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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 elliottbnewcombjr

elliottbnewcombjr
5,236 posts
 
Just to clarify, are you adjusting the bias after you change the tube types each time?

My Cayin came with used 6550’s. I switched to new KT88’s, preferred them right away and after broken in. Wonderful sounding tube amp.

I did not change the bias. I just now had the bias adjusted properly by VAS. While waiting, I used my tube amps with EL34’s, got out test tracks, refined my speakers L-Pads this way and that, love the EL-34’s sound as much as the Cayin.

Now, Cayin ’properly’ biased, I like the sound a bit less.

Using the Cayin, I’m going to re-refine my speaker’s L-Pads with a newly purchased sound meter, test tracks, and familiar music. Then? If not thrilled, unplugged, I can note where Steve put them, and I can put them back where I had them, right in the middle of the adjuster’s range!

If the sound changes, I will be a big proponent of adjusting bias for sound rather than tube life.
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