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?

Out of curiosity I swopped in a matched set of Genelex Gold Lion KT88s replacing the T-S KT120s.  Perhaps a hint more of sweetness, but definitely a loss of authority, that I liked so much to begin with.  Went back to the 120s soon after.
I believe that if an amp can work well with 6550's and can also accommodate KT120's...then the KT120's would be the superior tube in that amp. However, the big question is whether the amp can actually support KT120's without damage...as most that were designed for 6550's cannot. I went from an amp with 6550's to a mono block with KT150's...very big difference. 
Monoogan ;

I had the same experience when I tried KT 120's in my Prima Luna . 
Pretty good top and bottom frequency output but the all important midrange left me wanting . 

Good luck

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.
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. 
@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. 

OP, I worked with Kevin and Jarrod at Upscale Audio on Matching tubes with my PRimaLuna HP Integrated (Kevin is part owner of PL) 

Through talking to them and reading the Upscale Audio website and watching Kevin’s short videos on many types of tubes , not only KT120s, KT150s, etc, but specifically tubes for ARC amps. 

Kevin has a video specifically for tubes for ARC owners. I don’t recall much of it because I don’t have arc gear, but he talked a lot about current, electricity matching with the electronics to not only get good sound but good tube longevity in ARC gear

-no affiliation-

I think PrimaLuna and Rogue have pushed Audio Research to come up with amps like the VT80. ARC power amps normally are seriously fussy about tubes which I'm sure was no end of frustration for Upscale. They've sold enough tubes over time to figure out what will work and what won't.

I'm sure the VT100 scares the crap out of them.

What Audio Research has been doing since like the mid 90s is direct coupling the driver tube (the last small tube) to the output tubes. So when you bias an ARC power amp you're biasing the driver tube which then biases the output tubes. If the driver tube goes weak or fails or is just way off from the tube you swapped out you lose bias and then it's all smoke and burned boards.

No tube fuses, we die like real men!

Meanwhile everybody else is going from EL34s to KT150s with a single credit card swipe, so you get the VT80 or a Rogue or PrimaLuna.

The plus side is the coupling capacitor moves back to between the last gain stage and the driver which I'm guessing helps with the sound they are after. But now there is more direct interplay between the driver tube and outputs which is why ARC power amps are so darn fussy about tubes.

If the driver tube goes weak or fails or is just way off from the tube you swapped out you lose bias and then it's all smoke and burned boards.
@monoogan This statement is false. If the driver tube dies, the power tubes are held in cutoff and are not damaged. If the driver tube is weak, you may have troubles getting enough bias current on the power tubes. So in this *particular* regard, the amp is fail safe.
lalitk, I would suggest checking with ARC first before rolling in the KT120's and particularly the KT150's. I would be surprised if they told you that your VT-80 was compatible with these tubes. Best to get ARC's blessing before you damage your amp.
According to ARC, both the VT-80 and VT-80SE can run KT150s. Both amps ship with KT120s now. And all four tubes are fused.

Bad assumption on my part. I'm guessing if the driver fails the output grids get pulled down by the negative supply cutting off the tubes. instead of just losing bias.

Sorry for the misinformation.
Tommuch, I see that the VT80 can apparently run with KT150’s. What is odd is that the newer VT80SE includes the KT150’s, and so I’m not sure what is the difference between the 80 and the 80 SE??

The VT80 was originally designed with the KT120, the predecessor to the KT150, but these two tubes are electrically interchangeable and the demand for delivering the VT80 with the KT150 could not be ignored. So ARC decided to introduce VT-80 with KT-150 tubes and dubbed the amp as SE version.
Lalitk, I believe the KT150 requires a higher draw, as such it can damage the transformer if that is not up to the task. Presumably, the transformer in the VT80 can handle the draw like in the VT80SE?
Very old thread, but I thought I’d add something I saw on the whatsbest forum site about the KT120s.
The poster (who has a 100k+ system) noticed that the KT120 were not "particularly transparent."
I would have to agree with that, having had them for months in my Audio Research Vsi60. The "sound" can impress you, but the actual transparency? It’s not, in my book, top of the world. And I say this as someone who once had a Goldmund Mimesis 9 (back in 1990), THE most transparent amp (although solid state) I have EVER had. NOTHING escaped its resolution. Terrific transparency, too. (It cost $11k back in 1990, so it would cost $23,000 (used an inflation calculator) in today’s dollars. I’ve heard some of today’s "super amps." The Goldmund only loses out in the bass frequencies, and some microdynamic nuances and delicacy.

