Dirt sounds bad. It doesn't even sound good in a bad / good dirty way. So let's talk about how to get the crud out of your amp. I intend for this to be the first of a series of posts on things a player can do to understand, maintain and improve his own equipment. Not that I don't want the work, you can always just send it to me or one of my colleagues around the country. We're always happy to do this stuff. Even if you do just send it in, now you'll know what we're doing and why, and why we charge what we do for it.
What is this crap and where did it come from?
Open up any well used guitar or bass amp and you'll usually be greeted by something far dirtier than its external appearance and conditions of use would have you believe. The short answer is that large positive voltages attract crap from the air. Notice how quickly dust finds its way to your TV or computer monitor, like that but slower because of the lower voltages. It isn't uncommon to look in something like an Ampeg SVT and see a trail of filth on the circuit board that outlines the path of the copper traces on the other side of the board.
Fans (like in that SVT) are a part of that. Along with the cooling air, they blow in all the contaminants and dust in that air. Ever play a bar that served food? The airborne grease from the deep fryer has found its way onto and into your amp. If you can smell something, food, cigarette smoke, stage fog it can end up in your amp.
Crud and filth also get into an amp through more obvious methods. At the end of a show, what do you do? Unplug the guitar, wipe it down and put it away (you do wipe it down, right), then you go to the amp, turn it off and start wrapping your cords. While you're standing there winding your cable watch the plug drag across the stage picking up spilled beer, bassist sweat, punk singer spit and just plain crap tracked in from the parking lot when you loaded in. That filthy plug will go back into your guitar or amp the next time you play and it will foul the jacks. (Guinness will totally mess up a plug, that's experience talkin')
So what's so bad about this crud?
Some of this crud acts as an insulator that prevents signal from passing; some forms of crud, especially some of the corrosion found on contacts acts as a semiconductor. I've seen tarnished FX loop jacks that acted like a diode distortion circuit. How does that happen? Many jacks in guitar amps do more than just connect to the cable that you plug in. The input jacks almost always have some sort of switching contact that shorts the input to ground when nothing is plugged in. This is to quiet the amp when you aren't using it. When that shorting contact gets dirty the amp will tend to buzz or pick up radio stations when nothing is plugged into it. That wouldn't be so bad on an amp with only one input jack, but most amps have more and we don't use them all. So the unused jack is sitting there introducing noise and static while you're playing into the other.
Some amps have an FX loop or a set of jacks labeled "preamp out" "power amp in." Those are called looping jacks and have a set of switch contacts built in that bypass the jack when nothing is plugged in. When those tarnish or get dirty, the amp cuts out or distorts in really bad ways. If you have an amp that is cutting out that has any sort of looping jack the first thing to test is to plug a short cable between the in/out or send/return jacks and see if that fixes the symptom. Practice amps that have a "Headphone" jack can also suffer from this type of problem since the headphone jack is usually designed to kill the signal to the speaker.
What can I do about it?
There are two answers, one is preventative the other remedial. The most important preventative is exercise, that is use all of the jacks and controls on your amp. The guys who design the components are pretty smart and most switching contacts are what is called "self-wiping" which leaving out all potty references means they rub against one another when you use them abrading away tarnish and crud. At least once or twice a year make sure to exercise every switch, knob, control and jack. With the power off, flip every switch back and forth about ten times, especially the ones you never use. You shouldn't need to worry about the power and standby switches, but all those channels and stuff you don't use, exercise those switches. Turn every control pot back and forth about ten times, especially the ones you haven't touched since you got it. For those you can have the amp powered up with the master volume set sorta low and listen to see if they're noisy and if they clean up. For the jacks, again more exercise, run a plug in and out of each one about ten times. Don't feel awkward if you get a little excited doing this, it's only natural. And I do mean every jack, speaker jacks (with the power off, of course) FX loops, footswitch jacks, all of them. Again pay special attention to the ones you never use or just leave plugged in all the time, like the speaker jack in a combo amp.
The same thing applies to tube sockets. Partially pull each tube and push it back in two or three or four times. Careful with the big ones, you don't want to break the index pin, so be gentle. Just partially pull and re-insert each one.
For something like a mixing console this could take awhile. But if you don't do it, you will pay someone else to do it and that someone will charge between $50 and $125 an hour. Your call. This will prevent most of the problems that repair techs see in older equipment.
Summary.
Keeping your amp in clean, dry air helps. But that isn't where most of the gigs are. Wipe down the plugs on your cables before or after using them.
Exercising all of the jacks, pots, switches, every control on the front and back panel will prevent problems. Idleness does more harm than wear, so at least once a year run everything through its paces.
I'll be back next time with how to cure the problems that exercise alone doesn't fix.
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