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TUBE REPLACEMENT
Precautions:
Always unplug the amplifier and allow the tubes to cool completely before
touching them.
It is absolutely necessary to adjust the bias when replacing the output tubes.
When to replace tubes:
12AX7 preamp tubes will generally last for many thousands of hours but occasionally
problems can development before they "wear out". If you hear crackling, popping,
humming, ringing, or other strange noises, try replacing your preamp tubes one at a
time. This may resolve the problem.
Output tubes usually have a shorter life. They will often start to sound bad long before
they fail. If you notice that your amp is gradually loosing volume, headroom or
"punch", or if you notice that low notes are sounding a little "flabby", it might be time
for a new matched set of high quality output tubes. This could happen after a several
months or it might take several years depending on how much and how hard the amp
is played. Don't forget about biasing the output tubes.
Similar symptoms can be caused by a worn out speaker so be sure to try the amp on
another cabinet to be sure.
Sometimes a tube will fail before it "wears out". A tube failure will usually result in
blown fuse. If you find a blown fuse in your amp, the most likely cause is a bad output
tube. If your amp is blowing fuses, try a new matched set of output tubes and adjust
the bias. Always use the correct fuse rating.
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