How to Bias a Guitar Tube Amplifier

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732-738-7776

Congratulations! By owning a Mansfield Guitars Bias Probe you're not at the mercy of a tech anymore and

you am now in control of your tone.

Our probes measure directly in DC Milliamps so you will need to set your multimeter to the DCA mode(DC Amps, NOT VOLTS!) and if the meter is not auto sensing then select the 200mA scale.

Insert the black plug from the probe into the "Common" input and the red plug into the positive input whichis usually marked mA.

DO NOT CHANGE THE FUNCTION SWITCH ON YOUR MULTIMETER WHILE IT IS HOOKED UPTO THE AMP! This will blow the fuse in the meter and you will need to replace it! Make sure the meter isset properly and the probe is plugged into the meter before you plug the probe into your amp.

Remove one of your power tubes (6V6, 6L6, EL34, E34L, KT77, KT66 or KT88) and plug the probe in.Place the tube into the top of the probe.

If you had to remove the amp from the chassis then make sure that you have plugged the speaker back in.You must have a speaker load on the amp when you are adjusting the bias!

Turn on the meter and then turn on the amp leaving it in the standby mode for about a minute, then switchto the play mode. Your meter will read out the plate current in milliamps. Turn the trim pot to adjust theplate current to the desired setting. Wait about 5 to 10 minutes with the amp in the play mode and do a finalbias adjustment aûer the tubes are hot.

Congratulations again! You have just biased your amp!

If you measure the DC plate voltage from pin #3 of any power tube to ground then you can use the formulabelow to calculate the current draw. (Remember to switch the mode on your multimeter to measure de volts)

The formula for biasing is the plate dissipation of the tube (25watts for 6L6's, EL34's, E34L's or KT77'sor for 6V6's use 14 watts and for KT88's use 35 watts) divided by the plate voltage (letsuse 480 for thisexample) times

.7

will justget you out of crossover distortion and you can go as high as.85

percent but youwill wear out the tubes quicker!

25 divided by 480 =.0520

or 52 milliamps X.7

=.036

or 36 milliampsFeel free to experiment with different settings to find the sweet spot. Always keep a close eye on the tubesto make surer the are not red plating. If they are then your bias is set to hot and damage to the tubes willresult. 70% is usually hot enough and we never recommend going over 85%.

Most 50 and 100 watt amps using these tubes will have between 450 to 480 plate volts so even if you do notmeasure your plate voltage you will find that a setting between 35 to 40mA will be acceptable.

Bias NotesTorres Engineering

If you have a TORRES BULLET PROOF KIT as part of your project, install it first.Follow the enclosed BULLET PROOF INSTALLATION INSTRUCTIONS and install thewhole kit in the amp.

You will need a VOM meter to read Bias voltages. I use a digital VOM (actually,afrequently "on sale" item from Radio Shack) but an older "needle" VOM is just fine (andalsoallows you to do some tricks in PERFECT GUITAR #4 that the digital VOM can't do.)

All of these instructions are for.Fender and Marshall type amps. A complete coursecovering all the other variations (suchas Ampegs and Gibsons) would be impossible to prepare.There are complete charts and lots more information in my book, "Inside Tube Amps" available

fromTorres Engineering and many book stores.

OK, THE EASY WAYFind4he Bias trim pot. It is the only trim pot in the amp (usually.)Marshall has it over on

the left hand, upper section of the circuit board. A Fender has an"upside down" pot mounted to

the chassis over behind the speed and intensity controls, or behind the master volume. In anycase, if the amp's bias can be adjusted there will be a bias pot somewhere. It is connected to a"bias supply" that includes (atminimum) a reversed diode and reversed electrolytic capacitor toprovide the negative voltage the bias requires.

If you have a Digital VOM you can hook it up normally. But if you have an analog"needle" VOM you need to hook it up BACKWARDS when reading Bias (redlead to ground.)Most analog meters can't display negative voltage.

Plug the amp in, turn it on be sure the speaker is plugged in. - No Guitar or any kind ofinput.

You will be working around real high, deadly voltages, and with the amp ON!. If you are notexperienced with electronics and don't know proper safety precautions you SHOULD NOT BEDOING THIS. Get someone who knows how to do it for you. It isn't worth $20bucks to getinjured, burned or killed. TORRES ENGINEERING is not responsible for any injury or damagedue to following any instructions we provide. These instructions are provided with theunderstanding that you know enough about high voltage electronics to avoid injury.

Set up your VOM meter ( on the 100 volts scale) with clips to read the bias on pin 5 of

one of the tube sockets (doesn'tmatter which one.)Watch the bias go up on the meter. It takes a Marshall A LONG TIME to come up to

correct bias (upto 3 minutes!.) Once you see this happen you won't start playing as soon as itcome on anymore, it is a while before it is RIGHT. Set the bias with the trim pot to the voltages

on the chart. There are voltage ranges and recommended voltages for most of the amps we deal

Amp Bias adjustment Page 1

with. Read the voltage at pin 5 of a power tube.

Amplifier Tube Range Recommended

MARSHALLS and others with thesame tubes

100 Watt EL34-36

to-45

-42

50 Watt EL34-34

to - 40-36, -37

100 watt 6550-44

to - 55-52

50 watt 6550-38

to - 48-44, -46

FENDERS and others with the sametubes

100 Watt with Reverb 6L6-48

to-53

-52

100 Watt Showman, or no reverb amp 6L6-48

to-53

-52

Pre CBS 4 X 10" Concert. 40 w 6L6 (2)-44

to-50 -48

Old Tweed 4 X 10" Bassman 6L6, 5881-46

to-50 -48

Bassman Head (blackor silver) 6L6 60W-45

to-49 -48

Deville and Hotrod Amps

Bandmaster head (black,silver) 6L6 40 W-46

to - 50-48

CBS Bandmaster Reverb 6L6-48

to-53

-52

Deluxe Reverb (20some watts) 6V6-32

to - 38-35

* Hint. As the amp is coming up to voltage, you can lower the trim pot if it goes too far.Forexample, if it goes on up to

-50

or more, and is still rising, turn the trim pot down a little right

away. No reason to send the tubes a higher bias is you can help it.

Now, take a look at the tubes. Are they glowing Red Hot? If so, That is severe underbias,

turn it off and find out what is wrong. Most likely the bias circuit has gotten completelydisconnected.

If they are normal and running justa bit too hot to touch (ifthey are cold it is overbiased,set it correctly,) you are ready to start testing.

Set your amp up for a real clean sound and your guitar for a fat, full range tone. Yes, Iknow you don't always play real clean, but we are setting bias, not checking distortion levels,don't cheat!


Play a few chords and notes and vary the bias up and down a couple of volts each way totune in the cleanest, tightest sound. If it is fuzzy, dirty or low on power it is overbiased. If thetubes are red hot, and/or the amp hums, it is under biased.You are looking for a clean, clear tone with "perfect bass" and crisp highs.

Take note to be sure it is still within the correct range and maybe attach a note to theinside of the amp to indicate your settings.

No matter how much technical equipment, scopes, meters, etc. are used, I still alwayscheck bias by ear. We are musicians, not technicians, the sound is the most important.

Once the amp is biased on the one tube you are working with, check the others anddetermine if they are also within the correct range. They should be, or you have some bad tubes(atleast mismatched tubes.) If they are new and from a good company, the variation won't bemore than a couple of volts from highest to lowest. The new parts included in the Bullet ProofKit (ifyou have one) will help.

Play a little while with the amp still out of the cabinet. Turn it up, get some distortion,checkit out. Listen and Watch the tubes as you test. If something goes wrong, shut it off rightaway and check it out.

After 1/2 to 1 hour of play testing, check the bias again and reset if necessary. Tubes needa little break in to learn just what you expect from them.

I always recommend changing the "driver tube," the last 12AX7, or 12AT7 next to thefirst power tube, whenever I replace the power tubes. The driver works real hard and needs to bechanged frequently for the amp to keep up to full power.

More Detail and PrecisionSome people like to work at a higher level of detail, and may have more electronic skills

and knowledge to apply to this project. Here are some more"wrinkles" to work with.

After setting bias in the method above, and testing the amp as suggested, we can install a"Bias Check" resistor on each tube to allow you to make quick, accurate checks of the bias. Thisis only for amps with adjustable bias. 70's on up Fenders often have a

"balance bias"system that does not actually adjust the bias. You can't do it with these amps. See the drawingfor instructions on converting these amps to an adjustable bias (absolutelynecessary whenchanging to EL34 tubes.)

Pin 8 of the power tubes is soldered to ground. Clip the wire at the tube pin, and insert anaccurate 1 ohm, 2 watt resistor between pin 8 and ground for each tube. Put one end of theresistor on pin 8 and solder the other end to the clipped wire, still attached to the amp chassis.(You have to do this to all of the tubes or none!)

Now you can check the bias in millivolts of current across these resistors. This is far moreaccurate and exact, and accounts for more variations.

An amp with EL34's or 6CA7's, should read between 25 and 30 millivolts across theresistor, with optimum at about 28. 6550's and KT88's read a bit higher at 28 to 35 millivolts.

An amp with 6L6's should read between 28 and 34 millivolts across the resistor. Optimumis at 28.

An amp with 6V6's (twoof them) should read between 18 and 22 millivolts (19optimum.) Champ amps are

"cathode biased" and already have a resistor at pin 8. Leave them asthey are.


They can be set, but it is a more complex project.

Again, You have to look at the tubes, and make a sound checkCheck all the tubes and use the highest one as your reference. Put the ones that read low in themiddle and the ones that read high on the outside. With good Matched tubes they should all bewithin 20% of each other. If they are WAY OFF, you have some sort of problem, weak tubesusually or a fault in the bias supply. If one side is off and the other side is fine, check to see thatthe two bias resistors are the correct values and not shorted out.

I recommend checking and setting bias by both methods AND sound testing to get theamp perfectly in tune. You can vary the bias (withinthe correct ranges of course) to get differentsounds or a different level of "crunch." Always test voltages again after making "sound

adjustments."Take your time, check several times, switch tubes around, install a BULLET PROOF

KIT, and keep good tubes in your amp and it will give you great sound for years and years.

Copyright Dan Torres 1993

Bias Balance (Fenders)

I I

Tap onDRAWING BV the potDAN TORRES

Adustable Bias

I '

Torres Engineering1630 Palm AveSan Mateo CA 94402


General ConcernsA

round the dawnof modern musical time There are two main partsto everyamp: thepreamp, Manyof the earlysolid-stateamps had their transis-(the fifties), when electronic devices where thetone is created and shaped, and the power tors mountedin sockets, justliketubes.On theotherwere so simple evenyour mother could amp, where the volume comes from.These are two hand,most modern solid-stateampsdon't actuallyfixa TV set, there was a great wealth of very distinct parts, so much so that on many use single(discrete)transistors,butinsteaduse inte-

knowledgeabout the "user-serviceable

parts" inside schematics (circuitmaps) they're shown on sepa- gratedcircuits (ICs) and microchips. This makeshome electronic devices.Unfortunately,the inner rate pages.Yourstereo mayconsist ofseparate pre- them virtually impossibleto fixor eventroubleshootworkings of today's solid-stateamps and digitalEFX amp and power amp modules;understandthisand at home.Fortunately,thereare some thingsyou canprocessorsare incomprehensible to all but the few you're there. do to at leasteliminatesomepossibilities,and we'llrocket scientistswho designand fixthem. This is a concentrateon these.real shame, becauseto know howsomethingworks Because thev don't handlethe intense highvoltageis to knowhowto get the most out of it. Lucky forus, and current that poweramps do,tube preamps don'twe guitaristsstill use a good bit of ancient technolo- require as much maintenance. As you'll see in thegythat evena mother could understand. next chapter, preamp maintenance is clear-cut and

simple. Power amps. on the other hand, requirefaithfulmonitoringand care to alwavsbe their best.

