Problem with my GK Fusion 550
 

I own a Fusion 550 and I have problem with this amp. When I turned it on there a big "pop" and after there a kind of fuzz when I play bass. This "fuzz" disapeared after like 15-20 minutes of playing. I've check everything (basse, cable, cab, the ac outlet) and came to conclusion that the problem come from my amp. I've bought it new and it is no more on the warranty. There is something I can do before putting it in the hands of a technician. thank,
Marc

You can try new preamp tubes first. The top cover not only has screws on top but a couple on the back. The tube shields need to be rotated CCW to remove them, normal care when pulling the tubes and re-inserting the new ones.

I will look for the tubes. How can I test a tube to know if its broken. Thanks,
Marc

The MOST reliable way without test equipment is to swap known good new or used tubes in the amp (one at a time).