Reparacion y Modificacion Tube Amp Building

8/13/2019 Reparacion y Modificacion Tube Amp Building

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Musical Instrument Tube Amp Building, Maintaining

and Modifying FAQ

Much of this material applies to building or re-building hi-fi equipment, as well but it was

originally intended for musical instrument crazies.

Authored, assembled and edited by R.G. Keen, een!geofe".comMost recent re#ision le#el is $ersion %.&', ()%*)'%

+he first edition of this A was first released in &%.

Contributors

+hans to the contributors who helped and taught me/

0undreds of fols who taught M1 stuff when 2 didn3t now a triode from a +ri-A"is4 2can3t remember all of your names, and it all comes out as general nowledge now, but 2appreciate it. A few names in that category stand out/

+om 5alon 6a#id Mourning Mar 0ammer

And people who ha#e contributed things that 2 ha#e included as part of the actual te"t/

6ennis 738eill

8athan 9tewart

George Kaschner 6a#id Kohn

Michael 1delman

:en Mosowitz

;ohn 9toes

5rian


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&. 5asics of +ube Amps for irst +imers

o >hy use a tube amp?

o Are +ube Amps louder than solid state amps of the same power?

o >hat is @9tandby@ for?

o 0ow should 2 turn it on and off?o 0ow long do tubes last? >hen should 2 replace my tubes?

o >hat things will damage my tube amp, what3s safe and what3s not?

&. *** SAFT! "A#$I$% ***R1A6 +029 2R9+%. >here can 2 learn about building tube amps?(. >here can 2 find parts to build)repair amplifiers?B. 0ow can 2 modify my amp to be more powerful?*. 0ow can 2 e"tend my tube life?C. 0ow do 2 get...

5lues distortion? Marshall)metal)5oogie)etc. distortion?

Good distortion at lowDerE #olumes?

&. >here can 2 find plans for a 5elchfire)Ma"imo)etc. speaer cabinet?%. +ransformer questions/

&. 6o output transformers really sound different from one another? 2f so,why?

%. Are the impedance tolerances the reason one 7+ may sound better thananother?

(. Are paper bobbins really better than plastic bobbins in output

transformers? 0ow about power transformers?B. Are differences in transformer sound in the pattern of the windings? 7r

something else?*. +o copy a great sounding output transformer do you copy impedance or

winding pattern?C. 2s hand winding superior to machine winding for output transformers?F. >hat are @gaps@ in output transformers, and why are they good?. hat is an @ultralinear@ output transformer?&'.


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&C. >ill it hurt my amp)output transformer)tubes to use a mismatched speaerload?

&F. hat are the things about output transformer that cause the differences in

tone?&. 0ow can 2 tell if my output transformer is li#e or dead?%'. >here can 2 get a good replacement output transformer?%&. 2 want to mae my own power and output transformers. 0ow do 2 do this?)

>here can 2 find information about this?%%. 9hould 2 replace my stoc transformer with a new)old)#intage)purple one

for better clean)grunge)grit)etc. sound?%(. Are potted ) impregnated transformers better? 2s potting ) impregnation

necessary?

=ower +ransformers

(. >hat is the easiest way to get tube sound at a good price?B. 0ow can 2 modify my tube amp to ... ? Dalso see recommended mods, belowE

o get lower hum?

o ha#e higher gain)more distortion?

o ha#e a smoother, less buzzy distortion?

&. >hen should 2 bias my amp and how do 2 do this?

o >hat is @bias@?o >hen should 2 bias my amp?

o 0ow do 2 bias my amp?

o Matched output tubes - do you need them?

&. Amplifier Modifications

o 7K)Recommended amp modifications

o 87+ Recommended amp modifications

&. +ube


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o >hy do my tube ha#e a blue glow inside them?

o 7ther 2ssues

&. Appendi" A. +ube 9tuff 9uppliers%. Appendi" 5. +ube Maers =roducing +oday D1ric 5arbour news postingE

5ac to the G17 0ome =age

Basics of Tube Amps for First Timers

>hy use a tube amp?

+ube amps ha#e always been the amplifier of choice for the woring musician.Musical myth has ascribed almost magical tone to them. >hile the results maynot be entirely magic, tubes do ha#e a sound that is different from solid state

amplifiers, and one that happens to mae amplified music sound better to thehuman ears and brain. +here are lots of technical and psuedo-technicale"planations for why this should be true, but there is enough solid e#idence that itis a real effect to trust it. +he real reason to use tube amps is simply that theysound better. or that ad#antage, we put up with the poor supply of parts, highprices, fragility and e"cess heat that they produce.

Are +ube Amps louder than solid state amps of the same power?

8o. 0owe#er they do 97I86 louder. :et me e"plain.

9ome e"cellent scientific wor on tube preamplifiers and their distortion products

has turned up the mechanism for this. >hen tubes are dri#en outside their linearregion, for the first &%db or so of o#erdri#e the harmonics that they produce tricthe human ear into thining that the sounds are getting louder, when in fact thesound is getting progressi#ely more distorted.

2t is this acoustic tric that can mae tube amps sound up to &%db louder than theyactually are compared to a perfect, undistorted amplifier. A solid state amplifier ofthe same power as a tube amp may distort at the same signal le#el as the tubeamp, but the distortions are not subtle, and we hear them as distortion, not as aslightly louder sound. A solid state amplifier of much greater power would remainundistorted at higher le#els, and the tube amp would sound comparably loud to

the larger solid state amp.

+hey sound larger than they are.

