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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
decooney
2,358 posts
 

@mapman  ... I use a ARC tube pre-amp with Class D amps in my system. That combo adds just a subtle touch of warmth (compared to similar setup I have with no tubes) and with no softening or rounding of the sound. Perfect for me!

 

Interesting. I posed this idea of pairing a really good 6SN7 triode tube preamp with good Class D amplifiers over on the ASR forum. They just about took my head completely off over there with their regular "perfect measurement" and "distortion" debates, before "listening".  I ran my tube preamp and SS amps before going back to all tube monos. I bet your setup sounds very nice.  Good for you mapman  :)  

rossb OP
193 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.

mapman's avatar
mapman
22,979 posts
 

@decooney 

The technical key to integrating a tube preamp (high output impedance typically) with any SS amp including Class D is the impedance matching.  Some basic  class D amps have only 10 kohm input impedance, which is not a good match.  40kohm input impedance on the amp or higher.  Input impedance of my Bel Canto ref1000m amps is 100kohm.  Bingo!  Prior ref1000 model (lower cost) was only 10kohm, same as input impedance of the Icepower Class d module used. 

decooney
2,358 posts
 

@mapman my tube preamp is 1 volt output line stage = 20 dB, and input impedance on my Cary SA-200.2 solid state amplifier was 22K Ohms RCA-unbalanced. This got very LOUD at 10-11’oclock, starting at 7’oclock on the volume control. Had plenty of gain there. Agree, maybe 10k ohms is not enough on the amp side depending on the preamp. Will keep that in mind if I test some class D amps later. Thx.

mapman's avatar
mapman
22,979 posts
 

The voltage would have more to do with the gain whereas the impedance more to do with distortion levels and dynamics. 

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