KT120 replacement options in a Cronus Magnum II

Agree, replace all 4 tubes and re-bias when you're changing types. Also agree the Gold Lion KT88's are great tubes. I've rolled them into my Rogue Apollo Darks and I think there's a good argument to be made they're better than the stock KT120. The GL 88's have a sweeter midrange and an overall more refined sound. 

In the past I compared EH KT90 versus KT120 - the KT90 are brighter and leaner. I greatly preferred the KT120, though systems which lean a bit dark (need a lift in air and sparkle) could conceivably benefit from the KT90.

In other amps (VAC amps - not Rogue) I've compared EH KT88 versus GL KT88. Here the EH KT88 are a bit more rock 'n roll; the GL are a bit more refined and smooth. I put the Gold Lions back in, but I really like the EH 88 and keep them around for a change of flavor/pace some day. I don't think you can go wrong with any of the modern Russian KT88, and the Gold Lions are an easy general recommendation. 

That sucks about the KT120 shortage - a lot of tube amp makers have been using that as a "stock" tube in their amps for a while now, and I wonder if the tooling (dating back to 2009 / 10?) is getting worn down :( 

Budget is a bit of an issue. I understand it may not be ideal, but what is the downside to getting a pair of KT88s and holding the remaining working KT120 for when the next KT120 goes? 

I don't think the tubes are as much tired as the previous owner never heard of biasing them. There was 1 position where the bias was way off! I'll bet that's where the now dead one sat. The amp let's me bias per tube and it was the first thing I did after an initial warm up period and how I discovered I was down 1 tube. Okay, yeah, I did not notice that 1 tube was not glowing. I was just playing the amp quietly and working across the room. My listening room is also my work from home office.

As a data point, the amp sounds absolutely amazing even with only 1 tube on the one channel. Yes, if I drive it hard the one channel starts to deteriorate, but that is not normal for me. My, also new to me, Cerwin Vega E312s are very efficient and I don't need to push the amp to more than fill my room. 

I'm looking forward to rolling the other tubes, but I need to let the buying settle a bit first. In the past month I got the Cronus Magnum II, Denon DN-300Z, and E312's. My credit card is a bit warm. ;) I was hoping to upgrade my stylus to the AT-VM95SH. I have to weigh the sonic impact of 4 tubes and no stylus upgrade with 2 tubes and a stylus upgrade. 

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.