>hat is @9tandby@ for?9tandby is used to mae sure the amplifier is quiet and lower the powerdissipation inside the amplifier during times when you will not need it for someperiod of time. 5reas in song sets are the ideal illustration. 2f you3ll be coming
http://www.persci.com/~larrysb/blue_glow.htmlhttp://www.geofex.com/tubeampfaq/TUBEFAQ.htm#AppendixA%23AppendixAhttp://www.geofex.com/tubeampfaq/TUBEFAQ.htm#AppendixA%23AppendixAhttp://www.geofex.com/tubeampfaq/TUBEFAQ.htm#AppendixB%23AppendixBhttp://www.geofex.com/tubeampfaq/TUBEFAQ.htm#AppendixB%23AppendixBhttp://www.geofex.com/http://www.persci.com/~larrysb/blue_glow.htmlhttp://www.geofex.com/tubeampfaq/TUBEFAQ.htm#AppendixA%23AppendixAhttp://www.geofex.com/tubeampfaq/TUBEFAQ.htm#AppendixB%23AppendixBhttp://www.geofex.com/


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bac to the amp within a short period of time Dunder an hour, sayE, flip it tostandby.

9tandby also ser#es as a softer power up switch. +o get the most possible lifefrom a set of tubes, the tube heaters should really be hot before the main power

supply is applied to the tubes. >hen tube rectifiers are used, the rectifiers do thisalmost automatically. 0owe#er, when solid state rectifiers are used in an amp, themain power supply comes up almost instantly, and this can shorten tube life bythe somewhat-esoteric means of cathode stripping. +he standby switch can beused to pre#ent this.

0ow should 2 turn it on and off?

2f it has a standby switch, flip this to standby, and then turn the power switch on ifit is separate, or if the power switch is simply 78-7-9tandby, turn it tostandby. :ea#e it this way for &*-(' seconds, then turn it to normal operation.+his gets the heaters hot before the main power supply comes up.

+o turn it off, simply flip the power switch to @off@, don3t use standby. +his letsthe still-conducting tubes bring down the high #oltages in the power supply.

0ow long do tubes last? >hen should 2 replace my tubes?

2t depends hea#ily on use. 2n a closet, the tubes will last fore#er, of course. orpractice in a bedroom a couple of times a wee at modest #olumes, you3llprobably get fi#e to ten years out of them. 2f you practice twice a wee for acouple of hours at full #olume and play two gigs a weeend, count on one to twoyears out of a set of output tubes. 8ote that this assumes that you got good ones tobegin with and that you had them properly biased when they were put in.

+ubes wear out by sheer hours of being turned on, by how hard they3re wored,how hot they get from Hust the heat in the bo", by the number of times they3returned on and off Dthermal shocE. 8otice that being played at ma"imum warpinto a dummy load Dor power brae, or attenuator, etc.E counts as being playedhard, and that because you can3t hear all the sound, you may not thin that you3reworing them hard.

Jour ears tell you when to replace them. >hen they no longer sound quite aspunch and sweet as they used to, start thining about changing them.

2 ha#e a somewhat more e"treme approach, myself. +he best time to get newtubes is when you 6783+ need them. Jou get the chance to find the best tubes atthe best price without time pressure. 2 prefer to eep a whole spare set ready. +hatway, a sudden burn out will not cripple the amp, and 2 can readily tell when oneof them is Hust not sounding right by subbing in a replacement that 2 already ha#e.5e prepared


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=reamp tubes - sure +hey3re in socets, any replacement tube of the same typewill at least not damage the amp.

1#en preamp tubes that are not e"actly the same type can often be substituted aslong as they ha#e the same pin connections. or the commonest type, &%AF,

there are many types that ha#e the same pinout and can be put in the same socetfor different gains and tone. or instance, you might be able to use &%AIF,&%A+F, &%AJF, 1


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o 8e#er ignore a red glow other than the small orange ends of the filaments.

A red glow o#er a large part of the internal plates of the output tubesmeans they3re about to melt Dyes, really melt - heat is our enemyE. 2f younotice this, shut it down and get a tech to help you find out what it wrong.


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to now this for the wor anyway. :ea#e the Humper in place whileyou do your wor D high #oltage electrolytics caps can @regrow@#oltage lie a battery sometimes. Really. E Remember to remo#e itwhen you finish your wor.

TST IT +ae your multimeter and ground the D-E lead. =robe the

high #oltage caps and be sure the #oltage across them is down,preferably to less than &'$. B)TT-$ IT BAC ) FI#ST +ae the shorting Humper out.

=ut the chassis bac in the cabinet, maing sure all of your tools,stray bits of solder, wire, etc. are out of it. Jou don3t ha#e toactually put all the screws and so forth bac in if you belie#e morewor might be needed, but mae sure that the chassis is sittingstably in the cabinet and won3t fall out. At the end of a listeningtest, either continue buttoning up if you3re done, or go bac toI8=:IG 2+ 2R9+.

5asics of +ube Amps for 5eginning Isers


".ere can I learn about building tube amps/

Get one or more of the following references Dnote that these boos are mostly old,and highly sought after, and so may be e"pensi#e and hard to findE/

@+he Iltimate +one@ by Ke#in 73


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Glass Audio magazine, 7ld


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D+om mentions a %nd and (rd #olume in the distant future - co#ering low powerpentodes N oddball tubes, and =ower N 5eam =ower pentodes respecti#ely.E

:earn about the manual and safety aspects of woring on tube

amplifier circuits. Read the ARR: handboo, or better yet, get tonow a ham radio operator who will gi#e you some guidance andteaching. 6o not simp on the safety aspects. +ube circuits containdeadly #oltages. Jou can - O 621 O - if you mess up or arecareless. 2t is your personal responsibility to learn how to do thissafely.

Get to now a guitar repair technician, perhaps do some freeapprentice grunt wor for them in return for some teaching.

