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?

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It could be a tube issue, why don't you change out that KT90 for a new one. If the problem goes away, you know it was a bad tube, if the problem persists, then you have another problem altogether. One that would probably require professional service.
I got the same crap, ain't the fuse though. Something more evil. Pops are tube like at least in my short experience, mine was two thumps. Anybody know what the thumps may have been ?? I got some cheap PCC88's from across the pond, a well known brand, I forget I would have to dig them out, pop and that was the end of the tube. It would still work but with static. A couple of those tubes popping and they came out.

I suspect my thumps were caused by moving my speakers a few inches, at least that is what I was doing when it happened.
Seems to me that it is possible that when your first tube shorted out that it burned out a resistor in the bias circuit for that tube. A very common occurence in tube amps for this to happen. Some times the tube comes up when changed but the bias is not controlled and the second tube was overloaded. Ask Rouge if this is possible for your amp. If so find a techie to replace the resistor. It's not a big deal if you have a local shop. If not Rogue should be able to help.
I am going to move the suspect KT90 to a different tube socket. If I continue to blow the V1 fuse it's not the tube. If I blow the fuse corresponding to wherebI move the tube, It's the tube-
could be a loose solder joint on the socket - that happened to me with a tube preamp
It sounds like the tube is shorting and blowing the fuse. It may be time for new tubes.
I'll state the obvious, have you called Rogue back?
I agree with the comments above. My amp does not have separate fuses for each tube. So, if I blow tube because it arcs, there's a good chance I took a bias resister along for the ride too. And I agree that changing out a bias resister is an easy fix.

As an aside, I've had a brand new tube arc and blow a bias resister. But more often it's old power tubes that are more likely to arc.

ARC recommends that I change out power tubes every 2000 hours. Don't know what Rogue recommends, but you mention in the OP that your amp is 4 years old. Have you changed out your power tubes yet. If not, you may be way, way overdue.

Otherwise, the advice above is reasonable, including calling Rogue if you are not able to easily resolve the problem. FWIW, tube amps are pretty robust. The only repair issue I ever had with my amp was a blown bias resister -- and that can happen anytime.
In case this may help. I just resolved a somewhat similar problem today with one of my AirTight ATM3 monoblocks.

Earlier this week, the right amp popped, and I gradually lost volume. I shut down the amp and as the amp has built in bias meters, powered back up looking for any issues with the output tubes. All seemed ok, but I was getting no bias readings at all.

Tested the power and input tubes and discovered a 12ax7 tube was weak. I alsoI tested both fuses and noted one showed slightly open but still "good"

I replaced that fuse with a new one and am back up and running.
I hadn't called Rogue- I plan on doing so on Monday, but I already heard from Mark. On the Rogue each tube is individually fused and biased. So far after switching things around the fuse is still holding but I have been exceedingly careful to play music at very low levels. I feel the errant tube is shorting when pushed and thus blowing a fuse.

Supeisingly I have not had any success yet finding replacement fuses. Nothing at Radio Shack or at a local musical instrument store in stock of the correct value.
I get my fuses as well as tubes from Mark (Rogue). If you order tubes from him he may not even charge you for fuses and just toss a couple in as a kind gesture. Amazing customer service.
Tubes were ordered today. Fuses too.
It's not the amp, it's the tubes! Replace em