Help me diagnose the problem

I've got a Rogue Tempest II Magnum. A couple weeks ago I heard a 'pop' and then noticed that the left channel wasn't quite right. Further probing led me to the KT90 in the V1 position- it was not hot to the touch. Mark at Rogue suggested I check the fuse to V1. Sure enough, fuse blew. Tip off was the tube could not be biased. Put in a new fuse, and we're good.

Today- I heard something not quite right from the left channel again. Popped the lid, saw I could not bias V1. Pulled the fuse and it was blown.

2nd V1 fuse blow in as many weeks.

Why am I blowing the V1 fuse? FWIW, the amp is nearly 4 years old. Could a tube issue be causing the V1 fuse to blow?

Thanks!