5ac to the +he &%* $acuum +ube =itch +hereminG170+he &%* $acuum +ube =itch +hereminome =age

".ere can I find parts to build0repair amplifiers/

>hen 2 first wrote the +ube Amp A almost ten years ago, tubes were a dyingbreed. 2n that decade, the tube world has turned around. +oday, there are maHorsources for tubes and parts. +he list is long enough that 23m se#erely cutting bacthis section.

8ew tube parts and supplies were steadily getting harder to find in the '3s, but inthe last decade this has turned around radically. +here are now many companiesoffering new parts, especially power and output transformers. 2t is still true thatused parts are often nominal cost or free. +he hard parts to find in high quality arethe transformers.

2f you3re building, 2 recommend getting your transformers first. 2f you are getting#intage parts, they are liely to be one-of-a-ind. 2f you3#e Hust ordered new ones,the transformers will ha#e a massi#e effect on your chassis3s mechanical layout.

+he easiest but most e"pensi#e source for parts is at your retail musical

instrument store as @repair@ parts. 7ther sources/

Musical instrument repair shops will sometimes order parts or sellyou parts out of their stoc.

Amp maers3 repair parts departments. Many manufacturers willsell their parts to @repair shops@ to fi" their amps. 9ome of themare better than others about this, so be polite and businesslie.
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old, broen, or unlo#ed equipment. +his may be free, or units-of-dollars. Jou get transformers, socets, tubes, and chassis in thedeal. May require cruising garage sales or di#ing in dumpsters.+rash e#ery part e"cept the tubes, transformers, socets andchassis. 2 got a &* >att mono amp)preamp intended for mono hi-fi

music for %' at a local garage sale. 8eeds only some tweaing tobe a 9tudio .%% or an A


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(. only positions in the amp that ha#e both high 6< #oltage and wide signalswings as in & and % will gi#e you enough resistor distortion to benefit from4 otherplaces should be chosen for low noise and)or economy.B. resistor power rating should be the minimum needed to wor for a reasonablelife in the circuit to ma"imize resistor distortion. Maybe a good guideline is that

the dissipation should be selected to be as close to two times the a#eragedissipation as possible.*. as a corollary to the power guideline, we should be prepared to replace , and a &'P change for biggerresistors. +hat3s one of the thrusts of guideline B - pic the smallest dissipationresistor you can, to ma"imize the coefficient.

7f course, that3s as big as the effect can get, and you would ha#e to carefully setup the situation to get that much resistor distortion. 2n an amp, you probably won3tbe able to get that close to ma" #oltages or signal le#els. Realistic le#els might be%''$ across a &)%> resistor, and a F*$ signal swing. +hat would gi#e you a%.CP distortion - enough to be audible as sweetening. +hat3s the point of guideline

( - you ha#e to ha#e a big enough signal swing across the resistor to ha#e thesignal distorted significantly by the #oltage coefficient.

5ut with a &'$ signal, you only get '.(*P distortion, and it starts down theslippery slope to inaudibility. More importantly, these percentages represent thema"imum beyond which a resistor would ha#e been reHected in the &*'3s.+oday3s


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le#el is typically too low. A &%AF can be dri#en from cutoff to positi#e grid#oltage with a couple of #olts of signal, so the grid resistor ne#er has a big enoughsignal to be distorted appreciably.


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An amplifier is properly thought of as primarily a big power supply that has somee"tra Hun taced onto it to carefully let a little of the power out to the speaersunder special, controlled circumstances.

Jou might be able to Hust pull a couple of tubes 7I+ of a high power amp to

mae it quieter, under some conditions. 73


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7#erdri#ing a tube stage too much gi#es you harsher clipping, not the singing,sweet distortion we want. +o really get sweet, crunchy distortion, eep each stagethat goes into distortion no more than C-db into distortion.

Marshall)metal)5oogie)etc. distortion? Made by massi#ely

o#erdri#ing preamp tubes until the original wa#eform is massi#elycompressed and clipped. Isually followed with a moderate amountof high frequency cut to remo#e some of the @insect attracting@o#ertones generated in the clipping process. +here is commonlysome output tube o#erdri#e in this process, too.

Good distortion at lowDerE #olumes? o#erdri#e preamp tubes untilyou get the clipping you want, then feed a limited amount of this toa power amp stage to get the loudness you want. +his is howmaster #olume controls wor.


".ere can I find plans for a Belc.fire0Maximo0etc6 spea7er cabinet/

1lectro$oice sells D?E maes a#ailable D?E plans for cabinets fortheir speaers.


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0igh and :ow requency Roll-offs/ +ransformers all by themsel#es ha#e a highend rolloff point and a low end rolloff point, and a broad flat region between thetwo. +he e"act frequencies where the highs and lows roll off are a characteristicof the iron, copper, and how they3re wound and staced together and treated ormis-treated.

2t is possible to get a resonance or two between the two ends, but most often thereis only a self-resonance point abo#e the high frequency end. Isually anyreasonably well-made output transformer has no oddities within the high and lowfrequency roll-off points.

Gi#en enough info about the transformer and the use Dpower through it,temperature, 6< bias current, etc,E you can use the parasitic parts of thetransformer3s response, the una#oidable self capacitances and leaageinductances, with other components outside the transformer to shape thefrequency response inside the relati#ely flat pass band4 this is more lie shaping it

e"ternally than designing the response into the transformer, though. +he parasiticsare largely what cause the high and low frequency cutoffs, so it3s not easy to mo#etheir effects out into the passband with simple circuits.

Jou can design in a high and)or low frequency response point, and you may beable to tiner inside the passband a little, but e#en an e"perienced designer can3teasily Hust design in a scoop of arbitrary depth at a gi#en frequency with only thetransformer windings to wor with.

