Easy DIY Maintenance Tips
Here are a few things you can do to keep your amp on the gig and out of the shop.
1. Don't defeat the ground pin on your amp's ac plug. This is really important, as you can end up shocked or electrocuted! If an amp has a ground pin, it was designed to safely function with it connected (if it's an older unit with a two prong cord, please take it to the shop as soon as possible to have one installed). If you defeat it by using a "cheater plug" or clipping it off, the chassis is no longer at the building's ground potential; it could be at 120VAC above it. In addition to the shock hazard, the amp will probably also be much noisier.
Bands should complain to clubs that don't provide safe, grounded outlets on the stage, and should take the time to look around to find another source of power. You can pick up an outlet tester for a few dollars at Radio Shack that will tell you the status of the outlet you are connecting to. Use one, and alert the building owner to any hazardous conditions.
While we're on this subject, each time you plug in, always check that the line cord or plug hasn't become frayed or damaged in any way.
2. If a fuse blows, there's a good chance that something is wrong. Replace it only with the exact same type (you should always carry spare fuses for all your gear) as specified by the manufacturer. If the replacement blows, you definitely have a serious problem that will require a trip to the repair shop. Never use a higher rated fuse just to get through a gig or practice -- your equipment will be damaged and could catch on fire or shock you. The fuse is the most important safety feature in an amp; make sure it's the correct value.
3. Speaker cable is not instrument cable, and vice versa. Speaker cable is of heavier gauge and is not coaxial. It carries high current to the speakers from the output of the amp. Instrument (small signal) cable is optimized for very low level signals, and is of a coaxial design to limit noise pickup. They are not interchangeable! Use of speaker cable as signal cable will result in an increase in noise, probably a loud hum, but it won't hurt anything. Using signal cable for speaker leads can result in failure of the amplifier. It is not designed to handle the current required. Take care of all your cables; spool them neatly, don't tie them in knots or leave them tangled. A defective cable can be difficult to find on a dark stage.
Sometimes this can be confusing, as both types of cable can have a very similar external appearance. They are labeled though, so take the time to be sure you are using the correct cable for the application.
4. Keep everything mechanically tight. Tighten your speakers frequently. Speakers will loosen up over time; this will result in a loss of tone, and an increase in buzzing and rattling. Tighten a speaker the way you tighten lug nuts when you change a tire -- in a "star" pattern. A few turns on one screw or nut, then a few on the opposite one. Work your way around slowly until they are all tight, but don't overdo it.
While you're at it, this is a good time to check all the other screws, nuts and bolts that secure the amp's components and cabinet. Make sure the handles are tight and in good condition. Make sure the amp chassis is tight, particularly in combo amps. Check the screws holding the reverb tank bag in. Check for loose components such as transformers, particularly in amps where they hang down underneath. We've seen amps with the output transformer dangling down by its wires!
Another very important thing to check is the nuts that hold the input, speaker and other jacks into the amp. On some amps the jack is soldered to a pc board, and if it's loose and you plug in, the board can easily be damaged. This is a common cause of failure in amps with plastic jacks. If the nut is missing, don't use the jack until it's replaced. Check all switches and tube sockets as well.
5. Be careful if you lay your amp down for transport. On many amps, there are switches on the front and rear panels, and knobs and jacks that protrude beyond the cabinet. If you lay the amp down and slide it on its face or back, they can break. This is a common cause of reverb failure, as the cable connectors on the rear panel can be broken this way.
6. Use the standby switch properly. Here's the way to use the standby switch to get the maximum life from your tubes: When you are first turning the amp on, make sure the standby switch is off. Allow several minutes for the filaments to heat up before turning it on. If you are taking a break between sets, leave the amp on, but switch the standby off; it will be ready for the next set without requiring any warm up time. When you're finished for the night, first turn the power off while leaving the standby on. This will drain the high voltage supply down from the filter capacitors. You can leave the amp in this state if you remember to turn the standby off before going back through the power up sequence above, or after a few minutes turn the standby switch off so you're ready for the next use of the amp.
7. Don't move a hot tube amp. When you're finished playing for the night, load out the tube gear last. While the tubes are still hot, they are in a more fragile condition and much more prone to damage. Let them cool off before moving the amp to the truck for the bumpy ride home. They should be sufficiently cooled off by the time all the cables, instruments and other gear are packed.
8. Match the amp and speaker impedances. Tube amps are designed to be loaded by a specific impedance. Depending on the type, the wiring, and the number of speakers or cabinets you're using, the load presented to the amp can vary from too low to optimum to too high. Some amps have an impedance selector switch or several jacks which are labeled for different loads, while some amps have one or two jacks and a single recommended (usually the minimum) load. What you should do is calculate the load of your speaker setup and try to match it as closely as possible to the amp's output. Check out the following table for some common speaker connections and how to figure out the load seen by your amp. [link to ohm's law equations and chart].
9. Never run your tube amp into an open load. If you turn the amp on and there is no sound, the first impulse is to turn everything up full blast. Instead, power down and check to make sure the speakers are properly connected. Tube amps don't like to run into an open load, and can also be damaged by plugging in the speaker while you are playing. Also be careful with power attenuators, dummy loads, etc.. Always follow the manufacturer's instructions, and be sure your amp is compatible with the attenuator unit.
10. When using gain devices in the amp's signal path, be careful not to damage your speakers. Many amps have speakers rated for the normal output of the amp, without much margin. When you use a pedal, rack preamp, or other device, either between the instrument and the amp or in its effects loop, and which produces a dramatic increase in gain or distortion, the speakers can be on the verge of overload. This is particularly true of older low-power amps having speakers that weren't designed for use in high gain situations, or that by virtue of their age are relatively fragile to begin with.
11. Bring your amp to the shop once a year even if it's working fine. A complete going-over by a qualified technician can find problems before they cause a failure. Even if you've been diligent with the above tips, a good check inside can reveal loose wires, capacitors about to go bad, weak tubes, and more. A checkup will prevent failures that always seem to occur at the worst time. The amp will also get a thorough cleaning, a voltage test, and a bias adjustment. This usually only costs an hour or two of bench time and will keep your amp running at optimum performance. It's cheap insurance for your valuable gear.