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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 1 response by ghasley

ghasley
2,437 posts
 
IIRC there were a series of modifications required for the initial version of the VSI75 to properly handle KT150's. You should call ARC, have them check your serial number to ensure you are in possession of a properly spec'd VSI75. If it shipped originally with KT120's, you probably have an original version.

1800 hours on a quad of kt150's is no big deal but rest assured, the VSI75 should not sound the way you describe. Your VSI75 may need a trip to Minnesota but you will be glad you did. At the same time its never a bad idea to have a new tube set fitted.
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