KT120 replacement options in a Cronus Magnum II

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.

Indeed, I had the resort to eBay and there is only 1 seller shipping from China. I just ordered 2 so I have a spare. I suspect the war is causing import issues. No sign that will be over any time soon. Now I wait 3 weeks.

Avanti1960, did you roll any of the pre-amp tubes? That was my plan come next winter. I saw 1 thread indicating some success. I'll bookmark them for when my wallet recovers.

@markcasazza yep, clearly a New Sensor TungSol tube and supplier shortage again for sad reasons of war, yes, likely. Recently I checked my former sources and a few others. Some TS tube suppliers I know of are reporting the next round of TS output tube supplies might come back in stock in April/May. But, I would not count on it.

Also just for grins I’m checking with a former overseas source provider of the prior available KT120s that use to be with label by PSVANE as KT120s, saw those a few years ago but never tried them myself. Some other non-TS options would be nice.

As a backup I recently acquired up an older familiar SS amp to rotate in once in a while any time I want to give the KT120s and KT150s a rest. If that does not go well, will plan on obtaining an EL34 amp or something. Other options nice to have.

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.