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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
tsushima1
1,165 posts
 
@rossb 

Thats grand , these are also proving popular amongst the ARC crowd

https://www.ebay.co.uk/itm/313453792172?hash=item48fb4d4bac:g:fO0AAOSw-QthRxh5
rossb OP
188 posts
 
Thanks, they seem reasonably priced so I may give them a try.
decooney
2,276 posts
 

@rshad0000 re: KT150s, ..."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..."

 

These comments here and others help to confirm what I’m hearing too between these two power tubes. I’m re-testing KT120s again after running KT150s for the past year. "Neutral" is a good descriptive word for the KT150s in my amps too, I agree. I was yearning for a bit more of the notable midrange flavor from prior amps I had before with EL34s and KT88s. My current mono amps are designed to run only KT120s or KT150s with larger transformers and higher plate voltage.

The mistake I made the first time was not giving KT120s enough burn-in time in brand new amps with all new caps not settled in yet. Retrying KT120s now with better caps and better input/signal tubes fully burned in. This time around the KT120s have some time on them. Now, the midrange is more present, nice tones, not characteristically "neutral" like my KT150s are. The KT120s are hedging closer back towards KT88 sound a little, sounding better the 2nd time around. I think I understand a little more why some people prefer them. I like both for different reasons.  Nice to switch back and forth every once in a while. 

 

 

 

jetter
3,070 posts
 

@rossb 

I would be curious to learn if you eventually spoke with ARC and how they advised you?  Did you stick with the 150s or the 120s?  Thanks

runwell
284 posts
 

Even audio research Ref 75se, originally with  kt150 amplifier, you can choose kt120 for warm and sweet sound.

With the tube amplifier, I always think of downgrade the tube, but never upgrade the tube,as it will depend on the power of the transformer.

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