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Audio Research VSi75 - End of Tube Life? KT120 vs KT150?

I have recently bought a second hand ARC VSi75 amp. It came with both the original KT120s, which have about 250 hours of use, and some after-market KT150s, which have about 1800 hours of use.
With the KT150s, the sound is quite hard, cold sounding, tonally a little bleached, but with great dynamics, extended bass and treble, and much more three dimensional. KT150s are claimed to have a tube life of about 3000 hours, so these should be not much past half their life. There is no distortion or noise.

The KT120s sound warm, tonally rich, colorful and much more musical. But they also don't have the dynamics or frequency extension. Nevertheless I much prefer this sound.

Does it sound like my KT150s are at the end of their life after 1800 hours? This cold, steely, colourless sound does not match what I read about these tubes, but it also doesn't sound like the symptoms of normal tube aging.
A quad of KT150s is reasonably expensive. Is what I am hearing just the normal difference between KT120s and KT150s? If so, it is probably not worth the cost of buying another set of KT150s to find out.
rossb
ghdprentice
6,354 posts
 
I’m guessing the KT120’s just sound more relaxed. I have a ARC REF160 and I operate it in the triode mode… so little difference in impact (half the wattage) but ever so slightly warmer character. I’m guessing reaching for more power is coming at the expense of natural character. Stick with the KT120s if they sound better. Tubes with 1,500 hours should sound as good as they get.
docknow
302 posts
 
You might want to call ARC and see what their thoughts are about KT120s vs KT150s.  It is possible that the power supply is marginal for the 150s.
rshad0000
265 posts
 
Funny…today I’ve been comparing the KT120 to KT150 tubes in Octave MRE220s.  Overall I heard the exact same thing you described other than the bleached sound on the 150s.  I’d call it more neutral sounding.  Definitely not as warm as the 120s.  In my system I have warmer cables so the KT150s appealed to me more and seem to have better synergy. Both are great but different.   Go with the 120s if you like them.  They have a fantastically sweet mid range.
rossb OP
183 posts
 
Thanks, for the comments.

@docknow The VSi75 power supply is fine with KT150s, and in fact the current version now ships with KT150s as stock.

@rshad0000 That is reassuring, although from what I have read the vast majority of people who have done the comparison prefer the KT150s. A comment often made is that the KT150s have a purer midrange, which led me to think that they might be even smoother in the midrange than the KT120s. What I am experiencing is a bright, hard sound, particularly in the midrange, which is quite colourless - not at all what I was expecting. But perhaps you are right, it is just a difference of presentation between the tubes.
tuberist's avatar
tuberist
951 posts
 
In my Prima Luna Dialogue Integrated I used KT 120s for about a year before installing  KT 150s. I had the exact opposite experience you described with your tubes. The KT 120s had great detail and dynamics but sounded sterile and analytical to me. The KT 150s provided better dynamics and much more realistic tone and musicality. This is just another example of how everybody’s ears and equipment are different. Like others have said, just go with whatever pleases you the most.
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