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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

Showing 5 responses by rossb

rossb OP
183 posts
 
Thanks, they seem reasonably priced so I may give them a try.
rossb OP
183 posts
 

@jetter 

I ended up selling the amp. Both the KT150s and 120s sounded good, just different. I preferred the KT120s, but realised I the heat put out by the amp in my small room was a bit too much, and I decided to go back to the simplicity of a solid state power amp.

I bought a Pass XA25 and an EAR 868 preamp and I am now happy with this combination.

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.
rossb OP
183 posts
 
Thanks, it is interesting that there are certainly different perspectives on the KT150s, with the overwhelming majority preferring them over the KT120s. This includes the previous owner of my current amp, which suggests that it is a matter of preference rather than an issue with the amp.
My understanding is that the only differences between the current VSi75SE and the previous VSi75 are that the pcb has been lowered to allow for the height of the KT150s, and it now ships with KT150s as stock.  However, probably I should probably check with ARC that there is no other issue to be aware of when changing over.
rossb OP
183 posts
 
@elliottbnewcombjr I have been adjusting the bias, but primarily to maintain it at the level recommended by ARC. I am not all that comfortable changing bias too far from the recommended levels. The previous owner did recommend biasing the KT150s a little lower for sound quality reasons, which I have tried but it did not make much difference.


@tsushima1 I actually do have a pair of the Reflektor 6H30-DRs that I bought years ago for some reason (and now extremely hard to find), as well as the EH Gold 6h30Pi and the stock Sovtek tubes. I find the EH and Reflektor tubes sound quite similar, and both are an improvement on the Sovtek tubes.
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