I have a Marshall Origin 50H, its about 7 months old and I've used it perhaps  maybe 10 times total for some pretty low volume playing and never used on anything but the lowest 5w setting. I'm using it with a proper 16 ohm Celestion cabinet and proper cables and playing for about 40yrs so know what I'm doing, well, most days that is lol! :)

Anywho,  after about 10 minutes the volume drops WAY off and I'm assuming its a power tube since the level of gain doesn't change at all,  just the volume and it gets real compressed. 

Since I've only had the amp 7 months, would the tubes be covered under warranty in any way? I would hope so due to how little I've used it, thanks for any insight.