Sophia Royal Princess 300b quality

One of my Sophia Royal Princess 300b tubes if I shake it gently I can feel the internal plate mount is loose. I contacted sophia but they refuse to replace, just simply claim do not shake the tube. The tube has long period of pop mechanical sound during warm up. Very disappointing.
What is Sophia's warranty period? If it is still under warrantee, you should get an exchange.
Warranty period is one year if the tube fails.