2nherent Dor 2ronE 6istortion/ 2t3s not widely understood, but transformers ha#etheir own distortion generating mechanism. +he iron in a transformer has to be

dri#en to a certain magnetic flu" le#el to couple signals to the secondaries. +heoutput tubes must supply this magnetizing power as well as the power to thesecondaries. A well designed transformer can mae this power #ery small.0owe#er, the nature of the iron itself is that the magnetizing power is non-linear.2t taes proportionately more or less magnetizing power at different magnetizationle#els, so the iron siphons off more or less of the signal.

+his depends on the core material linearity and magnetic softness, and how closethe transformer is dri#en to saturation. +he amount of distortion depends, amongother things, on the source impedance dri#ing it, as the distortion comes from thenonlinear shunting effect of the transformer3s primary inductance. A #oltagesource dri#ing a transformer will be able to be distortion free on the secondary. Ahigh impedance dri#e source Dlie a pentode plate, unfortunatelyE will not be ableto pro#ide all the current the core needs to eep the #oltage linear, and so somedistortion will show up on the secondary as a result. +his distoriton is relati#elysmall, probably %-*P below the beginning of saturation, and is primarily thirdharmonic.


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1"cess 6istortion/ 7nly in push-pull


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:et me be clear - 2+39 87+ +01 575528, 2+39 +01 6192G8 A86A991M5:J.

/Are differences in transformer sound in the pattern of the windings? 7rsomething else?

A/0a#ing pooh-poohed the idea of paper bobbins being better with the @a turn3s aturn@ argument, 2 hesitate to get into this, but 2 guess 2 should. +he thing thatmaes for an 7+ tone is the sum total of the contributions of all those parasiticelements in the transformer - the series wire resistance, the leaage inductance inthe magnetic path, the turn-to-turn capacitance, the layering and distribution oflayers in the window, the depth of the core stac, the thicness of the iron in thelaminations the -Omagnetic softnessO- of the iron in the laminations and howcarefully they ha#e been interlea#ed and butted together, and how free thelaminations are from electrical contacts.

=aper bobbins are one of the few things that 6783+ affect a transformer3s tone, allother things being equal.

2t is really a no-brainer to copy a transformer, but you ha#e to now about and beable to copy all of the details about how it3s made. 7nce you do that, you getfairly repeatable results, with the e"ception of the handling of the iron.+ransformer iron is annealed to be physically and magnetically soft Dwhich gotogetherE, and an o"ide layer is grown on it to pre#ent the layers from electricalcontact, which affects how much core loss happens. 2f the iron is roughly handled,it gets hardened and has more core losses and higher distortion, as well as ha#ingthe o"ide scraped and or punctured, which increases eddy current losses. +hese

will ha#e an effect. Jou can detect the changes in magnetic characteristics oftransformer iron by testing the iron into a test coil, then striing it sharply 1$1878


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/ 2s hand winding superior to machine winding for output transformers? >hen anown @good sounding@ #intage or modern transformer is duplicated, will modernequipment should be able to consistently reproduce them without #arying results?2s hand winding being superior to machine winding is a myth?

A/ +he de#il is in the details. Machines can produce goods with a consistency thata human hand)eye)brain simply can3t, so where pure replication is required, bet onthe machine. >hene#er the materials)situation)adHustments, etc. requireHudgement and on-the-spot compensation, bet on the human - 2 the human is ane"pert at whate#er the situation is.

2 would say that historically, the real situation is that cheaply produced and poorlydesigned Dfor instance, non-interlea#edE machine wound transformers wereaccurately reproduced, and sure enough, the sound was reliably bad. A good handwinder repro-ing a carefully staced interlea#ed 7+ could, in spite of theine#itable slight #ariations and flaws produce a much better sounding

transformers - at a much higher price - in low quantities. A myth gets started - @2used so-and-so3s hand wound 7+, and it ran rings around the machine-woundreplacements,@ which is demonstrably true, but then the leap to myth is made@therefore only hand wound transformers are good.@ +his train of reasoning is soseducti#e in the @#intage-is-god@ en#ironment that it almost instantly becomescommon wisdom, putting the merit on hand winding when in fact the differenceswere in different designs.

or duplicating windings, a well made, well adHusted and well maintainedmachine will produce more consistently identical windings than a human. 8otethat 2 put a number of ca#eats on that statement.

An e"pert human with little or no equipment will produce better, more consistentgoods than a bored, ill-trained, poorly paid operator coping with the wrong wiresize on a poorly adHusted forty year old machine, which is how some Despeciallythe cheaperE transformers get made.

9o - is the notion that handwound transformers are A:>AJ9 superior51


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/ 23#e read references to modifying == 7+s, adding a mysterious @gap@Dmysterious to me at leastE for 91 use. >hat does that mean?

A/=robably this/ process of adding a gap means carefully unstacing aninterlea#ed == transformer, restacing the 13s and 23s into non-interlea#ed chuns

and reinserting the bobbin with a spacer. +his gets you a transformer that has alower primary inductance, but that won3t saturate as easily. 2t3s probably not whatyou would originally ha#e designed for any gi#en 91 circuit, as the primaryinductance is now lower, but it might wor. And it R1A::J appeals to the home-handyman tube hacer crafstman instinct.

/ >hat are @gaps@ in output transformers, and why are they good?

A/ +he @gap@ in single ended transformers is Hust that - a space where there is noiron. +he way this is done with 1-2 cores is to stac all the e3s together and insertthem into the bobbin in one chun, then put a paper or fiber spacer across the end

of the @1@3s, and then the 23s are laid on in a chun. +he point of this is that there isnow a gap where there is no iron bridging it that maes the magnetic field Humpacross. +his linearizes the magnetic properties of the structure as a whole, as theproperties of the gap are so different from iron that they dominate, and the gapcannot saturate lie the iron can.

