Questions on KT150 vs KT120

@paulcreed for whatever reason and comparing to my local audio shop friend’s same amps, driving back and forth several times to compare, I found his same amps with KT150s were notably smoother and more musical than mine brand new when I got them. With my brand new KT150s I noticed slight changes at 100-150hrs, and he kept telling me "nope" try again at 250-300hrs and see if they smooth out and sound better, and they did at 300hrs.  I’ve sinced boxed them up and went back to KT120s again, yet I do like both sets and the KT150s can sound good with the right front end tubes in the preamp and with the amps too. It can be a balancing act.  

FYI, what I did finally realize with others, some amplifiers are NOT actually designed to run KT150s properly, with insufficient plate voltage.  My amps are designed to run KT50s, KT120s, and I cannot run KT88s as the plate voltage is too high and will burn them up prematurely. 

Ask your amp mfg what the plate voltage is on the amps to see if they are actually optimized to run KT150s or not.  Many tend to ignore this because they will plug in and play, maybe not properly though, fwiw, worth asking your amp designers.  

Great advice @decooney 

I run KT150’s in my mono block amps, which were designed with that tube in mind. The KT150’s sound great, but you have to balance them with the preamp and the driver tubes. They are very sensitive as to what driver works best, I am still experimenting with drivers even after a few years. So far, the KT150’s seem to prefer a robust tube, the RCA clear tops are good, as are the NOS Telefunken’s, The newer reissue tubes can work, but i find them to have less life when driving the KT150’s.

Peter have not tried 150’s on VAC, I found the 150’s tipped up on the Audio Hungary so it may ever be worse on the VAC.
 

decooney I contacted Audio Hungary and a75i class A is shipped with both 120’s and 150’s depending on customers preference. Why did you end up preferring the 120’s, at this point I’m in the same camp. They do have big bass but at times it’s a little too much. Also agree on front end tubes play a big part on tube I found was Mullard work the best, Telefunkens,RCA, Mazda and others did not play as well.

I prefer the 150s over 120s in my Quicksilver V4 monos 

"@paulcreed Why did you end up preferring the 120’s, at this point I’m in the same camp."

I may have an unhelpful answer, in my case.  First, has to do with my custom built speakers.  Secondly, old habit, familiarity, maybe. I'm an old EL34 fan too.  I like KT150 & KT120s for different reasons. I end up tuning the system to be a tad more rolled off on the top end, slightly, always focusing listening pleasure more on midrange frequencies.  It's partly a preference kind of thing as I've aged. I cringe hearing uber-detailed systems. Piercing horn speakers hurt my ears, for example. I grew up listening, building, electrostatic AMT hybrid speakers.   

My speakers generate a lot of sound output above 1200hz in my particualar listening room. Revealing.  Some of my more neutral front-end and power tube combinations can be too much of a good thing to my ears.  I might have some hearing sensitivity in upper midrange and high frequency ranges, so I tend to lean towards a smooth dac, smooth interconnects, tubes (input our output) that tend to recess high frequency, slightly. I have the same challenge and preference with my solid state amplifiers. Got that dialed now too, to my preference.   If I decide I want a tad more detail and depth, I pop in the KT150s, but i've been stuck back on KT120s again past few years. Bought three quad pairs of them as spares. I may try the KT150s again over the holidays. When I replaced the main power capacitors and coupling capacitors with really nice upgraded units, even more detail came through - back to the balancing act.  As my buddy says, "its all preference at the end of the day for each individual".  I tend to agree.   Its all good. :)