KT150 to KT120..or KT120 to KT150

Check with the amp designer if you can before buying KT150s, fwiw.


Test-1:
In reverse, just for grins I tried KT120s in my monoblock amps specifically designed and optimized to run KT150s. Day-1 the amps were designed to run KT150s, big transformers, increase in plate voltage, etc.  

The KT120s were a bit more dry sounding, not as musical, bass was less full in THESE particular amps. Re-installed the KT150s and wallah, nice! Was it too much plate voltage, too much bias, or a less optimum circuit for the KT120s, possibly.

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Test-2:
A buddy had tried KT150s in his nice upgraded integrated amp originally designed to run KT88s. He’d been complaining it did not sound right to him. We popped in some new production PAVANE CV-181s, wallah, nice! Was it due to an insufficient amount of plate voltage, smaller transformers, an amp never designed to run KT150s, possibly.

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If unsure, defer to the amp designer about "what output tubes" to run.

Hi Dave

I have had a KT150 amp on loan to compare with my KT120 amp. 
I prefer the KT120. 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 have to add that the KT120 takes hundreds of hours to settle in and I think that’s why people dismiss them to soon. 
All the best. 

I ask the question in my OP because there is a well known and respected (justifiably so, IMO) amp designer, who believes that the KT120 is superior sounding to the KT150. This has NOT been my experience, but i am questioning if others have experienced the same results as the designer claims.

Would love to know the amp designer. I have been saying the same thing but everyone seems to be on the KT150 bandwagon. 
Keep us posted if you give it a try. I think you are safe to swap them in your Jadis. Might just need rebiasing?

@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.