If you're sure its the amp thats crackling, and not just because of loose jack connections, the most likely culprits are the tubes and the power filter caps. You can try tapping gently on the tubes with a plastic chopstick or a wooden dowel to see if the noise gets worse.

This is the sort of thing that makes a good tech worth their fees. See if Gary at Steve's Music Store on Rideau Street near King Edward can steer you to someone good. Or look up Greg Wilson.