KT150 to KT120..or KT120 to KT150

@daveyf 

if I learned anything more since this thread, its back to what a few mentors have shared with me, it's about the designer and the circuit...

If the amp was optimized to sound a particular way with a certain input or output tube (in this case), merely changing output tubes up the chain might not necessarily improve the sound.  

Also, I've been varying bias on my own KT150s between 40,45,50,55 ma in my mono amps.  While I could not get the KT120s to sound quite right in my particular amps, trying different bias has changed the sound, tone, sound stage and character of the presentation to keep it interesting and enjoyable to listen to. Back down to 40ma now and a tad more laid back, less forward, which I tend to prefer in my particular system and speakers.

@decooney  I do agree that the design of the circuit is a major factor in whether the KT150 or KT120 is going to be preferred. The amp designer in question is absolutely convinced that the KT120 is superior to the KT150 in SQ. i think while this might(?) apply to his particular amp circuit, it certainly is not a universal. ( even though this guy would like everyone to believe that it is!) 

@daveyf 
Good to know.  I have some friends with CJ older amps that swear the KT120 sounds better in THEIR particular amps, i.e. not designed to run KT150s.  I believe what your designer friend is saying maybe for that amp circuit. Fast forward, now CJ has amps designed for KT150s, 2x the cost, go figure.  Optimized to run KT150s.  All new amps, new circuits... 

In reverse, my current mono tube amps are designed to run KT150s natively, but I thought I'd be smart and try KT120s at first hoping they'd sound more like EL34s did in my old amps.  Amp designer said I could try KT120s and see.  Nope, no-go. They sounded more dry, less musical, kinda plain, some call it analytical, less musical - and lost some air up top. Just not engaging in these amps.   Yanked the KT120s and replaced with matched pairs of KT150s, and wallah (for these amps). More top air, more bottom, and a mild mannered midrange.  So, I then ended up back to rotating the former pairs of vintage input/drivers in my amps to bring out more midrange, it was better. Then, went to better coupling caps, and even better midrange with KT150s, now I was "there".  When I tried these same changes with KT120s, just did not help all that much. With KT150s, worked really well.  

And, opposite in my buddies integrated amp that was originally designed and voiced for KT88s.  The new KT150s were not cutting it in HIS amps.  Less musical. Yanked out KT150s after 6 months, then went with PSVANE KT88s, and wow they worked so much better in HIS amp.  

Finding the right tube(s) inpu/driver/output for the circuit seems to be the ticket for me.  I don't know if I believe in the whole "universal" amp theory and just changing the driver.. may work with some combos, not all.  My designer knew KT150s worked better in my amps circuit.  

What amuses me a little with the amp designer in question, is that not only does he preach that the KT150 is worse than the KT120, with no proviso that it could be in his circuit, but not others; but also the fact that he did not believe in tube rolling or NOS tubes until recently and was known for telling everyone he was in contact with about this. ( wherein his new marketing manager must have told him that this could be another profit source, and apparently magically changed his mind!). We really do have a lot of 'interesting' fellows in this hobby on the manufacturing side, IMHO.

I used both in my previous amp, a push pull 85wpc, and my current amp, a single ended SEP Class A...in both cases I also preferred the 120s as they seem relatively clean and clear sounding, where the 150s just sound forced somehow. Now I use neither as Gold Lion KT77s and 88s and Tung Sol 7581s all sound better in the SEP amp than 120s or 150s...currently running an old unused pair of Sovtek KT88s...they're great!