+he o#erall primary inductance is much lower now than if you had interlea#ed thelaminations, alternating the direction of the 13s in the center of the transformer,though, so you must use much more iron and copper to get as much primaryinductance and low frequency response as you would ha#e had if you3dinterlea#ed the laminations, so the transformer gets big e"pensi#e, and can

support much less output power for its size than it might otherwise. +en wattgapped single ended 7+3s for instance may be four to si" times as hea#y as


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designer, there are going to be cases where it will not wor or won3t wor well.1"periment if you lie, but be prepared to toss the transformer and)or output tubesif they fry, or tear it all down if it Hust doesn3t sound good.

/ >hat is an @ultralinear@ output transformer?

A/ 2t3s a transformer with a tap at about halfway between the 5 connection andthe plate connection to which you can attach the output tube3s screen grid. +hisconnection pro#ides some feedbac to the screen grid as well as a bias #oltageand has been found to act lie a connection halfway between pure pentode modesand pure triode mode, with lower distortion than either. 2t3s almost a requirementfor tube hifi ampliers. 5ass amplifiers use it to get large amounts of clean power.2t usually sounds too @sterile@ or clean for guitar players3 tastes.

/


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+he safe way to find out the phasing is to use two resistor-loads, one persecondary, and then to connect one end of each secondary together4 dri#e the ampwith a signal generator and measure the A< #oltage across the free ends. 2f the#oltage is % the #oltage on either secondary, you3re phased correctly, and youcan lea#e the center connection where it is, remo#e the resistor loads, and put one

speaer across the free ends.

2f the #oltage across the free ends is smaller than the #oltage across eithersecondary, they3re phased wrong, and you need to swap the ends of onesecondary.


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+ransformers transform impedances as a pure ratio. +hat is, a BB'' == to ohmtransformer maes any load on its secondary loo lie it3s **' times bigger to atube at the primary. An ohm secondary load then loos lie a BB'' ohm load atthe primary. 2t also maes a &C ohm load loo lie an '' ohm load if you hoo&C to it, %%'' if you hoo a B ohm load to it, and similarly for all #alues in

between. =ower tubes ha#e a power output that depends on matching - that is,they ha#e sweet spot load that they do best on, most power out, and other loadswill get less power because the tube itself limits how much power it will transferout. SActually there are two sweet spots, one for highest power, one for lowestdistortion4 the two spots are not the same for any nown tube. rom zero ohmsloading up to some ill-defined number of ohms higher than the optimum powerload, power tubes do not destroy themsel#es, they merely change how much theytransfer to the load. 9o - if you ha#e a tube amp with a tap for ohms, you willget the nominal power of the amp only with a @matched@ ohm load. 2f you hoo&C ohms there, the power tubes @see@ a proportionately higher ohmage on theirplates, and can only put out about half the nominal power. 2f you hoo up a B ohm

load to the ohm tap, the power tubes @see@ a load about half of the matched one,and again will put out only about half of the nominal power. +his @half thenominal@ power is not fi"ed because of the %/& change in load, but #aries fromamp to amp and tube to tube, and may not be e"actly %/&. 2n addition, speaers are87+ single impedance loads. 2t is con#enient to thin of @-ohm@ speaers, butthe plain fact is that the speaer3s impedance #aries with frequency and also withthe acoustic loading Dcabinet and other thingsE that the speaer sees. +hatimpedance meter is not going to be a huge help, because you ha#e to specify thefrequency being tested as well as the impedance to ha#e something meaningful.

/ >hy do 2 ha#e to match speaers to the output impedance of the amp?

A/ Jou3ll get the most power out of the amp if the load is matched.

/>ill it hurt my amp)output transformer)tubes to use a mismatched speaerload?9imple A/ >ithin reason, no.9ay for e"ample you ha#e two eight ohm speaers, and you want to hoo them upto an amp with B, , and &C ohm taps. 0ow do you hoo them up?

or most power out, put them in series and tie them to the &C ohm tap, or parallelthem and tie the pair to the B ohm load.

or tone? +ry it se#eral different ways and see which you lie best. @+one@ is nota single #alued quantity, either, and in fact depends hugely on the personlistening. +hat #ariation in impedance #ersus frequency and the #ariation inoutput power #ersus impedance and the #ariation in impedance with loadingconspire to mae the audio response cur#es a broad hump with ragged, humpedends, and those humps and dips are what maes for the @tone@ you hear andinterpret. >ill you hurt the transformer if you parallel them to four ohms andhoo them to the ohm tap? Almost certainly not. 2f you parallel them and hoo


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them to the &C ohm tap? 1"tremely unliely. 2n fact, you probably won3t hurt thetransformer if you short the outputs. 2f you series them and hoo them to the ohm or B ohm tap? Inliely - howe#er... the thing you hen the current in the primary is dri#en to bediscontinuous, you get inducti#e icbac from the leaage inductances in theform of a #oltage spie.

+his #oltage spie can punch through insulation or flash o#er socets, and thespie is sitting on top of 5, so it3s got a head start for a flasho#er to ground. 2fthe punchthrough was one time, it wouldn3t be a problem, but the burning residuesinside the transformer mae punchthrough easier at the same point on the ne"tcycle, and e#entually erode the insulation to mae a conducti#e path betweenlayers. +he sound goes south, and with an intermittent short you can get apermanent short, or the wire can burn though to gi#e you an open there, and nowyou ha#e a dead transformer.

