KT120 replacement options in a Cronus Magnum II

I have a "new to me" Cronus Magnum II" that came with a shot KT120. Thankfully the am still plays with half the power on the one channel. I'm having no luck finding this tube. Being made in Russia and in high demand I expect the situation to stay lean for some time. Rouge reported that the KT88’s, KT90’s, and KT77's are all compatible. I'm new to the tube amp space and I was hoping to read for a few months before going tube shopping, but I have a situation. 

Has anyone "downgraded" the Magnum II to one of these other tubes?  I'm guessing I should buy a pair so both channels have the same output. Is that right?

What are your thoughts for the best way forward? I spent most of my budget so a pair of KT150's is really not an option. 


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. 

@markcasazza do you feel confident its just the one KT120 going bad? Have you swapped R/L tubes to see if the issue follows that one tube?

If its just the one tube you might be able to find a single for now and let it play a while longer while saving back up and deciding to pick up a different set of GL KT77/KT88s later on. A nice luxury to be able to run these smaller musical tubes too in your amp.

Great questions. Here are the answers:

I have swamped the tube and the issue follows the tube. I get no current when trying to bias and the tube does not get warm has a burn spot on the its side unlike the "hot spot" of 3 other tubes. 

Rogue Audio said it is okay to operate the amp with the dead tube. It is inserted. I turned it off at first until they advised it was okay.

I finally tracked down a KT120 for sale. I'm now thinking to buy 2 KT120's and get the 3 remaining working tubes tested. Set the weakest aside as a spare. Delivery on a new tubes will be 3 weeks. 🙁 Getting the tubes tested is also not trivial as the nearest shop that I know of is an hour's drive away. I will keep looking closer to home, but I live in rural Ohio.

I like the idea of a full set of KT88s sitting in a drawer. That may be a Christmas present to myself. 



I forgot to ask this clarifying question:

Yes, I was thinking of 2 KT88s. The thought was can I run a KT120 and KT88 on each channel. It sounds like that answer is "No!".  I'm a newbie and not finding good content to figure this out for myself. 

Thank you for the replies!!!

do your best to find a KT120.  as long as they bias correctly you should not need to have them tested.  

KT150s will not work on the CMII.  

I had the CMII and tried KT88s.  It was an instant downgrade- weak and dull sound.  the KT120s are the real deal.