Filter capacitors are part of the power supply and bias supply in a guitar amp. They need to be changed after a certain number of years. Any amp 20 years old or more should have the filter capacitors changed.

Filter capacitors dry out after time. They can cause an amp to have "ghost notes". These occur when the capacitance in the power supply gets reduced. Some people like this tone, but it is usually a sign of an old and possibly ready to fail filter capacitor. Some newer amps are designed to give this tone. Another problem that old filter capacitors can cause is leaking your plate voltage to ground. This will give you less power and punch. Some people like this, but it is a sign of an old and possibly ready to fail filter capacitor once again. Sometimes you will hear a very strange version of tremolo or vibrato in your amp. This can be a sign of a bad bias filter cap. If the bias filter cap shorts out, you will burn up your power tubes! The bias supply capacitor needs to be changed too!

What happens when a filter capacitor fails? They can short out which can do a few things. If you have the correct size fuse in your amp, the fuse will probably blow and when you put in a new fuse, it will blow it again. If you try changing all the tubes in the amp (all of them!) and the fuse still blows, you very likely have a shorted filter cap. If you have too big a fuse in your amp (or a bunch of tin foil shoved in the fuse socket like I've seen a million times), your high voltage secondary will blow on the power transformer. This is a very expensive mistake. Don't use anything but the correct value fuse for your amp! Filter caps can also lose their filtering function with time, leaving your amp with less filtering than it needs to operate properly. Symptoms of decreased power supply filtering are hum, buzz, motorboating, loss of power, and strange noises.

Some people complain that the tone of their amp was lost when the filter caps got changed. In reality, that tone that they were getting was due to bad or failing filter caps and that tone should have never been there in the first place! There are some people that will advise you that changing the filter caps will "kill" your tone or that if you like the tone of your amp now, then the filter caps should not be changed. These people may also say that changing the filter caps will devalue your vintage amp. This is not good advice. Let me give a few analogies. You say that you like the way your car handles. But the tire pressures are at 5psi. This is your opinion of what a good handling car is. Is this a reasonable opinion? No! Get the pressures right or you're going to throw the carcass right off the tire! You say that your car has been running fine and you think you don't need to do anything to it. Is this any excuse not to get the oil or spark plugs changed? No! Filter caps are like changing the oil or tires on a car. It just needs to get done. People are spoiled by modern equipment that doesn't need service. Old amps need service, and after a certain amount of time, the filter capacitors need to be changed.

I use Sprague Atom filter capacitors for Fenders. These are the best caps you can get for the money. There are cheaper ones to be had out there, but I like to use the good stuff. However, Sprague does not make values that cover all amps. For Marshalls and Hiwatts, the original value LCR can caps are still available. I have a list up that should help decide what caps your amp needs.

Many amps use an electrolytic cap on the cathodes of preamp tubes, or cathode biased power tubes. The function of this cap is to help decrease sag and mush when the amp is pushed. Most Fenders use a cathode bypass cap when the tube is arranged in a standard plate follower configuration. Since these are electrolytic caps, they are subject to drying out. This can cause shorting, which will tend to burn up the particular tube that the cap is on. They can also start to drift upward in value which can cause unwanted headroom decrase and distortion.

In cathode biased amps, it's a good idea to increase the voltage rating of the cathode bypass cap on the power tube(s). For instance, in Champs the voltage rating of the cap is 25 volts. Somtimes the voltage present on the cap is higher in use. ALSO, since this cap is mounted close to the cathode resistor a lot of the time, the heat causes the cap's voltage rating to decrease (trust me on this). So in practice it's a good idea to increase the voltage rating of this cap to 25uf-50v (I sell these) and mount it higher off the board than the way they came stock.