9o how much loading is too high? or a well designed Dequals interlea#ed, tightly

coupled, low leaage inductances, lie a fine, high quality hifiE 7+, you caneasily withstand a %/& mismatch high.

or a poorly designed Dhigh leaage, poor coupling, not well insulated or pottedEtransformer, %/& may well be marginal. >orse, if you ha#e an intermittent contactin the path to the speaer, you will introduce transients that are sharper and hencecause higher #oltages. 2n that light, the speaer impedance selector switch couldill 7+3s if two ways - if it3s a brea befor mae, the transients cause punch


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through4 if it3s a mae before brea, the 7+ is intermittently shorted and thehigher currents cause burns on the switch that e#entually mae it into a breabefore mae. +urning the speaer impedance selector with an amp running issomething 2 would not chance, not once.

or why Marshalls are e"tra sensiti#e, could be the transformer design, could bethat selector switch. 2 personally would not worry too much about a %/& mismatchtoo low, but 2 might not do a mismatch high on Marshalls with the obser#ed datathat they are not all that sturdy under that load. 2n that light, pulling two tubes andlea#ing the impedance switch alone might not be too bad, as the remaining tubesare running into a too-low rather than too-high load.

/ hat you3re maing really 29 two class A output stages run in parallel. >ith

no magnetic coupling between the two half-primaries, there is no interaction onthe secondaries, either. Jou ha#e to run them


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A/ >hat you ha#e ased, translated into transformer-gee language, is @0ow do 2completely describe the equi#alent circuit of a transformer and the circuit itresides in?@

+o be truthful, there isn3t all that much mystery about transformers, but it3s not

lie the rest of your e#eryday electronic parts. +ransformers are susceptible toelectronic modelling, and once you get the model correct, you can twiddle the#alues until you get the @tone@ you want, including nonlinear effects. +he later#ersions of 9=2


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e"citation does the core start going into saturation, and from the composition ofthe iron, what is the irreducable energy loss per cycle to magnetizing losses,which shows up as pure third harmonic distortion.


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#oltage wa#eform is distorted on both primary and secondary. Infortunately, weneed the transfomer 51hat you


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CA)TI-$ CA)TI-$ CA)TI-$

5oth the filament transformer and the transformer under test will ha#e at least Ahat3s nice about the cathode resistor method is that you3re not dealing with high#oltages. +he cathode sits #ery close to ground so the chance of a dangerousmistae is lessened. Jou3re also reading each tube3s bias current indi#idually.


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-t.er Met.ods

9ome of the manufacturers say to set the bias #oltage to some specified #oltage,without any other measurements. =resumably some designer somewhere decidedhow much was good for you and wrote down @9et the bias to "" #olts@ as a good

compromise for all the tubes s)he e"pected. +his method ignores the #ariability oftransconductance in output tubes, and only gi#es good results for matched setsthat happen to be e"actly lie the @typical@ ones the designer thought they3d get.8ote that Gr!!#e +ubes tries to help by pro#iding matched tubes with a biasnumber from & to &'. 2f you ha#e G+3s with a @B@ bias number, and you replacewith a G+ @B@ set, they will ha#e selected only tubes that are properly biased atthat le#el, and no rebiasing will be necessary. 7f course, G+ e"pects to be repaida fair profit for this ser#ice to you...

Another way to set bias is to use a test signal, typically a sine wa#e. Monitor theoutput wa#eform on an oscilloscope and adHust the bias for minimum crosso#er

distortion. +he ob#ious problem is when has it @Hust disappeared@? Most fols doHust a bit more than @Hust disappeared@ and get their outputs too hot causingshortened tube life and o#erheating. 8ot #ery accurate or repeatable.

Jou can also use a special purpose instrument that nulls the input signal out of theoutput signal so that you can monitor Hust the distortion products. Jou then adHustthe bias to get the distortion to a realistic minimum without maing itdramatically less than the residual +06. +his is the premium method, but requiresa distortion analyzer - big bucs.

+hese methods can be more accurate than the first two methods but they require

e"pertise and tools that most fols don3t ha#e.

2f you are a circuit hacer, and li#e on solder fumes and cold coffee, you canmodify the amp with solid state ser#o bias adHusters that twiddle the bias to eachoutput tube on the fly on a continuous, real time basis to eep each tube -O e"actlyO- where it ought to be. 7nly recommended for real wiring fanatics...

%$#A+ BIAS %)I&+I$S >from Tremoluxaol6com?


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1:-B)C5* - %% to %F ma


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2f you mismatch, try to get the 6< current the same in both sides of the outputtransformer, and an imbalance in the A< gain of the tubes. +he logical limit ofthis A< mismatching is to remo#e all the A< dri#e from one output tube, which isa technique used by at least one commercial amp maer. +his effecti#ely eepsthe output transformer happy with respect to 6


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the grid lea resistors. +his increases the a#ailable current to the grids and eepsthe tube out of runaway.


Amplifier Modifications

A6 -0#ecommended amp modifications

Read the SAFT! "A#$I$%first, before you put your hands - or otherpersonal parts - into a tube amp.

=ut a fuse in the 5 line after the rectifierDsE and before the first capacitor filter. +his cansa#e burning out your power transformer and maybe your output transformer if you get ashorted filter cap, shorted output tube, or lose bias on an output tube. 2t -O might O- sa#e

an output tube that has lost bias e#en though it also might not. +he fuse current ratingshould be slightly larger than the ma" current rating for your output tubes, generallymuch less than &A.

=ut a &(' or &*' $olt M7$ surge protector across the A< line at the power transformerprimary to absorb spies from air conditioners and motors turning on and punchingthrough the primary insulation. Recent articles say that &('$ M7$3s will e#entuallyshort, recommending only &*'$ M7$3s.

Get rid of all two wire line cords and line switching arrangements. Refit with three wirecords, tieing the safety ground to the chassis. Jou3ll lo#e this the ne"t time you touch a

mie or stand while holding a guitar. 8o shocs. 7h, yeah. 6o it to A:: your equipmentto be safe.


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=ut &*''$olt, &A silicon diodes in series with the two sections of your rectifier tube Difyou ha#e a rectifier tubeE so that if the rectifier tube shorts, the silicon will sa#e theoutput tubes, and power and output transformer.

