KT120 replacement options in a Cronus Magnum II

@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.

+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.

@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.

@tubeguy76 I swapped the KT120 tubes in my CMIII with Gold Lion KT88s and I really like the sound. Seems "fuller" to me.

@goldprintaudio as much as I love KT120s in CMIIIs, I’ve really been enjoying the last few units that have come through with 88s. It does seem to be bit warmer and lush over the 120s.

Probably so. I’ve seen this comment when amps are not truly optimized to run KT120s or KT150 tubes properly within their optimum operating window. Just because the tube fits in the socket, does not always mean it’s going to sound its best in any amp. Worth researching more. Is the amp truly designed to run KT120/KT150s well? Ask the designer for the plate voltage, rec bias - check values of the tube spec vs. the amp capability. Compare. 

Running a KT120 or KT150 tube "cool" in an amp with lower plate voltage and low bias current, well under its optimum operating window, will have more of a sterile un-engaging sound. Drop in a smaller KT77/KT88 in the same amp, running it hotter inside its desired operating range - now sounds nice. How is that, hmmm.

Now, move that same set of KT120s or KT150s to an amp with bigger transformers, i.e. running 600v at the plate, biased at 60-70ma, moving up to 70-80% plate dissipation, now the same tubes running inside their optimum operating window, no longer being run "cold" or "cool", now sounds like a nice sounding tube amp again. The BlueGlow Electronics guy has a nice YouTube video about this, running tubes "in their window". No need to run big tubes in smaller low current amps not designed to run them properly - just a waste of money, fwiw.

@decooney The CM II or CM III are not low current amps. They come with KT120s but can run KT88s per Rogue.

@tubeguy76 Curious to know what the actual plate voltage is in the new CM III? Is it static, or does PV change when KT88s are installed? (if anyone here knows the design well) and has the real specs. I’ve looked for this before with Rogue, would be interesting to know and share here.

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Note: I’ve seen this complaint before about KT120 or KT150s not sounding as good as KT88s in the Rouge amps starting back with CMII, and with the CMIII too. Always wondered why not.

Sure, there are many amps that can run both with lower plate voltages, yet how the circuit, transformers, design is set up is key in how the larger KT output tubes behave, performs, sounds. The circuit design, voltage, transformers - matters.

i.e. I cannot run KT77/KT88s in my Quicksilver Mono 120s with the larger transformers and higher plate voltage specifically designed to run KT150s. KT120s are okay too. The big monos will burn up KT88s, right on the ragged edge at 600v for most KT88s. It’ll work, but will burn them up prematurely. Having owned amps with EL34/KT88s, KT120s, KT150s, can share KT120s sound glorious in the QS Mono 120s, sound full, powerful, plenty of midrange, can be quite liquid when the big iron gets warmed up and saturated. Similar to VAC, in the right amp design KT120s can sound very nice. Same with Conrad Johnson too with KT120s, nice!

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@mulveling I’m with ya man, only have so many spare quads of KT120s and KT150s lying around. If New Sensor does not figure out how to get them out, its only a matter of time before PSVANE and others start producing clones, Read something recently about Mike at NS/Sovtek reaching out to PSVANE regarding this. Maybe another path of getting production out to clients. Will be tracking this. 

@mulveling "Unfortunately, I think Psvane DID try to make KT120 several years ago - and it was a complete bust due to failures and general poor reliability."

They did, and yes, several years ago. Same was said about PSVANE 6SN7s, and KT88s too, and fast forward five years, some changes for the better.. New people and equipment acquired since. So far so good on my last pairs of CV181 MKII 6sN7s and helped a few friends re-tube with quads of PSVANE KT88s, no issues past two years.

And now making tubes for some high-end amp manufacturers, easy to spot, see them at my local audio dealer. Several tubes out now since. Quietly working in the background. Counted 75 different versions and levels of tubes in production now this week. They don't seem to be slowing down yet.