Check out fenderforum.com. People around here are into "home" tube amps, not guitar amps. You probably need new tubes.
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I've repaired and rebuilt a large number of guitar amps. The main reasons they distort:
1. weak tubes
2. dried out, leaky power supply caps and/or cathode bypass caps
3. carbon comp resistors that have increased in value
The most likely reason in your case depends on the age of your amp and tubes, and there are others but I would start there.
How old is your amp?
How old are your tubes?