Audio Corner

More musical instrument ampsand other MI equipmentBy Ron C. Johnson

Those of you who have been around theservice business for a while are probablynot surprised that tubes are still usedextensively in guitar amps. And youveprobably seen your share of tube circuitsfrom the old days of television. On theother hand, some of you younger guysmay find the idea pretty archaic. But, real-ly, were still using vacuum tubes asCRTs in televisions and monitors, sowere not all that far removed from thedays of vacuum tubes.

Servicing vacuum-tube amplifiersAs I mentioned in a previous article,

many guitar amps still use tube circuitsbecause musicians like the sound they getout of them. Solid state amps just seem tobe too clean. Not enough texture, somewould say. Whatever the reason, if youfind yourself repairing musical instru-ment electronics youre going to run intotube amps.

Normally, the first thing I would sayabout tube amps is to be careful. Four hun-dred volt power supplies require somecare and attention to avoid a shock. Butanyone who works on televisions on aregular basis should already be aware ofthe danger of high voltage. Just the same,dont let your guard down just becauseyoure working on an amplifier instead ofa TV. The results of an electrical shockcan run from mild discomfort all the wayto electrocution and the dangers of sec-ondary injuries can be just as serious.

A typical tube ampThe circuit shown in Figure 1 is a typ-

ical low power bass guitar amp made byFender. This one only uses four tubes: two7025s (dual triodes) and two 6V6GTAs(pentodes). Both of these are very com-mon (or at least used to be when tubeswere in general use). The 7025 is used asa signal amplifier and the 6V6s are poweroutputs. For those of you unfamiliar with

Johnson is a journeyman electronics servicing technicianand an instructor of technology at the Northern AlbertaInstitute of Technology in Edmonton, Alberta. Canada.

tube circuits, a few quick points follow.Vacuum tubes work on the basis that,

if you apply a potential between twometal electrodes in a vacuum, and thenprovide a source of free electrons (by theheating one of the electrodes), electronswill flow through the vacuum from thenegative (heated) electrode to the positive.

The positive electrode is called theanode (or plate) and the negative elec-trode is called the cathode. The heater, orfilament, is under or inside the cathode.It heats the cathode, releasing electronswhich are attracted to the plate.

The vacuum-tube diodeA tube with two electrodes constitutes

a diode. If the plate is positive and cath-ode negative, and there is a closed circuitbetween the two, external to the tube, cur-rent will flow. Within the tube, this cur-rent will be from positive to negative ifyou use conventional current flow, or viceversa if you use electron flow.

I personally use conventional currentflow although some still prefer the other.If the voltage is applied in the oppositepolarity no current flows. Because thecurrent flows if the voltage is applied inone direction, but doesnt flow when thevoltage is in the opposite direction, thevacuum-tube diode is a rectifier.

For amplification, vacuum tubes add atleast one more electrode, (called a grid),between the anode and cathode. Byapplying a voltage to the grid, the currentflow from anode to cathode can be con-trolled. If youre familiar with FETs youcan easily catch on to tubes. A voltage onthe gate (grid) controls the current flow(transconductance) of the device fromdrain (anode) to source (cathode).

The triode as amplifierThe circuit in Figure 1 shows the input

signal applied to the grid of V 1 A, the firsthalf of the first 7025 tube. We wont getinto how to determine gain of the amp butthe schematic conveniently tells us thatwith 3.5mV of signal in, there should be

195mV on the plate of the tube. The sig-nal is inverted because, as the plate cur-rent is modulated, the voltage drop acrossthe 1OOK plate resistor varies.

The output is capacitively coupled,with a relatively simple treble/mid/bassfilter section and volume control beforethe next gain stage. Again, typical signalvoltages are shown on this schematic.Notice that the grids of both stages areswitched to ground through the input jackwhen nothing is plugged into the amp.This keeps noise from being amplified.

The output signal of the second stageis capacitively coupled to a differenceamp made up of the two halves of V2.Although it may not look like it at first,this amp is very similar to the differenceamp shown in a previous amplifier art-cle. The signal is fed to the grid of V2Awhile the feedback signal is fed to the gridof V2B. Both cathodes are tied togetherand their common current flows (eventu-ally) to ground.

The effect of negative feedbackIf the cathode current of one side

increases, the voltage of the other cath-ode increases as well. This decreases thepotential from anode to cathode whichreduces the current through that half ofthe tube. Decreased current meansdecreased voltage. It amounts to negativefeedback. Thats how the feedback volt-age from the output can reduce the over-all gain of the circuit.

Negative feedback also minimizes dis-tortion and noise in the output stage.Assuming the input signal to the stage isclean, any distortion (or noise) which isadded by the output stage is fed back tothe input, out of phase. Since the distor-tion part of the signal is only found in thefeedback, it is amplified (out of phasewith the distortion added in the stage) andthis subtracts out, leaving a clean output.

Push-pull outputThe output stage is a push-pull config-

uration: one side drives the output on the

54 Electronic Servicing & Technology November 1993

positive excursion of the signal while theother side takes care of the negativeexcursion. Notice the additional -33Vpower supply which sets a bias on thegrids of each output tube.

