The first step in troubleshooting any tube amplifier is to determine if the power tubes have gone bad. Most high-quality amplifier tubes have a lifetime of about two years if they are cared for and maintained properly. Most guitar players do not know how to properly diagnose a bad tube and have subsequently wasted countless dollars on replacing tubes that were simply not installed properly. There are a number of telltale signs to help you accurately determine if your tubes need to be replaced
Check the fuse to see if it is blown. The fuse is usually located in the rear of the amp near the power cord. Unscrew the fuse. If there are burn marks around the fuse, then it is blown. This means your tube is either dead or dying, which forces the amplifier to draw power from the wall and overworks the fuse.
See if the tube is broken. A broken tube will have white, black or silver powder spots on its interior. If a tube has burned out (this rarely occurs), it will look very similar to a burnt-out light bulb, with burn marks all over it.
Turn on your amp and listen for a dull sound or what sounds like a blown, flabby speaker. If, after about five or ten minutes the sound goes away, that means your tubes are bad. When all of your tubes are completely dead, the sound will never go away.
Never try to replace just a single tube, if one tube goes out, you will have to replace all the tubes in your amp. This will ensure that your power is balanced correctly among all your tubes and will result in longer tube life.
When replacing a fuse, don't put in a stronger fuse to make up for a lack of power. Doing so will permanently damage the central power of your amp.