Is this the big "plate fuse" that's on the back panel? (I have premier 5's) If so then that's the high-voltate supply to the plate from the tranny, I think. CJ will send you a schematic via email if you need one. Agreed I would check tubes first. Best bet is if you have a tube tester, pull all the tubes and run them through, esp. checking for shorts between elements on the EL34s. No sense using the amp as a "tube tester" if you can help it! If you do need service, the place to use is http://www.musictechnology.com/ - owned and run by Bill Thalmann who is the original designer of these amps (he worked for CJ for 20+ years). Had good luck with him myself on the CJ Pr. 5s.
Good luck!
-Ed