The 6V6 pentode is a power tube. Itsextra internal grids give it better drivecapacity without the capacitive and gridlimitations of the smaller tubes. In thiscase the 400V power supply is connect-ed to the center tap of the output trans-former. On each signal excursion currentflows in alternate directions through theinput windings inducing a current in theoutput windings which are connected tothe speaker.

The transformer not only allows thisbias arrangement but it matches the highimpedance tube output circuit to the lowimpedance speaker load. The feedbacksignal is obtained from the output side ofthe transformer and sent back to the inputof the difference amp to control the gain.

ServicingTube circuits are actually pretty easy to

troubleshoot. There are two dc powersupplies here: the 400V main supply andthe -33V bias supply. The 400V supplyuses a full wave bridge while the nega-tive supply uses a single diode for half-wave rectification. Each has its owntransformer winding.

Watch for bad power supply capswhich will cause large levels of ripple onthe supply rails. Also, make sure the tubefilament voltage is present. This 6.3Vacis provided by a separate output windingon the power supply transformer and isconnected to each tube.

Tubes, of course, can fail. They tend tobe somewhat fragile, especially in an ampthat is hauled around to gigs. Sometimesthe internals come loose and the tube goesmicrophonic. Even though it amplifies,the slightest vibration will be amplifiedas well. In extreme cases the vibrationsfrom the speaker itself will feed backcausing the amp to howl.

The positive aspect of tubes is that theyare easily replaced and can be tested ifyou have a tester available. I wouldnttrust a tube tester too far though.

Signal tracingSignal tracing through a circuit like this

is fairly easy as well. You need to knowwhich pin is which on the tubes. I havean old GE tube manual which lists most

of the common tubes, their specs andpinouts. Schematics like this one givetypical signal levels which is helpful.Watch out for open capacitors, shorted oropen rectifiers in the power supply, anddirty, intermittent potentiometers in thevolume and tone control circuits. Alsolook for signs of abuse. Its fairly com-mon to find dried residue of beer and softdrinks inside the chassis.

The newer tube amplifiers have tubesockets mounted on printed circuitboards. The older ones still have point topoint wiring between sockets. These canbe a nightmare to troubleshoot and worsewhen youre soldering or desolderingcomponents. Usually, the easiest way toremove a component is to cut it out andthen remove the solder and the ends ofthe component leads.

Well, thats a pretty sketchy overviewof a typical tube amp but hopefully it willhelp somebody to feel more comfortabletackling one for the first time.

Mixing boards and effectsSound mixing boards are another kind

of musical instrument equipment thatyoull see quite often probably becausethey receive a fair amount of use andabuse. They run from small four channelmicrophone mixers to large road consoleswith 32 or more input channels, monitorand effects mixes, and several submixes.

The features on these are limitless.Some have built in phantom power (atechnique of simplexing dc power out tocondenser microphones), clipping indi-cators, LED bargraphs, solo and cueswitching, and so on. Ultimately, they alldo basically the same thing. They take au-dio signals and route them to various out-puts while controlling their levels.

One of the common problems withmixers is dirty or damaged potentiome-ters. Each channel has a volume faderor linear potentiometer which adjusts themain output level of that channel. Eachchannel will have several other rotarypotentiometers which adjust equalization(tone) in several ranges, signal level tomonitor and effects busses, input signaltrim controls (to avoid overdriving theinput circuitry), and other controls de-pending on the complexity of the board.

Cleaning noisy controlsInevitably youll have to clean up noisy

potentiometers using spray cleaners. I

recommend you find the best kind avail-able and use it liberally. Unfortunately,these pots wear out and eventually clean-ing them wont help. Youll have to re-place them. Also Ive only had limitedsuccess trying to clean linear faders.These have to operate smoothly and cantcreate any noise.

Usually trying to clean faders results ina quiet but sticky action which the cus-tomer wont accept. Keep a good stock ofreplacement faders for the brand of mixeryou repair. They are expensive but nec-essary to do the job right. Youll also findthat, often, these faders can only beobtained from the manufacturer of theequipment as they have been specifical-ly manufactured for that product.

Locating noisy componentsAnother common problem with mixers

is noise generated somewhere on theboard. This can be caused by bad filtercaps in the power supply, leaky signalcaps or leaky transistors or op amps. Thefrustrating part of troubleshooting theseproblems is in trying to find the source ofthe problem.

A heat gun and some freeze spray canbe handy here as some noise problems arecaused by thermal defects in components.Warm up the suspected area (not too hot)and then spray individual components.Sometimes this will show up a bad one.Dirty connections and cracked circuitboards are common sources of problemshere as well. Just moving the equipmentaround, or flexing the printed circuitboard, will show up some problems.

Although the newer (and more expen-sive) mixers are being engineered for eas-ier servicing, youll find half the job onsome repairs is just getting the thing open.Besides removing lots of screws youoften have to remove all the knobs andnuts From the potentiometers before youcan get to the solder side of the printedcircuit board. For one particular brandand model I used to have a special, home-made puller just to get the knobs offwithout damaging the mixer.

Finally, dont always believe what thecustomer tells you about a mixer. Mixersare complex pieces of equipment andsometimes the problems are caused by theoperators themselves. So insist that thecustomer gives a detailed explanation ofthe problem and then check it carefullyyourself before tearing the board apart tostart testing. n

58 Electronic Servicing & Technology November 1993