When solid-state ('transistor")

amps were Brstintroduced in a bigwas backin the mid sixties,theywere heraldedas nothing less than the salvationofmankind. Solid-state amps didn't require upkeep,

co and dik distort hke those nasty old

IC the chips, but I can't C rf anything inside 'em

"Return with us now to those thrilling days of is brokenyesteryear..."

Tube Amps !!!!! WANNING!!!!!Even in their most complex forms,tube amps are ! For thosewho don'tpayattention,poke aroundsimple devices.Tomake a fullyfeatured,"complex" where they don't belong,don't get enoughtube amp, the designerstrings togethera bunchof

_ _ _

sleep, drink alcohol, or take drugs or coldcommon,simplecircuits.Mostof these circuitswere medicineswhileworking on any amp, tube orperfectedyears ago; the real trick in designingan transistor,the possibilityof deathawaits!amp is assemblingthe puzzleand gettingall the cir- Salvation by silicon?cuits to work well together. If you can visualizea ! Allamps contain lethalvoltages;certain com-group of buildingblocksstrung togetherin a fashion Early solid-state amps like the Vox Super Beatle, ponents canstore a lethalcharge fordays!Thesimilar to the way modern EFXdevicesstring togeth- Baklwin Exterminator, Kustom 200, and purpose of thisbookis to showyou techniqueser blocksof EFX,you'vegot the picture. Rickenbacker Transonic were expensiveand big. and procedures you can perform safelyby

There were a couple of reasons for this:Theywere yourself.Allof theinstructionshereinare clearbeinghand-wiredliketubeamps, becausethiswas and to the point;if youwant to infer any otherthe waythe amp factorieswere set up; the transistors meanings or venture beyondthe scope of thisof the time were expensive;and trannyamps were book,you're on your own!newand hadlotsoffeatureslikefuzzand repeat per-cussion. Becauseof this, theywere placed at thetopof theline.

It didn'ttakelongfor guitariststo rebel against theclinical tonesproducedby early trannystui. Theystarted screaming for the return of their belovedtubes. The era of bigsolid-stateamps was gonein a

ou flash.Inevitably,though,as tubesgot harderto findand therefore more expensive, amp companiesturned theirR&Deffortsto makingthe now-inexpen-sivetransistorswork forguitars.Theresult is today'slargecrop ofeconomicallypriced,reasonablesound-ing, fullyfeatured,and reliablesolid-stateamps.

Like, tubular

GENERAL CONCERNS • COMPLETE GUIDE TO GUITAR & AMP MAINTENANCE • 51

Tube AmpsTubes the plate, and the plate current increases. When it's

made more negative, it slows the electrons down,

Tubes. These enigmaticdeviceshavebeen s FilAMENT and the plate current decreases. (The grid canpurged from virtually all modern elec- PIATE FIIAMENT becorneso negativethat the tube actuallyshuts off.)tronics except guitaramps and certain

"I,,HODEE

high-endaudio components. Though aGRID

matter of some Patent Officedebate,it is generallyheld that the

"thermionic

valve," or vacuum tube, PIATE

was invented byLee de Forest in 1907. He wasn'treally interested in amplifiers at the start; he was o un CATHODE

lookingfora betterwayof receivingradio signals.In1911 de Forest was granteda patent for a tube PIATE PIATE

amplifier. Others, such as Sir JohnFleming and 6^T"°°EFIIAMENT FllAMENT FIIAMENT

Edwin Armstrong, also deservemuch credit, and I.EADS

were instrumentalin advancingtubetechnology. DIRECTLYHFATED INDIRECTLYHEATED

Bythelateforties,tubeswere everywhere:in radios,Inside tube diodes (Basic Electronics) Triode construction (Basic Electronics)

TVs(already20 years old), computers (the30-ton number of electrons emitted is determinedby the There are lotof electrons floatingaround in a tube,ENIACwas completed in

'45), industrial machin- heatertemperatureand remains constant regardless waitingto make a lot of noise. The grid's voltage is

ery,... butscience marched on. The inventionof the ofother factors.Theelectronsthat are freedfromthe verylowand the plate's is veryhigh(e.g.,500VDC).cool-running and service-free transfer resistor, or cathodebytheheaterforma cloud, or space charge. Becausethe littlegridvoltage is controllingthe very

transistor,bya teamof physicistsat BellTelephone Electronshave a negativecharge.This cloud would big plate current, the signal (yourguitar) is nowLaboratories in 1948 spelled the beginningof the justhangtherelikea subatomicfog,exceptfor... amplified!Most modern guitar amps use dual tri-end of tubesin thereal world. (It wouldn't be till the odes (twotriodesin each glass), like 12AX7sandmid seventiesthat theywere trulygone.) Butwe gui- The ACwall current alternates betweenpositive(+) 12AT7s,in their preamps. Output tubes are mosttarists hangon stubbornlyuntil this day. and negative(-) (60timesa second, or Hertz (Hz], oftenpentodesor beampentodes,like 6L6,EL34,

in the US;50 Hz in most other places). The plate EL84,KT88,6550, 5881,... A pentode is a moreWhy?The very

"limitations" that make tubes bad to alternates between+ and - alongwith the wall cur- elaborate formof tube that adds a couple more elec-"spec heads"are what we love:tubesdistort,they rent. The plate is made of metals that don't emit trodes to increase efficiency.

are unpredictable under stress, their operating electrons at normal operating temperatures.characteristics change under different condi- Therefore, when the plate is positive, the electronstions,... Factis, we lovethe sound of tubesso much around the cathode are attracted to it, and current Troubleshootingwe'll put up with and even romanticize their very flows.When the plate is negative, the rectifier Because of the delicate nature of the tubes them-real drawbacks:tubes need expensive power and

"sleeps"

for a sixtiethof a second, and there is no selvesand the heavv-dutvmakeup of tube amp chas-

output transformersto do their job,the tubes them- flow.Whatyou wind up with is DC,or a current that sis, it's easy to locateany number of problems in aselvesare expensiveand need to be replaced often, flowsin onlyone direction.Thisnewlyrectifiedelec- faultytube amp.theygetvery hot,they're fragile,there are onlya few tricity is whisked away into a circuit to do somefactoriesstill making them,... Howdo thesemetal- work, and thatworking flowof electrons is calledfilledglass bottlesweavetheir magicspell?It's actu- theplatecurrent Atube that's in good workingorder usually

allyverysimple- hasa clean and clear lookto it.This describeshowa half-waverectifierworks. All .

To understand tube operation, you need to knowa modern amps use fidl-waverectifiers, like 1

fewbasicsfirst: GZ34/5AR4,SU4, 5Y3, or solid-state "bridges."

These are simplytwo half-waversin one glass; they• Electricityis made up of electrons. work in conjunction with a more sophisticated

power transformerto create a greater amount of• Whenelectronsmovefromone placeto another, usable current.

theycreatea current (measuredin amperes). 1. Look for a frosted inner coating; this isThe next step in tube designis the triode.A triode nice on MiniWheats, but in tubes it signi-

• Oppositesattract. Whenthe negativelycharged adds a third electrode, called the controlgrid (G, in fies that the vacuum is broken. Replaceand discard the tube. (Photo, left to nght:

electrons see a positive charge, they want to schematics). The grid is a spiral of thin wire placed frosted, frosted, good, frosted.)traveltowardsthatpositivecharge. between the cathode and the plate. "It givesto the

tube its most usefulfunction--theabilityto amplify" 2 2. sometimesa tube

The simplest formof tube is the diode.The diode (Basic Electronics). The spaces betweenthe spiral $enÎruse.blhi

consistsof twointernal conductingsurfaces,or elec- windings are big enough to let the electrons flow blue haze can be

trodes, and a filament,called the heater. The heater through them onto the plate, but close enough to either vague or- pronounced; it's

is what you seeglowingred-hot when you look into control their flow caused by gas

a tube.In guitaramps, diodesare used as rectifiers. leaking into theA rectifier converts the ACwall current into the DC Thegrid is chargedwitha negativevoltage (e.g.,

-38 " " eheenctrn

that amps use internally. VDC).This negativegridcharge is the bias voltage, passes throughand it's at the heartof tube operation. It varies up the gas, it ionizes

Inside a diode, the two electrodes are called the and down, or more and less negative,in a pattern gi nÊngof tcathode and theplate.Theheateris placed in close that corresponds to the music you play.Asthe grid end for this tube,proximityto the cathode (In somediodes,theheater voltage is made more positive (lessnegative), it thorkgh itKw rsti

is the cathode.), whichis speciallytreated to giveoff anows more and more electrons to pass throughto while.electrons upon reaching a certain temperature. The

RO e COMPLETE GUIDE TO GUITAR & AMP MAINTENANCE • TUBE AMPS

fterbeingdropped,hit,and overdriveninto oblivion,the littlebitsinside a tube can work themselvesloose.Acousticenergycan 6nditswaybackinto thechassis (combosare especiallyprone tothisbecauseof theproximityof the speak-

ers), resultingin a high-pitchedsqueal or clangingnoiseknownas microphonics. Although highvolumeand modern super-high-gainpreamps can push tubes to their limit,microphonicshauntbothclean and crunchyamps. A microphonictube needs\>elocatedand replaced.

1. Turn the amp on and plug in a guitar or a dummya. Turn the amp off and replace the tube that ispl to nac toewtheinput. (Turn the guitar's vol-

2. Using a wooden chopstick, tap lightly on most affected by your tapping.each tube (preampand power amp) until

(Most amps have switching inputs, which short you hear the microphonic sound. It may bewhen the plug is removed. This keeps the amp difficultto tellwhich tube is the real offender 4. Tum the amp back on and repeat the test. Ifquiet when it's on but not being used. To make a ifthey're close together,so the problem still exists, continue on down thedummy plug, take a standard ¼" plug and solder line until you've found the problem.the tip and sleeve terminals together.)

It's possible, after testingor replacingeverytubein an amp, that the problem stillexists.Thismeans that the noise is comingfromsomeplace else.

NoisES FROM SOMEPLACE ELSE

Ninetypercent of microphonic spits and pops are tube-related and will be cured bythe procedure above. Some othersources of noise are: loosewires, bad solder joints,poor shielding,dirtypots or jacks,and leakycapacitors. Manyof

theseare not easilyfoundbythe novice.

Read the warning at the end of the preceding chapter as many times as it takes for it to really sink inbefore you proceed with the followingsteps.

