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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
rossb OP
188 posts
 
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.
elliottbnewcombjr
5,721 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.
tsushima1
1,165 posts
 
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

http://www.aca.gr/index/hiend/hiendArticles?row=1988
rossb OP
188 posts
 
@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.
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
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