u/Commercial-Prize-521

Hi guys, I recently got a 50 caliber (not plus, the el84 One) for a very food price. I got It shipped and I think something went wrong during the travel. As you can hear in the video, the sound Is randomly cut and distorted. I tried with different guitars, cables and cabinets and it remains the same. The issue appears after 10-20 minutes from powering on the amp. I have a good tech close by but I would consider taking the amp to repair as my last option. What could It be? What should I check? I'm not a tech, and getting It off the cabinet scaries me a little. Would do It anyway to change tubes if it were the case.