@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.