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KT120 back to 6550

Seen a ton of threads praising the KT120 over the 6550 or KT88. I get the new tube's durability and design but given that your amp was originally designed around one of the older tubes has anybody gone back.

I recently purchased and ARC Ref 110 that came with a set of KT120s. The output tubes test and match well so they don't seem to be at the end of their life (same for the signal tubes). 

This was a change from a VT100 equipped with 6550s. The VT100 had good midrange but seemed a little lean otherwise. Percussion was fantastic and voices just right. Just deep bass a little thin. The REF 110 definitely fills in the deep bass but seem a little congested in the midrange. Both top and bottom are an improvement.

Both these amps were sandwiched between an LS25Mk1 and a pair of JBL 4430s. Could be a synergy thing, these speakers work very well with high power solid state (Adcom GFA 5802). I've been trying to work up a good tube amp for these but so far I'm not feeling the love.

So the question is could it just be this system would work better with 6550s instead of KT120s?

Thanks
monoogan
monoogan OP
55 posts
 
Thanks every one. This is actually pretty helpful.

So for Saki and Twoleft it sounds like the tradeoff is maybe midrange definition versus extension at both ends? I think that's what I'm hearing.

As a side note towards the end of Ref 110 production Audio Research started shipping the amps with KT120s instead of the 6550s. As far as I know there was no other change to circuit or transformers. I assume they liked the way they sounded in the amp.

Of course they may have just been running out of good 6550s.
salectric
755 posts
 
I recently compared Genalex KT-88 and Tung Sol KT-120 in my Emotive Audio Vita amps.  I have been using a set of KT-120s for the last 5 years but recently one of them was making some suspicious noises, so I swapped in my well-broken-in set of KT-88s while I waited for a new set of KT-120s to arrive.  Bottom line: in my amps the 88s lack the bass weight, power and authority of the 120s.  They don’t sound bad but the 120s sound a lot better. 
twoleftears's avatar
twoleftears
5,318 posts
 
@salectric that was pretty much my experience too.  In a tube integrated, you can take care of the sweetness factor with the pre-amp tube(s).
lalitk's avatar
lalitk
5,557 posts
 
I have been on fence with tube rolling in my ARC VT-80. With its versatility with different types of output tubes, including 6550, KT88, KT90, KT120 or KT150, I just can’t decide which one’s to try.

One thing worth pointing out, I am using VT-80 for my mid’s and high’s only in a bi-amp setup. The LF’s are being fed through ATI 543NC Class D amp. As is, everything sounds pretty amazing. I am using LS28 pre and speakers are B&W 800D2’s.

@saki70,
@twoleftears,

After reading your comments, I am now anxious to try Genelex Gold Lion KT-88’s over T-S KT150’s. What do you guys think?

http://audioresearch.com/en-us/products/power-amplifiers/vt80


monoogan OP
55 posts
 
That VT-80 is so user friendly (a really good direction for ARC) I could see buying both and rolling between them. It's only 4 tubes.

You're in a really good place. It sounds like you really don't need the bass extension of the KT120. You just need a tube with good midrange and top end performance.

The only downside I see is that in a 75 wpc amp the KT120s would last a lot longer. Again it's only 4 tubes.
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