KT150 to KT120..or KT120 to KT150

My Qualiton X200 came with KT120's and sound very nice.

Decided to go KT170 and guess what the 120's are sitting in a box. I also have KT150's but prefer the 170.

KT88's I bought cheap are never going back in.

I've tried GL KT88 (stock tubes), KT120, and KT150 in my VAC Signature 200iQ monos. The KT150 were the most recent acquisition - and they were terrible. Extremely powerful low frequencies and gobs of slam - which honestly was fun - but it completely swamped the rest of the spectrum and was a very unbalanced sound. 

The KT120 - I like them a lot. They're not as polished and refined as the GL KT88 in this amp, and so I think most will prefer the 88's here, but I also think the KT120 are at least competitive here (unlike KT150). I'd gotten so used to the KT120 sound over 10 years of use in Rogue Apollos, that I think it's been hard to wean myself off them in the VACs. They feel like home base lol. I'm currently, maybe, transitioning from KT120 to GL KT88 in my VACs.

I'm sure the KT150 performance is very amp dependent, and I believe they'd be fantastic in an amp properly designed for them - but not in the VACs.

[Initial burn-in matters on these tubes].  I underestimated this on the first try.  

KT150s:

AFTER 200hrs of play, I now consider the KT150 as a sort of a  "do no harm" output tube in my mono block amplifiers. Brand new, I did not like them very much.  Way better after 200-300hrs.  I had been comparing back and forth with my audio local dealer too in the same amps. Top end smoothed more at 300hrs, wearing in.   

KT120s:

In similar fashion, I did not like them brand new in the first 24-48hrs.  In fact, I passed over them too quickly jumping to KT150s prematurely. Missing velvety texture, tone, gave them another shot and super glad I did. At 72hrs+ started showing some hope with a tad rolled off highs/lows. Nice though. Easy listening.    

[Interconnect type pairing between these two output tubes] can help fine tone too.

If you trying both of these types of OTs, and you are looking for fine tuning of tone and prefer a good balance half way between ultra clear and rich tones, here are two combinations I landed on with interconnects after a few hundred hours of comparative testing fwiw. I don’t run silver over copper interconnects any more except with EL34 output tubes. This is only referring to KT120 vs. KT150s in neutral sounding amplifiers. Your results may vary of course.  Happy listening. 

KT150 / OFC ICs:

• Paired with OFC Oxygen Free Copper interconnects

KT120 / OCC ICs:

• Paired with OCC Ohno Continuous Copper Casting interconnects

@decooney   Intersting point. I tend to agree that ic's can make or break your system, depending on a number of variables. Your findings above do not actually jive with mine, as I am very impressed with a pair of OCC Ohno Continuous ic's with my KT150's. However, I am fairly sure that the design and make has a lot to do with the ultimate results. So, no real generalizations are possible here.