NOTE:If your amp is new,read your warrantvbeforeyou open up the cabinet.1. Make sure the amp is unpluggedi 2A •

2. Remove the amp chassis from the cabinet. Most chassis are held inusing one of the following methods:

A. Top straps. Popularized by Fender and used on most combo amps,top straps are simple to undo. Remove the back panels and unplugthe speaker and reverb wires, if any, making notes as to their position.(Don't try to remember! Write it down!!) Loosen the strap screws,making sure to catch any nuts that may fall loose. Check to see ifthere are any resting strips for the amp chassis to sit on when thestraps are removed; if there aren't, make sure you catch it!

s. Bottom screws. Marshall, Boogie, Soldano,... Most heads are held intheir cabinets by four screws on the bottom. Remove the back panelsand then unscrew the chassis. Don't turn the head upside down andunscrew it; this can cause twisting,which can bend the screws, andwhen you remove the last one, the chassis will crash upside downonto the top of the cabinet. Instead, stand the head on its side and 4hold the chassis tightagainst the cabinet as you unscrew it.

s. Place the amp chassis in a well-lit area on a sturdy table.4. Discharge the large filter capacitors on the power supply using the fol-

lowing method (See "Capacitors," later in this chapter, to learn how toidentify filter caps.):Using a large screwdriver with an insulated handle, short the caps to thechassis. (You may get a big poM still makes me jump; don't worryForget to do this and that spark willzap you instead of the screwdriver!)

s. Look the chassis over closely for signs of burning or discoloration. Onthe output tube sockets of Fender amps, look closely at the large1-Watt 470-0 resistors that are placed between pin 4 (screen grid[G2]) and pin 6 (tie-offto screen grid supply). If they're burnt orcracked, replace them.

e. Poke around with your chopstick to try to dislodge any hidden broken 6connections.7. Plug in the speaker. On combo chassis, this may require a male-

female extension cable. You can either make it, use a double femaleadapter and a store-bought speaker cable, or buy one. This cable 10must be made from speaker wire!

8. Plug in the amp and turn it on.e. Insert a dummy plug into the input to activate the preamp.

lo. Place one hand in your pocket (This makes you think before yougrab.) and with the other hand tap all the circuit bits and boards light-ly with your chopstick. Try to locate any loose components or badjoints by slowly working your way around the chassis and listening forany crackles or other noises caused by your poking. If you still can'tfind the problem spot, close it up and seek professional help.

TUBE AMPS • COMPLETE GUIDE TO GUITAR & AMP MAINTENANCE • 53

T he chart belowis reprinted, with permission, fromThe TubeAmp Book, byAspenPittman.

NOSOUND. DISTORTEDSOUND.

PILOTLAMPANDTUBESLIGHTED. PILOTLAMPANDTUBESNOTLIGHTED. ONALLCHANNELS, ONONLYONECHANNELORONSINGLE-CHANNELAMP. (MULTICHANNELAMP).

Checkcontrolsonampfor normalsettings. Checkwalloutlet;Instrument controlsshouldbe allthewayon. plug in something Checkpreamp tubes.

AlsocheckStandby switch,ifany. thatyou knowworks. Lowercontrolsettings(especiallyvolume). Try interchanging tubesfrom channelthatsoundsOK.

No sound. SoundOK. OutletOK. Novoltage.Poor sound. Sound OK. Poor sound. Sound OK.

Turnvolume Checkfor blownalltheway up house fuse and Pickup, instrument, orconnectingcable Takeampto

andlisten for hum. replace. maybe defective;substituteoneknown competentrepairtobe good (especiallycable). service.

Replacefuse onamp.No hum. Hum. (Fuse couldappearto be OK Sound OK;repair

andyet be defective.) Poor sound. pickup, instrument,or cable.

Substitute tubes Unplugcablefromwithonesknown instrument and Fuse OK. Fuse blows. See thatalltubes arefirmlyseatedin theirsockets

to be good, touchtipofplu0. andthatallinternalconnectorsaresecure.

No Check Poor sound. Sound OK.

No Sound change Loud line cord,plug, Checkfor shortsound. OK. in hum hum. and in amp.

level. Power switch.Checktubes.

Substitute tubesknownto be good, oneat a time,startingwith outputtubes.

Unsolder one Checkcablespeakerwire.Touch between Checkandrepair Substitute tubes9-Vbattery towife instrument and pickupon to locateandspeakerterm- amp.Repairor instrument. shortedone.

inal. Listenfor click. replace. No improvement. Sound OK.

No clicks. Fuse Replace rectifiertube.Defectivespeaker, Clicks Sound Amp blows

leads, oroutput audible. OK. OK. repeat-transformer. edly.

No improvement. Sound OK.

Touch battely Speaker, leads, and Takeamptodirectlyto speaker transformer . competentrepair

terminals. secondaiyOK. service. Followthesameprocedure with the remainingtubes-phase inverterandvoltageamplifiertubes.

Clicksaudible. Noclicks.No improvement. SoundOK.

Speaker OK:troubleis in Voicecoilonspeakeropen;speakerleads oroutputtransformer. replaceor reconespeaker. Disconnect thespeaker.

Use testspeakerortestampto determine which is distorting.

Takeamptocompetentrepair Repair leads. I Sound OK.

sentice. Speaker no good. Replaceor recone. Takeamptocompetentrepair service.

• COMPLETE GUIDE TO GUITAR & AMP MAINTENANCE • TUBE AMPS

Replacements and Equivalents CAþß¢$$OTSAstubefactoriesclose downaroundthe world, the Capacitors, caps, or condensers-they're all thesupplyof qualitytubes is dwindling.The new tubes same thing, and what they do is store electricity.availabletodayjustdon't sound as good as the old They'recomposed of twoelectrical conductors sep-ones, especiallyin older amps. This is a factwe all arated byan insulator calleda dielectric;thesecanmust accept, and not just the nostalgic rantings of bemadeout of all sorts ofstuff,likefoiland paper,some hystericalvintoid. Side-by-sidecomparisons metaland air, silverand mica,... Theycome in whatprove that a Fender amp with real RCA"BlackPlate

seems like a million sizes and shapes. The unit ofSpecials," or a Marshall with old stock Mullards, capacitance is the Farad. There are twobasictypesjust sounds better.Even more recent designs,like of caps in youramp: nonpolarized andelectrolytic.the MESA/BoogieMKI, II, and III, were designed The nonpolarized caps are relativelysmallandcanaround the classic Sylvania6L6GC.These ampsare looklikelittleboxes,rolled papertubes,littledisks,very simpledesignsthathaveno biaspot and relyon or (orangeandbrown)candydrops.Theyare usedoriginal-specequipment to sound theirbest. to

"voice" the amp and in the tone controls.Nonpolarizedcaps are also usedto blockcurrentfromgoingplacesit doesn'tbelong.

The followingis a listof some standardguitaramptubes and their industrial and militaryequivalents.Sometimesequivalentssoundbetter,sometimestheysoundworse; but theywill all "work." There's noway to predict the outcomefor anygiven amp orpersonaltaste.Try 'em and see.

6L6GC: 5881*,7581, EL37,KT66,5932They don't make 'em like they used to

KT88: 6550••So what can you do?There are a fewdealersofN.O.S.(NewOldStock) tubeslistedin theAppendix. EL34: KT77,6CA7You'llhaveto digdeep into your wallet to payfortubes froma dealer,butifyou'reserious about good EL84: 6BQ5,7189sound, it's money well spent. The other great Capacitors

sources of all things old and electronic are ham- 6V6G'n 5871, 7184, 7408festsand antique-radio swap meets. Youhaveto get Theelectrolyticsarethebigguysin your power sup-there at the crack of dawn,and you never know 12AT7: 6201,6679 ply.Theyare polarized, which means theyonly letwhether it'll be boomor bust,but that's where the current flowin one direction.Afterthewall currentbargainsare! I also must add thatspendinga week- 12AU7: 6189,ECC82,6680,7730,5814A is rectified,it stillhassomeAC"ripple" leftover.Theend morningin a parking lotwitha bunchof people electrolyticsact as a reservoir,fillingup fastwithrip-who share your amplio-archaeological interests, 12AX7: 7025, ECC83,5721,6057,7494, 7729, pled current on oneend and lettingouta smoothedsurrounded by electro-junque and drinkinglots-a 6681 streamontheother.Becausetheyremovetheripple,coffee,is goodforthesoul. they're often called filtercaps. On many amps,*01d-style lung-Sol or equivalent.Modern Russian 58818 they'rethebigbrownor colored cans thatstand upYetanother alternativeliesin the explorationof tube willwork, butthey're not anexact replacement; significant on the chassis or are laiddownoff to one side. Onequivalents.Overthe years there havebeenliterally powerlosswilloccurin old amps. Fenders (andsome otherAmericanamps) theelec-tensof thousandsof tubetypes.Someofwhatappear trolyticsare placedlyingdownunder a metal cover.to bedifferenttubes are actuallythesame or at least

** similar;some modificationmay be necessary. This is becausewhen caps dietheyhavea tendencysimilar tubes with differentnumbers. Why?At one to spew theirgutsall over theplace.timetherewere three major users of tubes:industry,consumer electronics, and the militaly.A lot of the Testingcapsrequires specializedandveryexpensivecircuits these three groups used were thesame, but equipment. (Some modern digitalmeters have atheconditionsunder which thetubeswere expected capacitancescale.Thisis useful,buta meter likethisto perform were not. For example:One of themostcommonpowertubesis the 6L6.Consumerswantedan inexpensive tube to use in theiramplifiers andradios-they got the6L6G(G = Glass).Themilitarywanted a fewversions: one that could takea beat-ing-the metal-cased6L6-and onethat could takea bit of a beatingand fit into a small space-the5881.The industrialists wanted a tube that didn'tneed to be changed too often in machinery-theygot the heavy-duty7581. Add to thisthe Europeanversions--KT66 and EL37-and you see that any .

number of replacement possibilitiesexist.

Filter caps on a Fender

TUBE AMPS • COMPLETE GuloE To GUITAR & AMP MAINTENANCE • 55

willset youbacksome.)Besides,caps on thecircuitboardsare usuallythelastthingsto breakon an amp.Filtercaps tendto showtheirdistressphysically. nfortunately,replacingfiltercaps is not as simpleaswe wouldwish it to be.The electrolyticmate-

U rial inside new filtercaps needs to be "formed" slowly.If you were simplyto cut out the old caps

Troubleshooting Filter Caps and solder in new ones, theymight not work well and could blowup in your face. (Really.)Newcaps

Filter caps are housedin either a metal or paperneed to be broughtup to operatingvoltageslowly.

cover.Whencaps are in goodworkingorder, they're To formcaps, you need another veryspecializedtool: a Variac.Besidesbeingused byEddievanHalenlikelyto look good as well. Electrolyticcapacitors and others to blowout power transformersin the name of tone, Variacshave a number of uses on theneed exercise to feelgood andwell "formed." When electronicswork bench.Variacsare most often employedto set the linevoltage at exactly117 VAC.Thisan amp is leftin storagefora longtime,theelements ensures thatall measurements, likebiasand wattage,are relativeover time. Variacs are expensive,butinside a capacitor can dry out and break down. can be foundin many electronics

"junk" shops and at ham-festsfor under $50.Variacsare rated byThere are twocommon problems that would make amperage-handlingability;you need one thathasan amperage rating higherthan your amplifier.you suspect badfiltercaps:an excessive,unchang-ing lowhum,and "false

tones."Both of theseprob 1. Make a drawing of thelems are caused byripple fromtheACleakinginto wire hookups to yourthe audio circuit. False tones are weird, traveling"ghost" sounds as the leakingAC "note" beats o ns

(exithet eeyhaa

against the notes you're playing,the same way tWO arrows, or a stripe atfrequenciesbeatagainst each other when you tune one end. These mark-your guitarwithharmonics. ings indicate the polarity

of the capacitor. If theNOTE:Don't confuse falsetoneswith intermodula¯ cap is installed back-tion distortion (IMD).Allampshavesomelevelof wards, it won't work-IMD,whichalso causes travelingnotes.Withexperi- and there could be otherence, over timeyouwilldeveloptheabilityto tellthe "reactions." Make cleartwoapart +- + + notes as to their direc-

tion, and check, check,and check again.