Gerald >eber ad#ocates using a %F'K)%FK resistor di#ider from 5 to raise the filament

windings in a 6< sense abo#e ground. +his eeps electrons from the filament fromhitting the plate, another source of hum.

=ut &*''$olt, &A silicon diodes in series with the two sections of your rectifier tube Difyou ha#e a rectifier tubeE so that if the rectifier tube shorts, the silicon will sa#e theoutput tubes, and power and output transformer. +he 5 will go up about *'$ when DifEthe rectifier tube shorts, so the amp will ha#e a little more power and run hotter. +his canstill hurt modern manufactured power tubes if it goes on too long, so chec the rectifiertube frequently.

5ill >ebb3s fa#orite tone mods for ender amps

at the $ibrato channel3s second gain stage, change the ceramic '.'%u coupling cap topolypropylene or polystyrene

replace the coupling cap at the input of the phase in#erter with a better capDpolypropylene - polystyrene - mylar in order of preferenceE4 change its #alue to '.''& tomae the amp @sparlier@ and to '.'& to mae the amp sound bigger and more midrangy

+he (.(M resistor which mi"es the dry and re#erb at the output of the (rd gain stage,#ibrato preamp, is paralleled by a &'p ceramic cap.


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Ising a #ariac to run it at a higher or lower line #oltage. +his might be 7K e"cept thatrunning it higher can o#erdissipate parts and burn them up or o#er#oltage things liefilter caps, which can short and burn out your -O e"pensi#e O- output transformers, aswell as burning out your tube filaments by putting too much current through them4 andrunning it lower star#es the filaments for current, so they can3t put out enough electrons,

and any remaining gas in the tube bombards the cathodes, poisoning the electron emittingmaterials on the cathode surface, and wearing the tubes out early.

Adding massi#e amounts of capacitance to the power supply filters to reduce hum.=robably 7K with solid state rectifiers, but in amps with tube rectifiers this can causecurrent spies in the rectifiers that e"ceed the instantaneous current rating of the rectifierand wear it out quicly.

8athan points out @2 seem to recall one of my +ube Amp Mentors telling me thatthis is pretty much only the case with the first filter cap after the rectifier, and thatthe impedance of the power supply was high enough that you could dump

hundreds of uf worth of filtering on latter stages Dthough the only place it3s ofmuch benefit is at the power tube plate supply point.E

replacing your rectifier tube with a solid-state plug in module replacement. +hiseffecti#ely Hust puts in a pair of silicon diodes which tae the place of the tube. 5ut it alsolets the 5 come up about *'$. +his won3t ill the amp immediately, but it runs theoutputs hotter. ender often put more than the rated ma"imum #oltage on the outputtubes to get moe power out of them4 old I9 and 1uro manufactured tubes would usuallyhandle it Hust fine. 9ome lower cost modern manufacture tubes AT and substitutes - preamp and dri#er tubes

&%A+F, &%AIF and subs, preamp and dri#er tubes

&%AJF - dri#er tubes

C1IF - dual triode used in some older amps for preamp tube
http://www.geofex.com/http://www.geofex.com/


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C:C types - power output tubes, up to *' watts)pair, a mainstay of ender

1:(B - 1uro power pentodes, up to *' watts)pair, many Marshalls

C$C - smaller, lower power cousin of the C:C, &'-&B watts per pair4 used in smaller

enders

1:B - fits a pin socet lie a &%AF but twice as tall4 miniature power pentode, goodfor &%-& watts per pair4 used in smaller $o" amps, and a quad of these dri#es the $o"A


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12A-7.!" $(()+/ 6072

2082

o2er tube substitutions9

C5*)1:B Dminiature pentode with pinout


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are now getting rarer and more e"pensi#e. +here are a number of other pentode)triodesthat can be substituted, but the pinouts are different and this will require require rewiringthe socet for the tube. 1"amples are the CA8 and the CI. +here is a Russian tube thatis labeled F& which may wor, although this is new.

F'%F)F'%FA - this is a high power tube similar to a C**'. +he supply of these is #erypoor.

F&)F&A - a higher power)#oltage #ersion of the C5*)1:B. 0ard to find. A stocC5*)1:B may wor if the power and #oltage conditions in the amp are not right out atthe limits of the tube design.


Maintenance Issues

hy?>hat3s a cap Hob? A technician may recommend you ha#e a @cap Hob@. +his meansthat he will replace e#ery single electrolytic capactor in the amp, from the powersupply right down to the cathode bypass caps.

+his is because electrolytic DpolarizedE capacitors ha#e an inherent wear-outmechanism and will e#entually die e#en if you don3t playdeath)metal)country)barbershop through them e#ery day - in fact they may wearout sooner if you lea#e it sitting in the attic. 0ere3s why.

A capacitor is essentially two conducti#e plates separated by an insulator. +hebigger the plate area and the thinner the insulator, the higher the @capacitance@ is.1lectrolytic capacitors get a #ery thin insulator by @growing@ an insulating layerof aluminum o"ide on the outside of a rolled up piece of aluminum foil.

+he o"ide layer is @formed@ at manufacture by feeding the aluminum foil a #erysmall and carefully controlled amount of current. +he current causes a chemicalreaction between the foil and the water solution Delectrolyte ... hey... is that wherethey got the name?? yep.E which maes an o"ide layer grow. As the layer grows,they use higher and higher #oltages to force the same small current through thelayer, which gets thicer and more resisti#e with time. >hen they ha#e to use the

full rated #oltage to get the forming current through, the cap is fully @formed@ andready to ship.

