KT120 replacement options in a Cronus Magnum II

First off replace all 4 power tubes together with 4 new tubes. Then bias according to manufacturer recommendations. If the amp is old and the tubes are that tired , I’d consider replacing the small signal tubes as well. Be careful not to damage the amp with a faulty tube, or powering it up without a speaker load. As far as your question , I’d consider the new production Gold Lion KT 88’s, great tube. They will be slightly less power and slightly less bass, but would be fine. I’ve also witnessed my friend going to Gold Lion KT 77’s in his M 180 mono blocks. Rogue ships a lot of its gear with JJ small tubes, and there is a ton of improvement to be had. This is all based on the assumption that your amp is healthy and has no other issues. Regards , Mike B. 

Are you running the amp with empty tube sockets ? So tubes are a wear item, they break in and then slowly erode after hundreds of hours of use. It’s never good to run a bunch of mismatched tubes. As per your question, I’d buy tubes before a new cartridge. If a tube fails it might result in catastrophic damage to the amp. I’ve had multiple tube failures in my lifetime, most are harmless, but I had one destroy the power supply and the amp had to be shipped to the builder for repairs. Try to access a tube tester and test your 120’s, then you’ll know for sure. Find a repair shop in your town and see if you can test them. You keep mentioning pairs, but you need a quad (4) . At least I’d get 4 new tubes and  if you get more 120’s you can test yours and keep for spares. The amp you have is a great amp, but they have a reputation for eating tubes. Good luck, Mike. 

You are correct, do not mix tubes of different values in your amp. Also great advice above from the member comparing the specific tubes in the same amp. As far as “ Hot Spots “, are you referring to the Ghetter Flashing ? Watch some Utube videos that show how this happens. You will see how they use inductance to heat the Ghetter material and how it collects the impurities and deposits it on the glass envelope. When a tube fails you will sometimes get a black deposit on the glass. Also if the envelope fails , you will get a grey or white deposit on the envelope. Since you now have a tube amp, take some time to search on the internet. Videos on tube manufacturing are fascinating and reading about how tubes function is very interesting. Having a minimal knowledge of how tubes are made and function has served me well. Also there are many members here that are extremely knowledgeable on this subject. As a kid I built some Dynaco stuff, as well as some SS kits. I found it very rewarding.  Good luck and happy listening. If your 120 purchase doesn’t materialize, PM me and I’ll help you out. Regards , Mike B.