1. With the amp unplugged, locate the filter ,I

caps and zap'em

as described under"Microphonics." 2. Replace the caps.

s. Set the Variac to OV andturn the amp on. (Speakers

2. Look for brown pimples, dots, or leaking aren't necessary, as long asslime. Paper caps should be dry or a little no signal is going throughwaxy. If the wax is runny or slimy, the cap the amp.)could be bad. On metal caps, look at thebottoms-around the terminals-for simi-Iar signs of degradation.

to VCvolta

should take between fiveand 10 minutes to do and

jumpornotrksnw

teho biga

s. Let the amp sit for a fewminutes, then try it out with

a guitar. It should soundclean, clear, and new.

NOTE:This method should also be employedwhen turning on an old amp that mayhavesat unused fora fewyears.

56 • COMPLETE GUIDE TO GUITAR 8: AMP MAINTENANCE • TUBE AMPS

Capping it Off ful?€ Ela,S MikeHill,and SteveGrindrod-and I am nowpass-Electrolyticsare sometimes foundin the main cir- ing this informationon to you in the same spirititcuitryinside an amp. The telltalesign is the pointing The subtitle of this section should be "The canof was givento me.

arrow or stripe that indicates polarity. Worms in Pandora's Box." As explained earlier inthischapter, without the control grid and therefore Fixed Bias

Nonpolarized caps, especiallypaper axial ones, can biasvoltage, therewouldbe no tubeoperation as we There are a number ofwaystofigurethe biassettingsuŒerfrommanyof thesame physicalsignsofdegra- knowit. There are a million analogiesaboutbias on a fixed-biasamp. I willexplainthe twomostcom-dationdescribed for electrolytics.When replacing For example: It's likesetting the idle on a car; you monlyused. But:The onlyone thatcan giveyouthecapacitorsit's very importantto takenote of the volt- can't make a Chevyinto a Ferrari, butyou sure can true pictureof what's happeningwith your specificage ratingof the original.Newercap typeswillprob- make a Ferrari into a Chevy.The one I likebestfor amp and any given set of tubes is theablybephysicallysmallerforthesameratingthanold a number of reasons is this: The control gridis like oscilloscopelsignal generator method. This is theones. Thevoltage ratingof the replacementshouldbe a faucet,the biasvoltage is likethe spigotor handle only method of biasing an amp that makesequal to or greater thanthe original.Ifa capis in the Asyouopen the faucetbyapplyinga more andmore allowancesfor the infinitedifferencesin transform-audio path, the replacement should be of boththe positivevoltage,more electrons flow.Thisis theori¯ ers and tubes. EverybodyI knowwho's learnedtheirsamevoltage and thesameconstructiontoensure that ginof the European term fora tube: valve tube-ologyin an engineering,college,Army,or busi-theamp's tone remains true to theoriginal.Manyof It's unusual forsuch an essentialpart ofa consumer ness setting,where all therightequipmentwas read-the cap values foundin older amps aren't made any deviceto be hidden awayinside where you can't get ily available, uses this method. The others startedmore. (Unliketubes, which don'tdegradewhen sit- to it. Biasingalso requires a highlevelof skill, con. out as hobbyistsand grewfromthere,bringingtheirtingon a shelfunused,youcan't useN.0.S.caps.) You fidence,and educated listeningability.For these and badhabitswiththem.will haveto get close and livewithit.

many other reasons, I feel that the whole "bias

The amount of biasvoltageneeded in anyparticularSG: "The smaller (1µfd or less) nonpo/arized thing"hasgottenwayout of control (nopun intend- situationis dependenton a fewfactors,the twomostcapacitors are usedfortone shaping and circuit ed). Here's mytake on it: If you get the equipment, important beingtubequalityand B+ (plate)voltage.coupling in amps. Thesecaps come in diferent learnthe procedures, take notes, and practice, over The B+ is determinedby the output transformer.varieties, and just to confuse you, each variety the course of time you will getthe hang of biasing Output transformersvarydue to design,age, condi-covers thesame capacity range as the others. But your amp tion,and manufacturingtolerances. AMESA/Boogiebe warned: just becauseeach value is covered by Most guitaramps are biasedin one of two ways: output transformer can deliveras littleB+ as 350each type,are theythe same? No way! Dtferent fixedbias or auto/self bias. Selfbias refers to an VDC,while a GTSolo 75 can haveas much as 525types have dtferent tone, and therefore,when older type of biasingscheme thatuses a high-cur- VDCat the plates. Bothare great-soundingamps thatreplacing capacitors you mustuse thesamevalue

rent resistor betweenthe cathode and groundand a are designedfor thesame tubes!The B+ measure-and type,or you could loseyour tone. constant negativegrid voltage.Asthename implies, ment by itself means nothing; it's the interplay

"If replacing, say, a silver-mica,polypropylene,or in self-biasingamps the tube itself determinesthe betweentheB+ and biasthat getsthingsgoing.polyestercapacitor in your favoriteamp, make bias,basedon its condition and operating circum-

sure you use theright one; it's therefora purpose. stances. In fixed-biasamps thecathode is grounded,They may cost more than ones your localparts andthe biasvoltageis controlledbya biaspotin thestore can offe but it really is worth it. grid circuit."Unfortunately, with the passingof time some Thesetwo terms can be confusing, since fixedbias is

classic capacitors are no longer manufactured, adjustable and auto/self bias is not. That's becauseand replacing them is a problem.For instanœ, you're thinkingfromyour perspective.Think likeathe wax/paper caps used in old Fenders am no tube and theseterms becomeclear.more. Myadvice would betogoforpolypropylene NOTE:Some may take exception to the followingas the best choiceforreplacement. And remem- informationbecauseI don'tuse their biasingproce-ber,always use the right voltage, as small capací dure.In myown defense,letme saythatI'm not try-tors can go bang.... ing to sell you anything;I don'tmake amps, I don't"Allthiscan sound confusing, and in many ways havea tubebusiness,and I don'trun a repair shop.it is, but thereare peopleout therewho knowthe I, likeyou,am justa musicianwho's in search of mydi¶erence, so if in doubt,findsomeone to ash. ultimate tone. But I've learnedfrom people whoMost peopleinvolvedprofessionallyin electronics generously shared with me their considerable Both of these transformers-Silvertone (L) and

can helpyou even tythey haveno idea ofguitars knowledgeof amps and electronics-people such Marshall (RMre intended for the same tube

and amps, as capacitors are used everywhere in as Dennis Kager,JessOliver,Mike Fliegler,George complement. The same? I don't think so.

the modern world. " Saer,AspenPittman,TomImparato, MarkSampson,

Y ou willneed thefollowingspecializedequipment toperform this procedure. It is expensivewhen

new,butcan be foundcheaplyat surplus dealersand swap meets.

• Oscilloscope-any cheap one that works will do. • Dummy load-depending on your amp, a 4-, • Sine-wave signal generator (audio)-can beYou can find an old DuMont at ham-fests for as 8-, or 16-O 50- or 100-Watt resistor, mounted in bought at surplus dealers or at swap meets.little as $10. a fashion that allows a free flow of air, is ideal. Some synthesizers have a sine wave as part of

• Variac-must be able to handle the current your • A plastic-handled screwdriver or alignment tool their palette; this will do fine.amp draws. (availablefrom Antique Electronic Supply). • Hookup wires-from the signal generator to your

amp's input, and a "Y" cable to go from youramp's output to the 'scope and dummy load.

TUBE AMPS • COMPLETE GUIDE To GUITAR & AMP MAINTENANCE • 57

Now to the procedure itself:

1. Remove the amp chassis from its cabinet using the method described earlier in

thischapter under "Microphonics." Zap the caps. Place the chassis on a firm table;

use wooden blocks, if necessary, to stabilize it.

2. Plug the signal generator output (setto =.5

mVAC) into the amp's input (volumeoff), plug the speaker output into the load and the oscilloscope (makesure theoutput impedance is set correctly), and plug the amp's AC cable into the Variac.Using your meter, set the Variac to read exactly 117 VAC at the AC input of theamp.

3. Set your meter to VDC at the 100 V scale if it's not auto-ranging; if it's automat-ic, set it to VDC. Place the - (negative)probe on a metal chassis point and the +

(positive)probe on pin 5 of one output tube (6L6, 6550, EL34,...; pin 2 onEL84/6BQS). If you can't attach the + probe easily through a pin-hole, you will

have to hold it or use well-insulated alligator leads.

4. Turn the amp on, but leave it on standby. When the amp is warm, turn the stand-by to operate.

s. Slowly turn up the amp's volume. You will see the sine wave develop on the'scope, and eventually it willget square on the top and bottom. Back off the vol-

ume until just before squaring.

e. Ifthe wave isn't symmetrical, play with the tone, presence, and volume controlsto get it as symmetrical as possible.

7. Ifyou're lucky, your amp willhave a bias pot located somewhere on the chassis.Use a schematic or layout drawing to locate it. If you don't have a bias pot, firstmake sure you have a fixed-bias amp: Look at pin 8 (pin3 on EL84/6BQ5): Is itgrounded, or is it connected to a big resistor and cap in parallel? If it's connect-ed to an RC net (resistor-capacitornetwork), you have a self-bias amp andshould read the "Auto/Self Bias" section, later in this chapter. If you have a fixed-bias amp without a bias pot, you need to have one installed. I recommend thisbe done by a service tech. Some Fender Silver Face amps have what looks tobe a bias pot, but it's actually a tube balance and won't work here.

s. Watch your meter and turn the bias pot up and down a bit to get a feel for theaction of the pot, and then set it for the highest negative number (-50being high-

er than -49). The waveform should get smaller, and may clip, or square off.Lower the volume until you have a clean wave.

ins. You should now notice two humps, on the leading and trailing slopes of the wave.

This is crossover distortion, and it's a by-product of push-pull operation (outputtubes working in pairs).

lo. Turn the bias pot in the positive direction (-38,-37,-36,...)

and watch the wave-form. It will get bigger and start to clip. Keep lowering the amp's volume to keepthe wave unclipped as you lower the bias voltage. As you do this, the crossovernotch will begin to disappear. When all you have left is the merest hint of a hump,

stop. Write down this voltage and label it V-A.

