|
Reprinted from Guitar World Tube Amplifier Care and Hygieneby Csar Diaz and Daniel Jacoubovitch
Like teeth, amplifiers are a lot less exciting to routinely clean and care for than they are to repair in an emergency.
Not that amps need daily flossing and brushing, but they do get gummed up with use. If the crud they pick up over time isn't cleaned off occasionally, they'll develop serious problems requiring professional care.
Many of the irritating, sound-robbing problems that plague older tube amplifiers can be traced to dirt and oxidation that have accumulated on contact surfaces over the years. Dirt and oxidation deposits in tube sockets and amplifier controls cause unwanted distortion, loss of volume and a wide range of crackling and popping noises.
The vintage Fender, Magnatone, Ampeg and other tube amplifiers thathave come to be prized by rock and roll players have all seen enough use and age to suffer from these symptoms.
Just because an amp says Fender on the front and is covered in tweed doesn't mean it will put out the sweet, singing tones that you expect from it. During the years that fifties and sixties tube amplifiers have been picking up value and acult following, most of them have also been picking up sound and power robbing dirt and oxidation.
Airborne dust that settles in amplifiers is conductive and electrostatically charged. Dust deposits interfere with signal transfer between components, causing stray noises, distortion, and generally dissatisfying amplifier performance. The dust's electrostatic charge attracts more airborne dust, causing more interference with the amp's circuitry, further hastening amp failure.
Oxidation deposits on amplifier-control and tube-socket contact surfaces also impede signal transfer, causing unsatisfactory amp performance. Sewing machine and 3-in-1 oil foolishly squirted into amp controls fouls the components andattracts more sound-robbing dirt. The more conductive contaminants settle on amplifier surfaces, the higher the risk of arcing; current taking a component destroying jump between normally unconnected electrodes.
In the worst case, dirt and oxidation can combine with moisture to cause the big "C"; catastrophic failure: the destruction of multiple components due to arcing and consequent short-circuiting across a large surface of amp circuitry. In the usual-less drastic-case, dirt and oxidation rob anamplifier of power and clarity.
A simple amp tune-up requiring only a few tools and supplies can cleanup the sound of any older tube amp considerably. The tune-up is part of themaintenance routine that a good service shop will do as part of most repair jobs. Many amps brought in for repair only need this basic tune-up, and perhaps a tube or so, to get back into service.
Doing the job at home will avoid needless service expenses by eliminating the common causes of poor amplifier performance. When the amp does need to go to the shop, having it clean will save you from paying for the bench time it would otherwise take to get that part of the job done. Also, from the repairman's point of view it's no fun to have to go wading through someone's audio toilet bowl, dust, grease, rust and beer that leaked in and crystallized on the chassis and circuitry, looking for a failed component.
After the amp is cleaned up, some attention to how it's stored andused will protect it from dirt and oxidation build-up, and keep it working and soundingright.
It's a good idea to let your amp sit unused for a couple of daysbefore the tune-up. This will give the filter capacitors some time to lose their charge and you a chance to round up supplies for the job.
Letting the capacitors discharge slowly is preferable to having them do it all at once through your body while you're poking around inside the amp. Having the supplies you need on hand makes the job go faster; you'll want screw drivers, some contact cleaner (Csar uses Gunk brand choke & carburetor cleaner and WD-40), a toothbrush and, unless you own an air compressor – an aerosol can ofcompressed air. If you know how to use them, soldering supplies and a volt/ohm metercan also come in handy, but aren't essential.
Start by unplugging the amp from the wall, the speakers from the amp,and removing the amp chassis from the case. For Fender amps, removing the chassis involves unscrewing the upper back case panel, then unscrewing the chassis from the top ofthe case. Put the screws, nuts, and lockwashers where they won't get lost.
With the chassis out of the case, you can locate the filter capacitors you'll want to avoid touching as you perform the tune-up. These will be cardboard cylinders (usually orange or tan or vintage Fenders) with "+" printed near oneterminal. It's also a good idea to steer clear of wiring connected to the power transformer (that's the large, heavy one); it's a discharge path for the filter capacitors.
If you haven't given the capacitors a couple of days to discharge, or if you just want to be cautious - you can use a short length of insulated wire with alligator clips on the ends to discharge them. First, make certain that the amp is unplugged. Attach one alligator clip to a capacitor'spositive (+) lead, then touch the other clip to the amp chassis several times. Repeat the procedure for each capacitor marked with a voltage (VDC) rating higher than 25. The time spent on this precaution can spare you from a numbing electric shock that you'll remember long after the amp is back together.
Now that the coast is clear, you can proceed with the tune-up. Start by cleaning dust and dirt off of chassis, component, and connection surfaces. Use compressed air to blow loose dirt away. For dust stuck to component and chassis surfaces, Csar works with a toothbrush in one hand and a can of compressed air. Don't waste time on crud that's leaked out of dead capacitors; it's there to stay.
As you clean off solder joints, examine each connection carefully to see if the wire is solidly joined to its terminal. Wires that wiggle loosely from their connections and solder joints showing visible breaks can degrade your sound. If youhave a volt/ohm meter and soldering supplies, this part of the operation is a good timefor checking circuit continuity and desoldering and resoldering worn connections.
