Tube amp down, need guidance from the experienced

When a blown tube takes out the resistor on my amp, new tube will work but can't bias and just runs very hot.

Being a current owner of an MC275 mk.V and having had several issues with tubes, I'm fairly certain that the circuit layout and tube designations are the same and I can tell you that you have one of 3 most likely causes for the KT88 tube to be dead.

1- The KT88 has failed outright.
2- The 12AT7 tube that supplies bias to the KT88 tube is defective.
3- The fuse that protects the biasing of the KT88 is blown
most likely from a defective 12AT7 tube.

Things to try before opening up the chassis to check for a blown bias fuse:

Swap one of the known good KT88's into the position of the dead KT88. Does it light up and hold steady? If yes, KT88 is the culprit. Replace dead KT88 tube.

KT88 tube swap results in the known good tube to be dead.
Swap 2 of the 12AT7's and if the known good KT88 in the suspect position is now functional, the 12AT7 is the culprit.

If the known good KT88 is still dead after the 12AT7 tube swap, you most likely have a blown bias fuse.

There are 4 bias fuses inside the chassis one for each output tube. Remove the bottom cover and check for a blown fuse and also check for any signs of component damage/overheating on the PCB. (It goes without saying but if you are not comfortable/familiar/knowledgeable with working on tube equipment, STOP and get experienced help.)

If no damage is noted, replace the fuse and the bottom cover. Retaining the swapped 12AT7 tubes and with the known good KT88 in the suspect position, apply power to the amp. Does the KT88 in the suspect position light up and hold steady? If yes, the circuit for that tube position is good.

Power down the amp and swap the 12AT7's back in their original positions. Power up the amp. Does the known good KT88 in the suspect position now light up and hold steady? If yes, the problem was the fuse. If no, you have a defective 12AT7 which probably took out the fuse again. This would also be indicated by the KT88/bias fuse being dead in the circuit that the defective 12AT7 was swapped to earlier.

Depending on what you find, replace the defective tube(s) but I would suggest you replace them all in a set/pair whether it's the 12AT7's or the KT88's. At the very least, buy and keep on hand a replacement tube for all the positions as well as fuses. That way you minimize your downtime and have known good tubes/fuses to troubleshoot with.

Cheers

5,000 to 6,000 hours on the KT88's is a *lot* of hours. My amp (ARC VS-115) has 6550's and ARC advises that 2,000 hours is the "expected" life of these power tubes.

With most pentode or tetrode power tubes these days 2000 hours is about right. After that you are lucky.

Ditto advice ARC gave me. 2000 on power tubes -- that's a max Q!!