11. Continue on down until the humps disappear-probably a volt or two more. Writedown this voltage, labeling itV-B.At this point the amp is running too hot, and badtubes could pop.

12. Disconnect the dummy load and plug in the speaker. Plug your guitar into the

amp. Reset the amp to V-Aand listen to it clean. Is the sound clear, or are lownotes

"gravelly"

sounding? The higher the negative number, the more gravelly theclean sound willget. Move the bias pot up and down a volt, but never lower thanaround V-B and no more than about two volts higher than V-A. Play and listen.Leave it where you like it best.

la. Now turn the amp up to distortion and play. Is it shrill and a little too clean? Toolow a negative voltage. Is it too mushy? Too high. Set it where you like it best. Mybet is that it's about half a volt higher than V-Aif you're a rock-type guy, and V-Aor a bit lower if your into country or blues.

14. Write all the numbers down in the log at the back of this book for futurereference-V-A, V-B, the final setting, and, for good measure, the B+ voltage asa reference point. Over the course of time you will see patterns emerge and bebetter able to draw conclusions before the fact.

is. Put the amp back together.

Thismethod requirespatienceand closelisteningWithsomepracticeit willyieldrepeat-ableresultsthatwilldeliverthemostpowerand thebesttonefromanygivenpairingof tubesand amplifier(Asyougetbetterwith your

'scope,

youwillalsobeableto see anddiagnoseall sortsofothermaladies,suchas high-frequencyoscillationsand phaseanomalies.)

58 • COMPLETE GUIDE To GUITAR & AMP MAINTENANCE • TUBE AMPS

nother commonlyused method of biasingis to measure theamount of current thetube draws.Thestream of electrons thatthecathode emitshas to come fromsomewhere;ifyoumeasure this

youcangetsomeidea ofthe tube's operation.Theonlyproblem withthismethodis thatit doesn'tallowforthe infinitevariablesin tubes and transformers. Onthe positiveside, thereis a tool available,calledtheBiasProbe=, thatcan make your lifemuch easier if you wish to use thismethod.

GS: "Thebetterway toperformthismethod is to use theBiasProbe, becauseit takesinto accountthe essentialrelationshiþ betweenplate voltage and grid voltage-i.e., bias. Thanks to Ohm'sLaw,you havea relationshiþ in which current is the same as voltagemeasured across a1-O resis-'°°tor Allyou need to do is establishtheamp you're working on, the tubesyou're working with, andthenconsult the charts that are provided.Youcan get within a ±5% range ofa biassetting with-

SidELD Ro Out d Scop. This is t'ery cool, becauseyou're using a method that doesn't allowyou to see whatyour signal looks like.

"Eten rehen using thesame tube-for instance, a 6550-in an SVTyou have thesame 6550that'sused in a Marshall, but becauseof thedifference in platetroltage (~600 Yin theSVTand~ 475Y in theMarshall) the bias is going to be difwent on the very same tube. With the BiasProbe,becauseit doestake the current draw system to afine-tuneddegree,you get a very close toler-ance. It can also be used to match tubesyourselfJustkeeppoppingtubes into an amp that's setcormctly and zeritedownthe current drawyou see on your meter Writethis number on the tube

The BiasProbe frith a Sharpie. Tubeswith the same (orclose) numbers are matched "

WITH THE BIASPROBE

1. Amp off!

2. Look up your amp in the current draw table orestimating chart below.

a. Connect the BiasProbe to a digital VOM and setthe range to "mV". Ifyour meter is not auto-rang-ing, set it to 200 mV.

4. Remove an output tube, install it in theBlasProbe, and install the BiasProbe in the tubesocket.

s. Locate the bias pot and turn the amp on.

e. Adjust the bias pot for the proper reading.

7. Turn the amp off and remove the tube from theBiasProhbe; wRaitforal

tecou

eirntheea ppothold-

e. If you're using matched tubes, you're done. Ifnot, repeat with the other tubes and get an aver-age. Ifthere is a large difference, you need to geta set that is more closely matched.

Asyou can see, there are certain parallels betweenBiasProbe Current Draw Table Estimating Chart-like tube groupings, such as in the Vibroluxand Two Output Thbes:Nom- Maxi- Super Reverb.All thingsbeingequal, twoamps with Plate

Amp Tube / inal mum similar plate voltagesand wattageswilldrawa sim- Voltage 350 V 400 V 450 V 500 V 550 VModel Quantity mV mV ilar amount of current. Unfortunately,all things WattageFender aren t equal. 50 W 41 mV 31 mV 31 mV 25 mV 25 mv

55 W 45 mV 34 mV 34 mV 27 mV i 27 mVBandmaster AD1269 6L6 / 2 25.3 30.3DualShowman AC568 6L6 / 4 26.9 32.3 The BiasProbemanual also hasa set of charts that 60 W 50 mV , 37 mV 37 mV 30 mV 30 mV

65 W 54 mV 40 mV 40 mV 32 mV 32 mVDeluxe6G3 6V6/ 2 15.1 18.1 show indows"of current drawforamps of a par 70 W 58 mV 43 mV 43 mV 35 mV 35 mVPro ReverbAA270 6L6/ 2 31.5 37.8 ticular wattage,tube complement,and plate voltage 75 W 62 mv 46 mV 46 mV 37 mV 37 mVSuper ReverbAA763 6L6 / 2 27.2 32.6 Some common examples follow.Simply find the Four Ou ut 'IbbesVibroluxRev AB568 6L6 / 2 29.9 35.9 plate voltage (measuredfrom plate to chaSSiS 70 W 29 mV 21 mV 21 mV 17 mV 17 mVMarshall ground; see the Appendix for tube pinouts) and 80 W 33 mv , 25 mv 25 mv 20 mv 20 mv100 W 6550 / 4 34.1 40.9 wattage foryour amp, and a general current figure 90 W 37 mV 28 mV 28 mV 22 mV 22 mV100 W EL34/ 4 28.4 34.1 will be at the intersect point. Remember,these are 100 W 41 mV 31 mV 31 mV 25 mV , 25 mV50 W 6550 / 2 34.4 41.3 110 W 45 mV 34 mV 34 mV 27 mV 27 mVstarting points; start cold and work up fromthere.50 W EL34/ 2 28.1 33.8 120 W 50 mV 37 mV 37 mV 30 mV 30 mVAnd need I add: Be Careful!

TUBE AMPS • COMPLETE GUIDE To GUITAR & AMP MAINTENANCE • 59

Auto/Self BiasAsstatedearlier, a tube in an auto-biasedampworks

Do-IT-YOURSELF in C0njunction with a high-current resistor and capto determineits own bias. Manyauto-biased amps,

T heBiasProbeitselfis a verysimple devicethatyou can make yourself;or your amp can bemodified like the Vox AC30and certain Fender tweed amps,tohavea permanentlyinstalled"BiasProbe".It's important to remember, though,thatwhen youbuy are particularlywonderful sounding. Unfortunately,

a BiasProbe you get a really greatmanual that lists many more amps and is a wealth of very specific many"tonellakes"

thinkthatauto biasis theone andinformation that'sbeyondthe scope of this book.The manual alone is worth theprice. (The BiasProbe only reason for theirgreat sound. If that is true, Iis availablefromGrooveTbbes.See the Appendixfor information.) offer thisquestion: The "NarrowPanel" 4x10tweed

Bassman is the Godfather of all vintage tone cre-Tomod your amp, youwill need togetsome 1-O, 1-Watt(orbigger)precision resistors (asmanyas your ators, yes?Well,it's a fixed-biasamp! Remember,anamp has output tubes).Precision resistors are accurate to 1%of stated value. If your electronics parts amp is an assemblageof manyparts and subcircuits;dealerwon't let you measure beforeyou buy(mostwon't) or doesn'thave 1%-ers, buya bunchand if conceivedas a whole bya knowledgeabledesign-measure them. Youneed exactly1 er, theresultingamp willsound good. Free yourself

fromsimplisticslogans;you listenand you decideforyourself.

Auto-biasingamps tend to run hotter and deliverlesspower than fixed-biasdesigns.They're harderon tubes,and since the tube itself determinesthe

1. Remove the wire or solder joints between biassetting, the bestwav I've foundto ensure greatthe cathode and ground on all output sound in an auto-bias amp is to use good-qualitytubes. (Pin 8 on 6L6, 6550, EL34,... pin 3 matched sets. These are gettingharder and harderon EL84/6BQ5.Check for your tubes in to find.Mostnewtubes simplycan't handle the extrathe Appendix.) grief an auto-bias amp inflicts.For best results, dig

deep and call one of the dealers listed in the2. Solder a 1-D resistor between the cathode Appendixforsome N.0.S.tubes.

and ground.

Ohm's Law states that I = E/R (current=

voltage ÷ resistance). If a resistance is 1O, then I = E, because any number divid- Onceyour amp is biased,youmightwant to see howed by i equals that number- much power it's putting out. This is very simple to

do. It calls for the same tools and hookupas thes. Plug in the guitar and speaker. Connect oscilloscopebiasmethod.

one of your meter leads to the chassisside and the other lead to the tube side ofthe 1-D resistor. Turn on the amp. Use theBiasProbe charts to determine theapproximate proper current draw for your 1. Get the biggestunclipped wave you can onamp. the screen byplavingwith the amp's volume

and tone controls. (Abitof bouncingripple4. Set the bias pot to draw a bit more current, at the top ofthe wave is OK,butno clipping.)

then play and listen. Then set it to draw abit less, and play and listen. As long asyou pay attention, you're OK. If the platesglow red, you're too hot. Ifthe clean soundis gravelly, you're too cold.

s. When you get your best sound on bothclean and dirty tones, leave the biaswhere it is and close up the amp.

2. Set your meter to MC and read the voltageAlwaysstart bysettingyourbiaspot at thehighest across the speaker terminals.negativenumberandworkdownfromthere.This a. Wattageis equal to the voltage2÷ imped-method ignores or assumesall other factorsthat ance. For example:a sine wave throughan oscilloscopewould tellyouat a glance- 17.5 MCsquared = 306.25;

,,3,......,3,,,, ,,,,i,

4The ultimate guide to biasingis your ear. Use 306.25÷ 8 R = 38.28,or ~ 38watts.these methodsto guideyou to a close point andthen listen. Thisis an RMSreading;peak poweris about two

timesRMS.Turnthe amp up all the wayand takeanotherreading to see what youget at fullbore.(These measurementsneed to be taken with anRMS-reading meter-which most modernmeters are.)

