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. 


@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. 


+1 @avanti1960

@markcasazza Best to follow this reply, from a known member with first hand experience on both sets of 88/120 tubes in that specific CMII amp and version you have. FYI tubes can perform and sound differently in different tube amp circuit designs and across different designer-builders. Glad to see this reply too, I run KT120s in my current mono amps myself. While my current quad of 120s and 150s are okay, and I have a spare quad of KT120s.  Checking this morning - my own tube source provider reports they may not see new quad sets of these larger KTs again until late April/May ’23. So, good news you found a KT120 or two. Worth trying first, yes.