Back to the KT 120s. The midrange is more "ice milk" than "ice cream." It CAN fool you because the resolution is good, but on stellar RCAs, Mercury Living Presence, Decca (all classical) of the ’50s and ’60s, the midrange does not have the lushness (and it’s there on the records). I’ve had Jadis, Goldmund, VTLs, VAC amplifiers, and Jadis, Convergent, Audio Research Sp-11, Rowland Coherence 1 preamplifiers (again, in the ’80s, ’90s, and into the new millenium) and none of them diluted the richness of tone that was on the album - if it was there to begin with. I think the (KT-120) midbass had punch and kick, BUT NOT the right amount of WEIGHT/BODY to make instruments solidify. And it is the midbass that is MAINLY responsible for the solidity of instrumental images, while the upper bass and lower midrange give the sound a kind of....well, lets use the words of Robert E. Greene, a former colleague at The Absolute Sound: " the power range" which makes music sound "BIG" when it swells up.

The KT-120s seem to have more ’push’ at the top and the bottom, but, after wondering if it was something else in my system (it wasn’t): I bought, over the last 2 years, top of the line Shunyata power cords (Sigmas), the Vsi60 (last year), Hana ML cartridge (last year), speakers (Nola), interconnects (two weeks ago) (jumped from Nordost Frey 2 to Nordost Tyr 2) and Shunyata Anaconda ZiTron speaker cables, I resigned myself to it being the sound of the KT-120s. Again, no component is perfect, so this is not meant to slam the tubes. I’m merely pointing out what I see? (hear?) as their inherent traits, so someone reading this can compares notes among all the posters here, many with equipment different than mine, and so the tubes will vary in other systems. Slightly. (The transparency factor, though? NO. That is a part of the tube itself.) But my comment is no more or less authoritative than anyone else’s. (It took me a while to come to the conclusion about its traits because I was changing equipment pretty rapidly over the last 7 months). The only things leftover from the previous system are the PS Audio Power Plant (and that never made anything sound thin), another (older) pair of Nola speakers, Shunyata Cobra ZiTron speaker cable and an Arcam FMJ23 CD player and that’s it. So, after hearing the tubes remain the same in terms of their virtues/flaws? It’s NOT my system!

If you have to "compensate" for sweetness - the kind that is in the music itself - than you’re adding a coloration to your system by getting something euphonic. No two ways about that. A truly neutral component will reveal "sweetness" if it’s already on the record. The component shouldn’t be doing it, because that’s a deviation from normal. And there’s no shame in building a system that way, but don’t kid yourself that your system is neutral if you do that. As soon as you upgrade/get some new component that is TRULY neutral, you’ll hear that euphonic component easily.

The KT-120s seem a bit "lightweight" in the midrange and also lacking in transparency. I can live with it, I guess. But I’m not happy about it. But glad I came here and saw that two posters noticed that "lightweight midrange" also. And I am comfortable with my experience as a listener of music through High End components over the past 40 years, since I bought close to state-of-the-art components (it speeded up my EDUCATION in High End components, for sure!), nor my time as a writer and editor in the high end industry.

I’m only writing this much print so someone else has an idea of what they might notice if they get these tubes. I like ’em, but not as much as some others. Oh, and the poster on the WhatsBest forum posited that the KT120s were not as transparent ( in the Vsi60) as the 6550 tubes, which, as it turns out, were the tubes installed in the amp when my old buddy, Wayne Garcia, wrote a review of the Vsi60 in The Absolute Sound. So, it would seem that with 6550s, you get the ultimate transparency; with the KT-120s, more slam and other qualities. Take your pick.