6o • COMPLETE GUIDE To GUITAR & AMP MAINTENANCE • TUBE AMPS

BIS€S Allamps havea Mains,or Power,fuse. (Solid-stateamps usually have circuit breaker) This fuse isOne fusecompany,I can't remember which, used to locatedright at thestart oftheamp'sACcircuit and fyour amp doesn'thavea B+ fuse,youmayhave their motto emblazoned on everypackage:monitorsthe overall flowof current into the amp. want to use an underrated power fuse.It"Electrics SafetyValve."Wellsaid, guys.Fusespro- This is useful for most problems.Other amps also may pop inconvenientlynow and then,but it

tect your amp frompotentialdisastet add a B+, or IIT,fuse.The B+ fuseis usuallyin-line will also blowthat much sooner when dangerwith the circuitry that feedsthe power to the output loomslarge,possiblythwartingan all-out dis-As explained in the biasingsection, a tube that's astettransformer. If a sick tube tries to drawtoo muchoperating properly will drawa certain amountofcurrent, arcs, or startsbreakingdownin some othercurrent. Numerous factorscan cause the amp or 1. Check the value of your fuse. Replace itway,pop! Another 50¢. This is a far cryfrom thetubes to try to suck up more wall current than is with one ½ Amp smaller.$75-150a new output transformercosts (plussafe; it's at this point that a fusewillblow.If the fuse installation).Allthis is bywayofsettingup the themeweren't there, any number of catastrophes could 2. Turn your amp on and play for a few min-of thissectionensue: blownoutput or power transformer, blown utes at stage volumes.

tubes, fried resistors and caps, or friedyou! Yes' Read and obey all fuse values.when it comesto fuses,you get a lotforyour50¢. a. Repeat this procedure, in descending ½-. Amp increments, until the fuses blow tooIf you must use a fuse that isn't the correct easily. Go back up ½ Amp or so andvalue, make it smaller,

you're set.

Never use a fuse that's larger than therecommended value.

SD: "Youwant to be 100% thorougheverysingletimeyou set up, becausewhat might takeyoufiveminutes to sort through nowmables, knobs,switches, etc.-can cause you gigantic embar-rassment when the spotlight's on and you'vegottofixor sort something out."

Inexpensive protection

TUBE AMPS • COMPLETE GUIDE TO GUITAR St AMP MAINTENANCE • 61

Solid-State AmpsR

eviled, stomped on, shown no respect, Anothercost-cuttingfactoron solid-stateamps is thatand universallyhated-solid-state amps they don'trequire an expensiveoutput transformer.outsell tube modelsat leastthreeto one! Transistors,becauseof their unique physics,can beCan theyactuallybe that bad?Ofcourse hooked up directlyto a speaker,whereas tubes need

not. Althoughit's a fact that most of the beautiful a transformer. I've heard a number of solid-statetones you hear on records, old and new, are tube- amps that incorporate a transformer in the outputdriven,it's also a factthat solid-stateampsare get- stage in an attempt to emulate all the

"deficiencies"

tingbetterall the time. And in blindfoldtests even of tube design,butto myears theyfail.Outputtrans-the most snobbish valvophilecan havetroubledis- formersdo not a

"tube-amp

sound" make.tinguishingthe differencebetweena good modernsolid-state amp and a tube one. Fact is, there are 1. Look closely for signs of e eating orsome uses where solid-stateamps excel: bass, jazz'

s we all know,solid-stateamps are made burns. If you see what appears to be acountry, and deathmetalall can benefit fromthe ,, burnt-out section, make a note and bnng

highlycontrolled and precise operation of solid- with "No User-ServiceableParts Inside, your amp to a tech. The burn may be theso we must "Refer All Servicingto Qualified tip of the iceberg, and replacing that sin-

state circuitry. Personnel." Unfortunately,with most all these gle part may start a chain reaction whenyou turn the amp on.

amps it's really true. The only hope a mere"" mortal can havewhen approaching a broken

paa2e solid-stateamp is thatsomethingreallyobvioushasbrokendown.

1. Remove the chassis from the cabinet orhead and place it on a well-lit table.

Rad2. Use your chopstick to poke around and

test all connections. If you find a looseone, resolder it.

a. Many modern amps use"ribbon,"

or"edge,"

connectors instead of solderjoints. Push them in tightlyto make surethey're all making contact.

4. Close the amp back up and plug it in.Test it with a guitar. If you've fi×ed it, youshould probably go out and buy a lotteryticket when you're done gloating.

Better than tubes-for some things(Joe Giron)

Earlysolid-stateamps used individualtransistorsincircuits thatweren't verydifferentfromtubedesigns.Todayindividual,or discrete,transistors are virtual-ly unheard-of, havingbeen replaced by integratedcircuits, or chiþs. Asinglechip can consist of thou-sands of parts that are photo-etched onto a verysmall surface. This cutsdownon laborand greatlyincreases reliability,making solid-stateamps a bar-gain to buyand own.

82 • COMPLETE GUIDE TO GUITAR & AMP MAINTENANCE • SOLID-STATE AMPS

Speaker CabinetsT

here are two common kindsof speaker Removingthe screws from a closed-back cabinet ¶jg jcabinets: open-back and closed-back. can bea real pain. I recommend the purchase of an .Almost all cabinetsare coveredin some kind of vinylThe open-back cabinets are most often electric screwdriver if you plan on futzingaroundor cloth.The"Tweed"clothcoveringof old was actu-foundin combo amps. Theyhavea wider with closed-back cabs.allyoriginallydesignedfor suitcases.Becausealmostspread of sound and fillup a small room better than

a closed-backcabinet.Open-backslack the bottom- The material a cabinet is made fromhas a definite all amp coveringhas a cloth backthat shrinks a littleend punch and projection of closed-backs. Closed- effecton thesound.Thiseffectissimilarto thewaydif-

when glued, it acts to keepthe cabinet jointstight inbacks are most oftenfoundin head-and-cabinetfor- ferentwoodsaffectthe tone of an acousticguitar.The additionto protectingor hidingthecabmet material.mats. Theyhave a tougher, more focusedtone and heavierand denserthewood,the betterthe toneof the Cabmet vinyl is easily cleaned and renewed. Thedon't sound very good off-axis (fromthe side). cabinet. How tightiva cabinet is screwed and fitted biggestworrywith cabinetvinyl is rips.Somecompanies,most notablyMESA/Boogie,make togetheris also a factor.Good-qualits,denseplywood

.

4x12s that combine open- and closed-back designs forcabinetsis expensive.and it (alongwith speaker Some cabs are covered in a carpet-type matenal.Whilemuch more durable than cloth-backed vinylto capitalizeon the strengths of both. Which one's quality)accountsfor a good ponion of the difference

for you is a matter of taste, venue, and what amp(s) in price betweenexpensiveand cheap cabinets. or cloth, it's so uglyto most players (includingyourstrulv) that it isn't worth it.you re using.

ny number of common householdcleaners can beused on cabinetvinvl. Simplyfollowthe directionson the cleaner

• For dust and light grime like beer, soda, burger grease,... plainsoap and water is best. lJse a big sponge with a few drops ofdishwashing liquid and water to wipe away the crud. If you usetoo much soap you might be rinsing for days; start with a dropand add more if necessary.

• For tougher dirt, graduate to glass cleaner and a sponge. Spraydirectly onto the surface and wipe.

• Other cleaners you might try on stubborn dirt are ammonia,Lestoil, etc. These cleaners are pretty tough stuff, and I recom-mend using them outside. Read labels carefully; these cleanerscan have dangerous reactions with certain chemicals.

A fteryou'vecleanedyour cabinet,you mayfind it's a littlehazylooking.The beststufffor rejuvenatingcab vinylis ArmorAll. The onlydrawbackhere is it can leavethe cab a little too shinyor

"faked" looking.There is a new typeof ArmorAllavailablethat hasa low-glossfinish;I recommend this one.

1 2 2. Let it sit for a while,or overnight.

ROTEC s. Wipe it off witha towel or rag.

1. Spray the Armor All onto the cabinet.

SPEAKER CABINETS • COMPLETE GUIDE To GuiTAR & AMP MAINTENANCE • 63

E ven if you're careful and use covers, rips are an inevitablepart of cabinet life.Dependingon how largea rip you have,try one of the followingmethods.

KRAZY GLUE

T hereare a number of differenttypesof Super or Krazyglues. Ifyou go to a good hardware store or hobbyshop, they'llhavea thicker typethat's describedas

"gap filling."Thisis the ideal stuff,and while the drug-storevarietywill work, it'sworth the trip to get the gap filler

1 2

1. Place a dot of glue on the wood in the center of the rip.

3 2. Using a guitar pick or a piece of waxed paper, press and hold theripped flap back over to its original spot.

Hold it there for a minute or two.

s. Place a piece of masking tape over the seam. Leave it alone

now-no peeking! Come back in a few hours.

a. Remove the tape.

s. If there are a few flips or ragged edges, place some glue in them'

. and repeat the process.

CONTACT CEMENT

F or biggerrips, contact cement is the order oftheday.

1. Paint the wood and the vinyl with an even, thincoat of cement and let it dry.

2. Press them together

I n an effort to look "big-time"

or to let theperson who steals their equipment knowtheirname,manyplayers emblazontheir cabinetswith stencils. Ifyou buyan amp or cabinet that hasbeendefiledin this manner, there are a couple of things

you can do to eradicate the mark of a formerowner.The usual fixon blackcabinets is to sprayblackpaint over the (usually)whitestencil.Thislooksprettybad,and the white let-tering tends to ghost through. Mosthardware stores (especiallyin cities) sell some kindof graffitiremover.Thisstuffwill peelthe paint offa battleship.The onlydrawbackis that sometimesit melts the vinylcovering.Testit firston a spot you can't see.Followthedirectionsto theletter,including the ones about ventilation,and your cabinetwillbeas good as new.A little"mop-

up" mightbenecessarywith window cleaner or Ajax to removepaint haze.

64 • COMPLETE GulDE To GUITAR & AMP MAINTENANCE • SPEAKER CABINETS

Sound Posts Ifyouplaymetaland youthinkyou're hearinga loud Dusty Speaker Holeslow-frequencybangeverynowand thenwhenyou'veGood-quality(read:expensive)4x12shavea wood- Over time, the space betweenthe front of your. . got your amp cranked, you probablyneed to secureen sound post thatruns fromback to front msidethe speaker cones and thebackof your grille cloth canthe backof your cabinet. A quick test is to haveacabinet. Thepurpose of the post is to exert pressure friendplaychunkyF#fifthsatshowvolume whileyoucollect all sorts ofdustand other unwantedmateri-on the backpanel and keepit fromflappingaround feelthebackofyour cab. If thebackblowsout moreal. This is particularly unsightlyif the grille clothand making noise. In daysof yore when smooth, than a half inch and feelslikeit's about to flyoffits happensto be black.Toclean this stuffout, use abluesytones were the thing, thesesound posts did vacuum cleaner and the round furry attachmentscrews,you need to secureit better.theirjoband did it well. Todays jackhammermetal intendedforvenetianblinds.Goeasyand don'tapplytones are a differentthing altogethera lotof pressure.

Wiring SpeakersA basicpart of all speakers is the voice coil. The

T hecure forthis problem is to add another screwto the center of thebackpanel. Thisis not as voice coil is a lengthof wire that's wound around aeasyas it seems,becausethe postmaynot beat the exactcenter of thecab, and it'susuallypret- hollowformthatis attached to the speaker's papertysmall.Followthe stepsbelow,butremember:Youonlygetone trytodrillthe hole in the rightplace. cone. Aroundtheformis a magnet.Whenyou playaMeasuretwice,cut once! note, your amp creates an alternating current thatcauses the voicecoil tochangepolarityaccordingtothe frequencyof the note. The voice coil and themagnet start attracting and repelling each other(likepolesrepel, oppositesattract), and thiscausesthe speaker cone to movebackand forth.

ofConei. Remove the back panel and locate the 2. Measure down to the center of the post. Permanentsound post. If you don't see one, close the Write the measurement down. (AT square Magnetcabinet back up and try to find the source of makes this job much easier, but it isn't * *

the noise (yourimagination?). necessary.) Electron FlowforAudio Signal3 a. Measure across to the center of the post.

