KT150 to KT120..or KT120 to KT150

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@ozzy for what its worth to you, while my purpose built mono block tube amps were designed specifically to run KT150s, nice, clear, pure, I am back to running KT120s again, now. Kinda missed the more textured (older) midrange, so I’ve alternated a few times. 150s are extra detailed on top, slightly larger and deeper sound stage if bias is turned up, but is the grass greener, maybe. The 150s can be so linear sometimes on my system, i miss that littlest of the midrange bulge reminiscent of EL34s, KT77s and maybe even KT88s. Popped the 120s back in, and there it is. Back and forth enough times now to know for sure in my setup.

Been playing again with bias on both 120s and 150s past six months. On my amps both tubes at 60ma are brighter. My ears are sensitive to bright sound. After 100-200hrs+ on both KT150s or KT120s, I decided on backing the bias down to 40-45ma per tube for low volume listening. Normally at 50ma. Just for grins, now sitting at 40ma with KT120s, lowest I’ll go. #1) saves the tubes a little longer, #2) takes a little of top edge off with bad recordings, and sounds nice. No fatigue at all. Save the $, keep enjoying those KT120s! I have zero desire to put the KT150s back in, and not for a good while. Like them enough that I had a spare quad set of KT120s stored too.

Give the KT 170’s a whirl.

I have 88’s 120’s 150’s and 170’s and they all have their positive attributes.

@jerryg123 are you running your KT170s in amps with proper transformers and plate voltage ability to take advantage of the extra 15 watts capability these tubes have over the KT150s? What "attributes" are you hearing differently from KT150s - if you can share more, thanks.

@decooney Yes Qualiton X200 by Audio Hungary.

Each of these tubes has their own sound characteristics. 

KT 120 is a great all around tube for all types of music. 

KT170 and 150 I like for Jazz, Rock, Blues, really like the sound stage and upper end.

KT88 I really enjoy with  classical it is fuller sounding than the 120's

Thanks @jerryg123 that helps understand more.Similar findings excluding 170s.  I've owned and listened to a few amps running KT150s with smaller transformers and lower plate voltage, and they did not sound very good to me. KT88s were nicer in amps designed for the smaller and lower power tubes.  

While my amps were designed for KT150s, and they sound unique vs. other smaller amps I've run KT150s in before, I have yet to hear a robust amp specifically designed for KT170s to get the most out of them.

Doing some research, came across a designer noting this about them:

"The Tung-Sol KT170 has a plate dissipation of 85 watts! ... making it the most powerful tube in the Tung-Sol family. A pair of these tubes in push-pull configuration can deliver power levels of 190 or more watts. When used at the parameters found in existing 6550/KT88/KT90 circuits, the Tung-Sol KT170 is impervious to overload, delivering peak power with extreme reliability and long tube life. However, taking advantage of the higher current handling capacity of these tubes, a very unique and super powerful and stable amp can be designed using the Tung-Sol KT-170." unquote. 

Hopefully other future manufacturers will come out with their own competitive versions of KT150s and KT170s to try. Will be on the lookout.