Q: "Why do I need to change guitar amplifier tubes?"
There are two main reasons why amplifier tubes need to be changed. The first reason is that the tube(s) simply burn out, causing a malfunction with the amplifier. On certain amps such as the Marshall JCM900 series amplifiers, there is the failsafe fuse, which is built in to indicate power tube failure. The tubes need to be changed immediately so damage to the transformer is avoided. Tubes have no definite life span and can burn out at any time. (Thus Marshall's 90-day warranty on them!)
The second reason and perhaps the more important, is to improve the performance (tone) of your amp. The effect the tubes have on your amp is much like the effect that strings have on your guitar. The more often you play and the more aggressive you play, the faster the tubes/strings will wear out. Playing loud and hard will cause your tubes to lose power and tone long before they finally die. This wear down process is very gradual and most people wouldn't recognize the difference in tone until new tubes were put in and biased by an authorized technician.
Tip: If a power tube goes, you should replace all of them so they will have equal wear over time. This will help provide a more even tone and back up tubes are always a good thing.
A common problem that we've all seen with tubes is a 'microphonic' pre amp tube. This is the culprit that causes that high end squealing even without a guitar plugged in. A way to check this is to unplug the amplifier, remove the back panel, turn the amp back on and then let the tubes warm up. Take a pencil and gently tap each pre amp tube to see which one has a loud ringing or crackling sound. They will all ring to a certain degree but the "bad" one will be much more noticeable. Replace this one immediately. A second way to find it is to pull out the suspect one(s) and replace it with new ones. This is simple because as you know there is no bias procedure for pre amps tubes and anyone can do this without hurting themselves, the amplifier or others. Plus it's always a good idea to have replacements for gigs anyway.
Q: "How do I know if my tubes are wearing down?"
A: These are the most common signs that tubes need replacement:
1) Excessive noise (hiss, hum) including squealing or microphonic tubes
2) Loss of high end. Little or no treble.
3) A muddy bottom end; Sounds like there is too much bass and note clarity is lost.
4) Erratic changes in the overall volume. Can go up and down but generally it goes much lower.
A blown H.T. fuse. (H.T. = high tension (measured in hundreds of volts)
5) The amp doesn't work!