Tighten down the power transformer screws. On tweed Fender amps, the only way that the transformers are grounded is through their mountings crews. A poorly grounded transformer can cause intermittence in the amp's output. You may need to substitute mounting screws one size larger than the old ones to restore a good connection between transformer and chassis.
With the underbrush cleared, you can start work on the tube sockets and toneand volume controls - areas particularly sensitive to interference from dirt andoxidation. You'll be using solvents and friction to clean contact surfaces. The cleaner you get and keep these surfaces, the louder and clearer your amp will sound.
Clean one tube socket at a time. Take out the tube, spraycontact cleaner - or Gunk followed by WD-40 - into the socket, thenre-insert it and shake it around in the socket to expose clean contact surface. Becareful with the spray, the idea is to use enough to clean the part you're working on, without saturating the surrounding area. If the socket was particularly dirty, repeat the procedure before moving on to the next one.
Clean the tone and volume controls - potentiometers, or"pots", for short - by spraying contract cleaner through the openings in their covers. Spray in a little cleaner, turn the shaft briskly back and forth a few times, then flush with another shot of cleaner. If you're using Gunk, flush with WD-40 right after you spray the carburetor cleaner; leaving Gunk on too long can freeze the control shaft.
Re-mount the amplifier in the case and reconnect the speaker. Now that your tube sockets and pots are clean, you can troubleshoot your tubes - another potential source of stray noise, distortion, and weakened sound.
All tube amplifiers have at least one power tube (such as 6L6,6V6, or 5881), and at least one preamplifier tube (such as 12AX7 or 7025). Most older amps will also have one rectifier tube (such as 5Y3GT, 5U4, or GZ34).
Drug store tube testers can be used to tell if tubes are dead oralive, but they don't tell you anything about whether a live tube is causing sound problems. Tube problems that won't show up on a standard tube tester can be diagnosed in the amp.
With all the tubes in their sockets, the tube covers off, and nothing plugged into the amp inputs, turn the power on and crank all the controls up to maximum.
Troubleshooting rectifier tubes is easy; they're either dead or alive. If the rectifier tube lights up when you turn the amp on, it works.
A power tube that doesn't light up is dead for sure. A healthy power tube will light up and give off a medium-intensity orange glow. Don't worry about power tubes that give off a dull blue glow; they don't necessarily need replacement unless they have other problems.
Power tubes that glow bright red, however, are a sign of trouble. Try another power tube of the correct value in the socket. If the new tube still glows bright red, there is probably a failed component in the circuitry to that tube, and you should take your amp to a professional to get the problem tracked down and fixed.
Tap each power tube with our fingernail, tubes that send a loud, low-frequency noise through the speaker when they're tapped should be replaced.
Inspect the shaft on the base of each rectifier and power tube to make certain that its guide pin - the ridge that slides into a groove in the socket's center hole, aligning the tube pins with their proper socket connections - is intact. Incorrectly inserted tubes due to broken guide pins areprobably the single most common cause of amplifier failure.
Use your fingernail to tap each preamp tube. If the tappingis amplified and comes out of the speaker as a loud, high-frequency noise, the tube iscausing problems. If there's another socket that takes the same value tube, try the questionable tube in that socket. Because of differences in circuitry leading to sockets that use identical tubes, a tube that makes noise in one socket can work just fine in another. If the tube causes problems in all the sockets you try, you'll need to replace it with a new one.
You can now check out the amp's speaker(s) as the last operation in your tune-up. Plug in your guitar and set the amplifier to a low volume; you won't be able to tell the difference between amplifier and speaker distortion at high volume. Slowly play a few full chords. If the speaker sounds distorted or gravelly it may need repair. Try the amp through another speaker (one you know is working right). If the problem goes away, go ahead and get the first speaker reconed.
Now that you've rid your amp of dirt and oxidation and substituted or replaced problem tubes, it should sound clearer, louder, and generally healthier than it did before the tune-up. a clean amp is a happy amp.
There are precautions you can take to keep it clean and happy. Protect it from dust by keeping it covered when it isn't in use. You can slow down oxidation build-up by keeping the amp away from damp places and protecting it from rapid temperature changes. These changes cause condensation inside the amp, depositing moisture on components and connections. The moisture is particularly harmful to components that are made with paper or cardboard - capacitors andtransformers - weakening them and hastening component failure. The moisture also cause oxidation, fouling the tube sockets and controls you've just finished cleaning.
Never power up a cold amplifier; let it come up to room temperature before you turn it on. Sudden heating can crack cold power tubes. Cold air inside the chassis condenses as the amp warms up, depositing moisture on component surfaces that can cause arcing and component failures.
Don't keep the amp anywhere you wouldn't be comfortable sleeping. Unless you're a lot weirder than most vintage amp owners, this means you'll never leave your amp in a car trunk overnight or store it in a damp basement.
Semi-annual tune-ups and regular attention to how you use and store your amplifier will keep it sounding sweet and strong and save you the painand expense of avoidable breakdowns.
|