Write the measurement down.4. Replace the back panel and estimate thelocation of the center of the post. Place línlike Poles

some tape there. Attract

s. Retrace your measurements and markthem on the tape. Find the spot where

sithey intersect. N

Motione. Drillthere. of conePermanent7. Remove the back panel and look at the Magnet

post. Ifyou were careful, the hole is in the - +center. Widen the panel hole by redrilling Reversedit to the same size as the others. Electron FlowtGh haar

wraersscrree Ranbdth sm ewLom Speakers: alternately repulsive and attractive

bar of soap to ease its way in. (If you (Television and Radio Repairing)can't find a similar screw, use one fromthe perimeter.) The only part of the speaker the amp

"sees"

is thes. Screw the back on again. voicecoil.Typically,voicecoilshavea DCresistanceof

s .4,8, or 16Ohms,and it'sfromthispointthatwestart.

OhmsGeorgSimonOhmwas a Germanphysicistwho dis-covered and explained a lot of the electrical phe-nomena we nowtakeforgranted.For example,a for-mula knownas Ohm'sLawis the premisebehindthe

6 BiasProbe,discussedin the"TbbeAmps"chapter.

Anotherpart of Ohm's legacywas a set of equationsfor calculating resistances or impedances.(Resistance is the wordused when measuring DCcircuits.Impedance is used whenmeasuringACcir-cuits. Theyare bothexpressed in Ohms [R]. Thereis no such word as

"ohmage." The signalyour ampsput out is AC,and therefore theword impedance isused in discussingspeakers in an amp.) Usingtheseequations, you can calculate the impedance of andfigureout howtowire anygroupof speakers.

SPEAKER CABINETS • COMPLETE GUIDE TO GUITAR & AMP MAINTENANCE • 65

• To calculate any number of impedances in Usually,4x12cabinetsare wired in a combinationof Differenttube amp companies havevarious nomen-series: R1 + R2 + R3... = O. series and parallel. The speakers are separated into clatures for setting the amp's impedance to match

twogroups of twospeakers wired in series.Those the cabinet(s). This can be confusing to a guitarist

two groups are then wired together in parallel. who justwants to play.I'll try to shed some lightby• Tocalculateany twoimpedances inparallel: Here's themath; if you get this,all theother combi- illustratingthe threemajor systems.

RlxR2= O. For more thantwoimpedances, nations are easy:

R1 + R2 • MESA/Boogie. Most Boogs have three jacksfigurein pairs andthentreateach pair as a sin- • Takefour16-O speakers. on the back,On most of these,two are labeled

gleimpedance. 4 O and one is labeled 8 O. The confusion• Wiretwo groups of twoin series: here is this:If you haveone8-R cabinet, you

If you thinkof the followingvisual cues, the terms 16 O + 16 O = 32O foreach group. plugit into the 8 O input; simple enough.But,

series andparallelare easyto understand. if you havetwo8-O cabinets, you plug eachone into one of the two4 O jacksand leavethe

• Seriescircuits are likea lineof dominoes.One• Wirethosetwogroupsin parallel: 8 O jackempty.Two8-O cabs (wiredin par-

dominoaffectsall theothers: if it falls,theyall 32 Ox 32 O _

1024 O 16 a. allel inside theamp) = 4 O.

fall;if a dominois removed,thechain reaction 32 O + 32O 64O

stops there.• Marshall. Marshallmakes it a littleless con-

fusingbyhavinga selector switch on the backof the amp. On older models there is a three-

R1 R2 R3 Speakers in Series/Parallel wayswitchthatselects4 O, 8 O, and 16Q. On

(4x15,4x12, 4x10) e1r6n

eew cchmbeothebs cehe ec

between4 O and8 O. There is a number inparentheses, either 4 or 16; this refers to an

• Parallel circuits are like railroad tracks.The80

¯

80- internal modificationthatcan beperformed to

rails carry the current, and the ties are the change thatswitchposition to theparentheticalimpedances. If one tie is missing, the others impedance value. Marshall output jacks arecan stilloperate. wired in parallel. If you havetwo 16-O cabs,

set theselector to 8 Q.

• Fender. Over the years Fender hasused a few

R1 R2 R3 SO¯

8Q- differentschemes.The one most oftenfoundis

two jacks in parallel. The one labeled "ExtSpkr" is switched,but it doesn't cause anychange inside theamp. Atthis point thereis animpedance mismatch, but nobody's com-

Practical Applications 8O plained for35years!C'est la vie.

Ninety-ninepercent of the time, an amp or cabinetwith twospeakers willhavethosespeakers wired in

Virtuallyeverytubeampfollowsone oftheseexamples.

parallel. This is a holdoverfrom the past, when There is a shortcutfor parallelimpedances. Anyspeakers couldn't handlea lot of power and would group of like impedances in parallel can be figured NÍßtElenduceblowout more than theydo now.Remember: if a out bydividingtheimpedanceof a singlespeakerby Speakers are fairlymaintenance-freedevices.Theyspeaker in series blows,theybothstop working. thenumber of speakers: liketobekeptdryand awayfromsunlight,don'tlike

to be poked at, and most of all theydon'tliketo do• For example,four16-Ospeakers in parallel:

more work thantheywere designedto do. When a= 4 O. speakeris asked to do too much work, it usually

Speakers in Parallel 4 returns the favorby blowingup. Before a speakerblowsit maysmoke, make a rubbing sound,or pro-ducea low-frequency"flubbing." If you experience

*80

¯ *80

- any ofthesesymptoms,turn it down.Or add morespeakers.If a speaker's power-handlingability israted at 25 Watts,two speakers can handle50Watts,and soon.Ifyou havefour25-Wattspeakers in yourcabinet, thatcabinet can handlea total of 100Watts.

4D

Speakers in Series

16D

& a naamat ETw Illnr TA IIITam Ae AMD MAIAITWAIAM e RPWAKFR CABINETS

Troubleshooting To Replace or toæRsencone?That is theA blownspeaker can take many forms.Sometimesthey justblowout and stop working. Withmodern speaker that'sbeentraumatizedbeyond Allof the speaker problems justmentionedrequirehigh-powered speakers, this isn't as common as it mbbingcan burn so hot it either melts replacing or reconing. If you havenew speakersonce was. Today'sspeakers usuallybreakin a more openor welds into a short. The easiest wayto withlittlesentimentalorvintagevalueand don'thaveinsidious fashion.Here are some symptoms and checkthisis with your Ohm meter.If you get a reconing expert in your vicinity,it may be mostchecks you can make to see if your speakers are OK. nothing across the terminals, it's open. If you cost-effectivefor you to replace. If you're dealinggetanythingother than theintended resistance, with original vintage equipment and want to pre-it may be shorted. In either case you haveto servesome of thevalue,reconingis a must.Avintagereplace or reconespeaker that's beenreconed willnot sound the same

as before.Thenew,toughercone will tightenup the

W hena speaker is overdrivenit starts to If you don'thavea meter, try this:tone a lot.Whatyou're reallybuyingwithreconingisoverheat.Thiscan cause the voicecoil 1

' the use of your old, aged magnet,withits individualformto overheatand deform.This in turn can tonaleffectand thevintagelook.cause the coil to rub or get stuck in the gap ittravelsin.

1. Take an ordinary 9-Voltbatteryand con-nect it across thespeaker terminals. If youhear a fairlyloudpop, there's hope.If you

a -

getnothing, it's gone.1. Remove the speaker from the cabinet and

place it, cone up, on a table.2

1. Look closely at the speaker cone for signsof damage. If there are any rips or punc-tures, this is bad. Speaker cone paperthat's been exposed to the sun too longcan dry out and crack. Ifa speaker's beenstored in a damp place it can rot.

2. PUmp the cone gently with the palm ofyour hand. If you hear any scratching orthe cone is stuck, it's blown.

SPEAKER CABINETS • COMPLETE GUIDE To GUITAR & AMP MAINTENANCE • 67

Wrench SizesFender

Bassand GuitarSaddles (US) .050"

Allen/hexSaddles (Japan) 1.5 mm Allen/hex

Floyd Rose

Saddles Va" Allen/hexStringClamp 2.5 mm Allen/hexNut 3 mm Allen/hex

Gibson

TrussRod Me" Socket

Ibanez

Trem ½2" Allen/hex

Kahler

Vibrato .050"

Allen/hexLockingNut Va" Allen/hex

PRS

TrussRod Vis" Socket

Rickenbacker

TrussRod ¼" SocketSaddle 2.5 mm Allen/hex

Guild

TrussRod ¼" Socket

somenacesto anystuffAngela Instruments Stewart-MacDonald10830 GuilfordRd. 21 N.ShaferSt.,Box900Suite309 Athens,OH45701AnnapolisJunction,MD20701 (800)848-2273(301)725-0451 Tools,guitarparts,...Tbbes,tube manuals, old books,..'

Wm. K. Walthers (Hobby Supplies Catalog)Antique Electronic Supply 5601W.FloristAve.6221MapleAve. Milwaukee,WI53218Tempe,AZ85283 Tools,littlebits-o-hardware,grease, oil, bushings,...(602)820-5411

Local Hardware, Hobby, and Auto Supply Stores1bbes, electronic parts and supplies, great vintage radio,,books Manythingsthat appearto bespecialized"guitar products,

such as certain polishes,abrasives, or files,are commonitems availablein a number of places.The sources listedMojo Music Supplyabove are convenient, but you pay for that convenience.524SanAnselmoAve. Shop around a bitfirst.

Suite204

SanAnselmo,CA.94960(707)255-8284

Schematics,amp parts,tubes,...

APPENDIX • COMPLETE GUIDE TO GUITAR Se AMP MAINTENANCE • 71

Cool Magazines with All Sorts of Info about Swap Meets and Listings ofParts Dealers

Antique Radio Classified (ARC) CQMagazine

P.O.Box2 76 N.Broadway

Carlisle,MA07141 Hicksville,NY11801

(508)371-0512 (516)681-2922

73--Amateur Radio Today

(800)289-0388

Resistor Color Codes

TOLERANCENUMBERS OF ZEROS ORDECIMAL MULTIPLIER2nd

SIGNIFICANT FIGURES1st

I

SIGNIFICANTFIGURE OR

COLOR NUMBER OF DECIMAL RESISTANCEZEROS MULTIPLIER TOLERANCE

PERCENT ±BLACK . - . . . OBROWN . . . . . 1RED -···--- 2ORANGE .... 3YELLOW . . . . 4

GREEN ..... 5BLUE ...... 6VIOLET . . . . . 7GRAY ...... 8 --- ---

WHITE . . . . . 9 --- ---

GOLD . . . . . . --- 0.1 5(J)

SILVER . . . . . ... --- 10(K)

NO COLOR . . . -.. --- 2O(M)

SYMBOL DESIGNATION ALTERNATE FOR COLOR

(Basic Electronics)

72 • COMPLETE GUIDE TO GUITAR & AMP MAINTENANCE • APPENDIX

rubeInfoThe followinginformationand diagramsare takenfromtheRCAReceiving TubeManual.

