Would love to hear head to head comparison of the two tubes. I've not heard the 150's but am quite happy with the 120's for now though a few weeks ago I almost pulled the trigger on a KT150 based Allnic and then decided to give the Carver a try and have been super impressed.
To MorganC, Fwiw, You might like Kt150s, hard to say. Maybe not. The amps to run them with is critical imo. Not all amps being created equal. Running them in Carvers would be a good try, if interested in comparing. With KT150s, It comes to preference and what you like to hear on
your speakers. Are you more in to detail and speed -or- a tad less detail, more full body, midrange, texture sound?. KT150s in my amps have great highs and bass, and cover the full spectrum, midrange is smooth, "neutral" midrange is how I hear it, not pronounced in my system. Known as a "balanced" output tube top to bottom in good amp designs. With sensitive hearing, some times i prefer a tad more rolled off high freq to let more midrange forward sound through - depending on the recordings I'm listening to. My last amp had (12) EL34s, KT88s. Too many tubes to make power. This is what drove me to try KT150 based amps, fewer tubes, less heat and fewer tubes to maintain. All my former tube amps had EL34s, KT88s. Tried some lower power triode wired 10w (lower plate voltage) amps with KT150s, did not care for that with
my 93db speakers. Several colleagues are SET fans with low watt 300B, 2A3, 45 amps. Helped them build some horns speakers. Low power SET is not ideal for my own 93db sens AMT based speakers I build. Down the road I may move to higher power SET or PSE 211/845 mono amps, maybe. Still on this KT120, KT150 trip for a while. May never leave it.
The KT150 based mono amps are a nice stepping stone, and/or stopping point, now having heard and owned them a few years. Cap tweaks help here too. I'll likely always have a pair of KT150 mono amps where I can run KT120s, KT150s with amps delivering the correct plate voltage for these tubes. This is key, as I've learned and heard it first hand. Best to ensure the amps, transformers, circuit, plate voltage is
designed to run and operate KT120s, KT150s properly, in their
sweet spot. While KT150s are amazingly open, smooth, textured, powerful, sometimes I yearn for the grass-is-greener older EL34 tube sound too, not overly so. PSVANE KT88s can get close too. I came from 35+ years of former [less detailed] sweeter sounding MOSFET dual mono SS amps. Amps with properly executed KT150 usage has been a nice segway 1/2 way between TUBE and SS amp worlds. Fine tuning the smaller input/driver tubes paired with KT150 based amps helps get closer to the desired sound. I'm re-visiting KT120 output tubes again to see which direction the needle leans forward or back to in terms of tone, texture, velvet sound in my mono amps.
Spatial: Curious to know how compatible properly designed kt120, kt150 based amps are with Spatial X3s, X5s. Or, will SET amps be a future target or not with them, not sure. Now seeing a wide range of folks with higher and lower power tube and solid state amps driving these Spatial speakers, which is a good sign of wide compatibility. Very cool! Feedback here helps, a lot. May try some X5s down the road...