KT120 replacement options in a Cronus Magnum II

I agree with tubeguy76......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.

@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. 
 

The Cronus II and up is for sure designed to run KT120s.  As mentioned above, they come stock (when KT120s are available at least) with 120s. 

KT120 had been stock in Rogue amps for many years now. You can run the KT120 in these amps at a higher bias (40 - 45mA, or even 50mA if you’re bold) than KT88 (35mA). In operation, they should be fairly close with more headroom (plate dissipation) on the KT120. For both the Rogue Apollos and VAC 200iQ’s I’ve rolled tubes in, Gold Lion KT88 vs. KT120 is just a matter of preference. KT88’s are more refined, slightly sweeter midrange; KT120 have more fullness and bass presence, overall a very voluptuous and "powerful" sound -- this is why Rogue switched to them (also they suffered through many faulty batches of Electro Harmonix KT88 in the 2000s). But the KT120’s upper mids can be a little rougher and drier in comparison. I didn’t even notice that for the first few years, but over time it wore on me a bit. That’s why I switched to Gold Lion KT88 in the Apollos, which aren’t naturally sweet like the VAC amps.

KT150 is a very different tube; they were safe to run in 200iQs but sounded awful. That’s where I buy the argument that an amp needs to designed *specifically* for that tube. But KT120 vs. KT88? Nah, they’re close enough for many models with adjustable bias and auto bias schemes. Roll and have fun, if you’ve got ’em.

Currently running Mullard KT88 in my 200iQ’s, and these tubes are excellent as well -- they will definitely sound great in the Rogue amps. But the KT120 are really excellent in those VACs. The VAC’s midrange covers for the KT120. I hope production files in again.