5AR4/GZ34 Fou,-WAVEVACUUMRECTIFIER 6V6 BEAMPOWERÏUBE

Glassoctal typeused in power supplies , Metal type 6V6 and glass octal type G24 5Gi

of televisionreceivers and other equip- ' 6V6GTAare used as output amplifiersin ament having high DC requirements. 6 automobile,battery-operated,and other .::·

Requires octal socket. It is especially , , receivers III which reduced plate-cur. H 2 7 H

important that this tube, like other, , rent drain is desirable.Require octal S:6V61 8

power-handling tubes, be adequately IC H,K SOcket. These tubes are equivalent in NC: 6V6GTA G3

ventilated. performance to type 6AQ5A. Type12V6GTis identicalwith type6V6GTAexceptforheaterratings.

Heater Voltage(AC/DC).......................... ...........................5.0

VHeater current...................................

...........................1.9

A 6V6 6V6GTA 12V6GTPeak Inverse Plate Voltage........................ .

............................1700

V Heater Voltage(AC/DC).............................6.3

6.3 12.6 YPeak Plate Current (perPlate) ............................

.........................825

mA Heater current.........................................0.450.45 0.225 AHeaterWarm-UpTime (Average)...............- 11 - secHeater-CathodeVoltage:

Peak Value...........................±200 max ±200 max ±200 max VSU4 Fou.-WWE SCLui RECTUERAverageValue

.........................100

max 100 max 100 max VGlassoctal type used in power suppliesof radio and color and black-and-white Po2 setelevision receivers having high DC * 6

12AT7ÆCC81 HIGu-NIU TWIN ÎRIODErequirements. Requires octal socket. NC

3 6PDT

This typemay be supplied with pins 3, F 2 7 NcMiniaturetypesused as push-pull cath- HT24

H5T16PT1

5, and 7 omitted. Verticalmounting is ode-drive amplifiers or frequencycon¯ KT2y GT1

preferred, but horizontalmounting is NC F V€ft€fS in the FMand televisionbroad-permissible if pins 1 and 4 are in verti- cast bands.Requireminiature9-contact GT2 2 8 KT1

cal plane. The coated filamentis designedto operate fromthe ACline socket. Each triodeunit is independent ey2' 'HM

through a step-downtransformer.Thevoltageat thefilamentterminals of the other except for the commonshould be 5 Volts at an averagelinevoltageof 117Volts. It is especial- heater.ly important that this tube, likeother power-handlingtubes, be ade-quatelyventilated.

Heater Voltage(AC/DC):Series ........................ .

..

..........................12.6

YFilamentVoltage(AC/DC)...........................

.............................5.0

V Parallel.......................... .

..............................6.3

yFilamentCurrent........................... .

...............................3.0

A Heater Current:Peak Inverse Plate Voltage......................... .

............................1550

y Series ........................

..............................0.15

APeak Plate Current (perPlate) .............................

............................l

A Parallel.......................... ..............................0.3

AHeater-CathodeVoltage:

Peak Value........................ . .

...........................±90

max V

6L6 BFat POWERTLBE

Metal type 6L6 and glass octal type 24 sG' 12AU7ÆCC82 MEDIUM-MUTWINÏRIODE6L6GCare used in the output stageof e ,

::. Miniaturetypesused as phase invertersaudio amplifyingequipment, especially,,,

HT1PT1

units designedto haveample reserve of H 2 7 H Or push-pull amplifiers in AC/DCradio 45

power-deliveringability. Require octal , , equipment and as multivibrators or KT2a GT1

socket. These tubes,like other power- NC C G3 Oscillators in industrial control devices.GT2 2 8 KT1

handlingtubes, should be adequately Alsoused as combined vertical oscilla-ventilated.Type6L6GCcan be used in placeof type 6L6and maybe tors and vertical-deiection amplifiers, PT2 HM

suppliedwithpin 1 omitted. and as horizontal-deflectionoscillators,in color andblack-and-whitetelevisionreceivers.Requireminiature9-contact socket. Each triodeunit is independent of theother exceptfor

Heater Voltage(AC/DC)....................................................6.3

Y the common heater.Types 7AU7and 9AU7are identical with typeHeater current........................... .

..........................0.9

A 12AU7and 12AU7A/ECC82exceptforheaterratings.Heater-CathodeVoltage: 6L6 6L6GC

Peak Value...............................................±180

max ±200 max VAverageValue.......................... ..................- 100 max V 12AU7

12AU7A/Heater Voltage(AC/DC): 7AU7 9AU7 ECC82

Series................................................7.0 9.4 12.6 YParallel .............................................3.5

4.7 6.3 YHeater Current:

Series................................................0.3 0.225 0.15 AParallel

.............................................0.6

0.45 0.3 AHeaterWarm-UpTime (Parallel, Avg.)........11 11 -- secHeater-CathodeVoltage:

Peak Value.............................±200 max ±200 max ±200 max VAverageValue

...........................100

max 100 max 100 max V

APPENDIX • COMPLETE GUIDE TO GUITAR & AMP MAINTENANCE • 73

12AX7/7025/ECC83 HIGH-MUTWINTRIODE 7591 BER POWER ÎUBE

Miniaturetypesused as phase inverters s,, Glassoctal typeused as audio-frequencyor twinresistance-coupledamplifiersin "T24 5 6PT1 p0wer-outputtubein high-qualityaudio G24 SG3,K

radio equipment or high-quality,high- KT2 3 7GT1 AppÏiC21iOns. Requiresoctal socket. e , _ _ _ e

G1

fidelityaudio amplifiers.Requireminia-¯¯¯¯

-::

GT2 2 8 KT1

ture 9-contactsocket. Each triodeunit " 2 7 N

is independent of the other except forPT2'

HM 1 8G2

the common heater.Type7025 is iden-ticalwith type 12AX7Aexcept that it hasa controlled equivalentnoiseand humcharacteristic. Heater Voltage(AC/DC)............................

..........................6.3

YHeater Current...........................

..........................0.8


HeaterVoltage(AC/DC): Peak Value......................... ..

.............................±200

max V

Series .........................

..........................12.6

Y AverageValue......................... . .

.........................100

max VParallel............................

.........................6.3

YHeater Current:

Series ...........................

....................................................0.15

AParallel.......................... ............................

.........................0.3

A EL34/60A7 POWER PENTODE

Heater-CathodeVoltage: G2 G1Peak Value.........

........±200

max V Glassoctaltypesused in theoutput stage 4 5

AverageValue............................ ..

.........................100

max V of audio-frequencyamplifiers. Require Pa _ _ e

NC

octal socket. -

H 2 7 H

1 8

6550 BEMI POWER TUBEG3 K

Glass octal type used in the output G2 G1HeaterVoltage(AC/DC)...........................

...........................6.3

Ystages of high-fidelityaudioamplifiers. 4 s Heater Current..........................

.........................1.5

ARequires octal socket. This tube, like "

a ...Heater-CathodeVoltage:

other power-handlingtubes,shouldbe-::

Peak value....................................................±200

max vadequatelyventilated. H 2 7 H

1 8S K

G3

Heater Voltage(AC/DC).....................................................6.3

Y EL84/6BQ5 POWERPENTODE

Heatercurrent................1.6

A Miniaturetypeused in the output stage HPeak Heater-CathodeVoltage: a e icof audio-frequencyamplifiers. Require 4 6

Heater negativewith respect to cathode........................300 max VHeater positivewith respect to cathode.......................*200 max V miniature 9-contactsocket. Types8BQS K.G3 3 -- 7

and 10BQ5are identicalwith type6BQ5 e io

* The DCcomponentmust not exceed 100 Y exceptforheaterratings.IC G2

EL84/

7027 BEMI POWERÎUBE 6BQ5 8BQ5 10BQ5HeaterVoltage(AC/DC)

.............................6.3

8.0 10.6 YGlassoctal typeused in push-pullpower G2 G1 Heater Current.........................................0.76 0.6 0.45 A

amplifier circuits of high-fidelityaudio P G1HeaterWarm-UpTime (Average)...............- 11 11 sec

equipment. Requires octal socket. This' ¯= 6 Heater-CathodeVoltage:

¯¯¯

Peak Value.............................±100

max ±100 max ±100 max Vtube,like other power-handlingtubes H 2 7 H' AverageValue............................100max 100 max 100 max Vshould be adequatelyventilated. 9 aG2 K,G3

HeaterVoltage(AC/DC).......................... .

........................6.3

YHeaterCurrent............................

...........................0.9


Peak Value........................... ..

.........................±200

max VAverageValue...........................

...........................100

max V

74 • CoMPLETE GUIDE To GUITAR & AMP MAINTENANCE • APPENDIX

Schematic Symbols

Electrolytic TubeResistor Potentiometers Capacitor Capacitor Pentode

Tube Solid-State Solid-State NPN PNPRectifier Diode (Diode)Rectifier Transistor Transistor

Standard Switched Single-PoleSpeaker Fuse Jack Jack Single-ThrowSwitch

Crircuitd Connection

Chassis NoGround Connection

Double-Pole Double-Pole Ground Circuit ShieldedSingle-ThrowSwitch Double-ThrowSwitch Connections Routing Cable

AC Plate 16Ohms

PowerSupply 8 Ohms110V CenterTap120 V 4 Ohms220 V Filament240V Plate Common

PowerTransformer Choke OutputTransformer

Things to Read

The 1bbe Amp Book, byAspenPittman Everything

Schematics,substitutioninfo, tube lore, mods,... Everybook,every magazine,every catalog; getthemall. Read them,save them,and reread them a year later.The more you read, thePublished by Groove Thbes (alsothe source for the BiasProbe).

(818)361-4500.$29.95 smarter you get; thesmarteryou get,themore you¶ be able to sepa-rate factfromlictionin what otherstellyou. Make friends,get in aband,get out of the house,talk about equipment untilyour girlfriendAMPS!The Other Half of Rock 'n' Roll, byRitchieFliegler(boyfriend)wants to kill you.Askquestions.Be a pain in the neck at

The world of amps according to me. Ibbes, troubleshooting,transis- the localmusic store (buysomethingeverynow and then). Smarter istors,history,...More funthan you can stand. Lots-apix! better!Don't beafraid!Nowgo!Published by Hal Leonard. At music and book stores everywhere.$24.95. FF: "Usecommon sense and don't beafraid to ask questions. Read

as much as you can. Read the manuah butdon'tget toogoodandFixing Up Nice Old Radios, byEd Romney steal mygigs!"

A book about radios? Yes. Under the guise of teachingyou howtorestore antique radios, Eddie boygivesyou thefulltreatmenton elec-tronic theoryand design.This is a great book,written withmore atti-tude than evenI can muster.Whew!

AvailablefromARCand Antique ElectronicSupply.$19.95.

APPENDIX • COMPLETE GUIDE To GUITAR & AMP MAINTENANCE • 75

Documents

How to Bias a Guitar Tube Amplifier