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?

@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).
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.


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?


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.

Good point, it’s only 4 tubes :-) 

I think I should try the Gold Lion KT-88’s since I am already familiar with the T-S sound of KT-120’s. Granted, KT-150 has more power and might be slightly more extended overall. 

@saki70 : the 120s may seem to make the mids thin, but not if you have a preAmp that can compensate for that. 
E.g. I have the PrimaLuna Dialogue HP integrated tube amp, and swapped the stock 12AU7s for NOS Brimars and Mullard’s. The Mullards in particular lived up to their rep with midrange beauty with the KT88s.

In fact, it was too much midrange. So I think the Mullards with KT120s would then have better balance in all highs, mids, lows while retaining the glory of each. Just my experience.