2f the capacitor is used regularly, has #oltage applied to it, and does not get toohot, the o"ide film lasts up to a few decades. 2f the capacitor is not used much, orgets too hot, the o"ide film slowly un-forms, the leaage current goes up, and itwill e#entually short.
http://www.geofex.com/http://www.geofex.com/


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1lectrolytic caps are designed to last ten years. 2t is a tribute to the quality ofmanufacture that they often last three, sometimes four times that.

7ld amps, particularly if they ha#e not been used regularly need to ha#e e#eryelectrolytic cap replaced. +his cap Hob may be needed e#ery ten or so years.

8on-electrolytic caps do not ha#e this wear out mechanism, and do not needreplaced for this reason. Modern capacitors can in some circumstances be muchbetter than old ones, and you can sometimes get a clearer, more sparly tone bychanging the non-electrolytic caps - assuming that is something you want to do.

6o new caps need to be formed?+here3s a lot of contro#ery on @reforming@ replacement caps. 0ere are a fewanswers.Manufacturers of caps design their caps for a ten year woring life, and a fi#eyear shelf life. +hat means that the stresses and heat of woring in equipment will

lea#e the #ast maHority of caps functioning 7K after ten years of normaloperation. After that, it3s gra#y to the buyer.+hey also design them to wor 7K after sitting on a shelf unused for fi#e years,meaning that the cap should not fail if it3s put into operation at rated #oltage aftersitting unused for fi#e years. As noted abo#e, the caps do slowly un-form withoutregular use.2f the electrolytic caps you use to fi" your amp are o#er fi#e years old asdetermined by the date code on them, you ought to at least worry about formingthem, and if they3re o#er ten years old Dlie 879 multisection cansE, definitely re-form them. 7ther than that, put them in and turn it on.0ow do 2 @re-form@ electrolytic caps?

Jou3ll hear fols tal about @bringing an amp up slowly on a #ariac@4 this canwor but is not particularly good for your tubes. A better way is this/

=ull out all the tubes.

if your amp has a tube rectifier, solder in temporarily some high #oltage silicon diodesacross the tube lugs to be a rectifier that does not depend on the filament #oltages. 2f youramp has silicon diodes, you can sip this.

open up the wire that goes from the rectifier tube Dor solid state diodesE to the first powersupply filter stage and solder in series with the wire a temporary &''K %- *> resistor.

+his resistor will limit the current that can flow into the caps and the amount of #oltagethat is applied to them to safe #alues that will cause the insulating layer to re-form.

clip your #oltmeter across the resistor

button it up. +urn it on Dno tubes in it, rememberE. >atch the #oltmeter.

when the #oltmeter reading drops to less than %'-('$6


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open it bac up and pull out those diodes and resistor, putting it bac in original shape.

+he forming could tae hours to days.

9ocets

9ocets get dirty, corroded, broen, and @arced@ +o recondition them, get a can ofspray contact cleaner, the ind that says @no residue@. 9quirt some in each socethole, then insert that tube in the socet, wiggle it around, and remo#e it se#eraltimes to get the crud off. +ae a thin tool lie a Heweler3s screwdri#er or ice picand gently bend the contacts inside each hole so they hold the pins better. 2f thesocet is craced, or has blacened lines from pin to pin Dwhere an electrical archas actually burned the socet into a carbon material that conducts electricityE,replace the socet.

/ Are plastic or ceramic socets better, or is there any difference?

A/ +he material is significant.

+hermoset plastics are what are usually used for socets. +he blac-brown stuffmost are made from is @baelite@ a trade name for a ind of clay-reinforcedphenolic. Maybe there3s a #ariety that is purer or more sturdy phenolic that ismore resistant - 23m fuzzy on that one.

All plastic socets are #ulnerable to arcing. >hen you get enough #oltage frompin to pin on an output tube to cause a spar to Hump from pin to pin Dlie whenyou run the output transformer unloadedE the spar runs along the surface of thesocet material and burns a trail on the surface. 9ince the plastics contain carbon,

there is often a carbon residue left on the surface. +his residue is partiallyconducti#e, and maes that path susceptible to arcing o#er at lower #oltages ne"ttime4 this can be so bad that it interferes with normal operation.


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fluorescence by the high #oltages in the tubes. Inless it is e"cessi#e, it is notharmful. +ubes with softer #acuums glow more.7ther 2ssues:ots of good info is contained in ;ac 6arr3s @1lectric Guitar Repair 5oo@, if youcan find a copy Dit3s now out of printE and in =ittman3s @+he +ube Amp 5oo@ and

>ebers @6estop Reference...@. :oo for/ - checing for capacitor leaagerom watching a tech wor on enders, 2 piced up a nice tidbit. +he eyeletboards in enders ha#e most components mounted across the eyelet board. A #eryfew parts run along the length of the eyelet board. 5ecause the eyelet boardfle"es, there is a lot of stress on the solder Hoints at the end of these lengthwisecomponents and the Hoints often crac. 1#ery time you open up a ender, tae aloo and maybe a soldering iron to these Hoints. 2f it3s your personal amp, youmight want to get a new part for these positions with long leads and bend a loopin the leads so that the leads can fle" and not put stress on the solder Hoints.


Appendix B6 Tube Ma7ers roducing Today >ric Barbour ne2s

posting?

D+he following is the te"t of a note posted to the alt.guitar news group by 1ric5arbour Vs#etengr!earthlin.netit is also badly out of date, and will be upgradedby 1ric soon.E

6ifferent maers of tubes use different designs. +here are si" maers of commonaudio tubes right now/

9huguang,


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or your guitar amp, 2 would recommend the @9o#te@ *&, it3s a really nice,rugged and smooth-sounding tube. 2t was a military type used in ser#o amps in Hetaircraft, so it has to be good. 2f you ha#e a Marshall or other 1:(B amp, the9o#te C

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Reparacion y Modificacion Tube Amp Building