KT150 to KT120..or KT120 to KT150

@dsholl1
There is just something about the KT150 I find too Hi-Fi sounding and cerebral.I wouldn’t characterise the Kt120 as dry at all. I think it’s a blend of a EL34 and a good 6L6. It just sounds a bit warmer tonally to me than the KY150 which does all the Hi-Fi tricks very well.
I’m sure it comes down more to implementation than the actual tube.

I’m a big EL34 amp fan myself in the right implementation. Was hopeful the KT120 would be the answer, for the exact same reasons you’ve outlined here - however MY particular amps just did okay with them. The amp designer told me to try KT120s knowing I was a big EL34 fan, but we eventually agreed the KT150 was a better synergy in my case.

New, yes, at first the KT150s had more of a "hi fi sound", yet at 200-300 hours run-in time on the KT150s output tubes + good vintage input/driver (12xx7), was a game changer. More of an EL34 sound now with a tad more air on top and definitely more weight on the bottom-end, less rounded off on the bottom end now - with THESE amps.

>> the real trick I found was getting the right vintage input tubes paired with KT150 output tubes to achieve the right balance, tone, texture <<

Pairing the good input tubes and the right silver-gold coupling caps in my amps truly made the KT150s a real pleasure to listen to now. Not until then. I’ve also heard the same amps with stock caps and stock input tubes with 500+ hours on the KT150s and they smoothed waaaaay out. KT150s do smooth down with longer run-in time.

Interesting, 
I have the Bob Carver Raven 350 mono blocks that use KT-120 tubes and I have been wonderin' if the KT 150 tubes would be better.
Bob Carver told me they would work fine with the amps but he was reluctant to say if they would be an improvement.

ozzy 

@dsholl1 I don’t want to tell who the amp designer is, but I really like his gear and I respect his opinion ( generally). However, my opinion is that he is mistaken when it comes to the differences. @decooney What you posted about the quality of the driver tube is exactly my experience as well. Right now,I am using stock EH driver tubes in my amp, but I have rolled in a variety of NOS vintage tubes...and the difference is not slight. The KT150’s really seem to light up bass on what is upstream. Since the amp designer I am talking about doesn’t believe in NOS vintage, I would strongly suspect that this is where he is basing his finding on..a mismatch between th driver tube and the KT150, leading to his favoring the KT120.( in his particular amp). Nonetheless, @dsholl1 in what context( amp..driver tube etc) did you find the KT120’s better?

@daveyf  I listened to the Jadis DA88smkii compared to the i50. I much preferred the 88. The 88 was considerably more expensive too. It just has such a musical sound with massive amounts of density. The i50 sounded very clean and Hi-Fi. The i50 has better bass but lacked density and warmth if that makes sense?

 I would’ve bought the 88 if it wasn’t so damn expensive where I come from. 

@dsholl1  Unfortunately, I don't believe that you can lay the differences that you heard between these two designs, the i50 and DA88sMk2, at the feet of the power tubes utilized by these amps. There are signifiant differences in amp abilities as you go up the Jadis line, just like most other manufacturers. There is a reason for the price differentials.