa
Silicondiodehascloseto0.7V atthesecurrents. The differencebetween thesevoltagedropswillbe
takenupbyRV3asithasenoughresistanceforthisadjustment.
A quickcalculationofthisfollows:-55VRMSisequivalentto55÷1.1107 =49.52VMean, whichhalfwave
rectified, becomes49.52 ÷2=24.76V MeanDC.
ForSETVgcalibrationpurposes,AVOstateintheircalibration procedurethat avoltageof20.8VMean
DCshouldbepresentacrosstheGridVoltscontrol, RV2; thisthenmeansthatthediodeintheCV140