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.
I’m guessing the KT120’s just sound more relaxed. I have a ARC REF160 and I operate it in the triode mode… so little difference in impact (half the wattage) but ever so slightly warmer character. I’m guessing reaching for more power is coming at the expense of natural character. Stick with the KT120s if they sound better. Tubes with 1,500 hours should sound as good as they get.
You might want to call ARC and see what their thoughts are about KT120s vs KT150s.  It is possible that the power supply is marginal for the 150s.
Funny…today I’ve been comparing the KT120 to KT150 tubes in Octave MRE220s.  Overall I heard the exact same thing you described other than the bleached sound on the 150s.  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.
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.
In my Prima Luna Dialogue Integrated I used KT 120s for about a year before installing  KT 150s. I had the exact opposite experience you described with your tubes. The KT 120s had great detail and dynamics but sounded sterile and analytical to me. The KT 150s provided better dynamics and much more realistic tone and musicality. This is just another example of how everybody’s ears and equipment are different. Like others have said, just go with whatever pleases you the most.
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.
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.
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.
A tad spendy , however I would recommend sourcing a matched ( if possible  ) quad of pre 1980’s  production Reflector Corps 6h30-DR ‘s 
they will take your VSi75 up a notch and should blend in very well with KT150’s 
Beware ... there are many unscrupulous individuals on line passing off new production as NOS DR’s ... this should help
@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.

@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. 





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

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.

The differences you describe do not sound unheard of in general when switching tubes. I stopped pursuing tube amps when I realized I was rolling tubes to get more towards a sound more inherent to SS: clean,,crisp and dynamic, not soft and warm. I would consult with the vendor for input in your particular case. Audio Research is one line I find I could live with. 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!

@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  :)  


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.


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. 

@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.

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

There was a discussion a few years ago at psaudio with folks and matching different preamps, output and input impedance matching, other considerations in addition to voltage. This is what i was referring to as well.

No issue with lower oi or ii on my tube preamp to SS, not quite 1/10th or 10x but still ok.

The KT120s or KT150 based amps no issue being driven, good matching helps for sure. Nothing today the KT120s still sounding nice with more burn-in time :)