ARC VS-115 Amp with new KT-120 Tubes

This post is for the benefit of ARC amp owners who are thinking about replacing the Winged C 6550 SED tubes with the new Tung Sol KT-120 tube. Discussions and caveats concerning the technical attributes of the KT-120 tube appear in others posts here on A'gon. I will not repeat, other than to say check with your amp manufacturer before doing a drop in.

The purpose of this post is to advise other ARC amp owners who may be thinking about a KT-120 tube drop-in about my experience with my ARC VS-115. I will update this OP with any additional comments as the new KT-120 tubes break in.

As a threshold matter, I called ARC to confirm that I could drop the KT-120 into my VS-115 amp without the need for a mod. No problem with this amp, however, as further discussed in other posts, some ARC amps will not take the tube without a mod, if at all.

I purchased the KT-120 tubes from Upscale Audio for $45/tube. The price seemed fair compared to other on-line tube vendors. I chose UA because they burn in their tubes for 72 hours before testing and matching and then match based on three variables: bias, transconductance and G2, whatever all that means.

At this point, I have just about 5 hours on the tubes. Biasing was a snap -- no problems yet. So far, there doesn't seems to be any unusual overheating problems with the power transformer - at least that I can detect. All tubes seem to be working -- no apparent arcing. That is a concern because of the possibility of burning resisters.

I understand that the tubes will need about 20 hours to break in. The tubes are already starting to sound better than when originally installed. Initial reactions: better dynamic range than the 6550s; my sense is that there is less distortion on dynamic transients when playing classical music (e.g., Stravinsky, Rites of Spring; Mahler, 1st Symphony - The Titan), possibly because of greater dynamic headroom - but let's face it, who really knows; bass seems a little tighter; overall seems to be better sounding than the 6550s.

That's all for now. If the situation changes, I'll report back. I'm keeping my fingers crossed that none of the new tubes will blow.
Bifwynne...thanks for the update. I believe ARC recommends maintaining the same bias as with the 6550s. Is that correct? Looking forward to your updated commentary as the tubes break in.
Cmalak, bias is still at 65mV -- at least in the VS-115. As an aside, in the VS-115, each tube pair has only one set bias tube, the other being a slave. So, in total, bias is set on only four tubes and checked on the others --in contrast to the VS-110, in which each tube's bias had to be set. I note that in the case of the KT-120s, the bias in the slave tube was much closer to the bias in the set tube than with the 6550s. Who knows, maybe it's all in my head, but that's what it seems to be.

I'm 9 hours in and the sound continues to improve. Maybe I'm just imagining things, but it seems that the sound stage is getting larger and moving back behind the speakers. Again, maybe it's all in my head??? Bass continues to improve -- one helluva lot of kick-slam. I may have been bleeding out of my ears from listening to Linda Ronstadt, Prisoner in Disguise LP. Lotta bass.

Then my wife start chasing me around the dining room table with a rolling pin because she was yelling something about the bass thumping from the stereo making a challah she was baking for the Jewish sabbath fall? How can bread fall when it's baking on an oven grill?? Where's it gonna fall to?? Any Rabbis out there in A'gon-land??
Byfwnne, I just replaced the 6550's in my arc VS-110 with KT-120's from Upscale Audio as well.  I only have 20 hrs on the tubes so far I am impressed. 

I had the unit checked by my dealer with the new tubes and he recommends running them at a lower bias around 60mv, which is a little lower than the arc recommendation.

Listening to classical music gives me a wider soundstage and sounds smoother and more effortless.  Listening to jazz the sound was more 3 dimensional.  I did not experience the sound to move further back but it is wider.  But the again you have a better amp than I do.

When I was using the amp in home theater watching blue ray movies, the transients and slam was immediately more noticeable.

I am looking forward to listening to more music to see how these tubes continue to sound.  Hope your experience is good as well.  

Ps I am not a Rabbi, but I am not sure that extra bass will make the challa to fall.  Good Shabbos!
Sdgron, good shabbos to you as well.

I'm not sure how much more the KT-120 tubes will break in after 20 hours. Just curious about your amp, why did you have your dealer check the VS-110 with the new tubes? Were you concerned about something? Also, any reason he suggested biasing the tubes at 60mV?? Will it result in extended tube life or better sound??

Hifigeek is an ARC repair tech. If he picks up this thread, perhaps he can weigh in on the questions on break in time and recommended bias.

BTW, I used to own the VS-110. I think it's a very nice piece of equipment. I hope you enjoy it. What are you using for a pre?
Bifwynne, I spoke to Audio Research before I purchased the KT-120's to see if it was ok to drop them into the VS-110. They said that they had not tested it with older equipment so I decided to buy them and have my dealer make sure everything was ok. I also took the opportunity to demo a pair of Vandersteen 5A's with my amp as a potential upgrade from my Quattro's.

My dealer recommends 60mv for sound first and some longer tube life. I am using an ARC Ref 3 as my pre.
Just a few quick comments with another brand amp. Chris at Parts Connection was doing his SE+ mod to my Sonic Frontiers Power 2 amp which he originally designed last November. He suggested using the KT120's. I had run 6550's an KT88's and preferred the 88's so the KT120's seemed a reasonable option.

BREAK IN: There was some break in and 20 hours would be a good guess as to when it ended. Early on, they had settled in and (as had I) and have remained consistant since. My experience was a slight but consistant early improvement that I think was equally the tubes and me getting aquainted with their sound. Than again, my amp had been upgraded which made a significant improvement and the new parts were getting burned in. As in most discussions about "sound" user experience will vary.

SOUND: I agree with all the above stated impressions but will highlight two rather dramatic areas. I run Wilson WP7's which can be a difficult load. I have a spare ML 23.5 that I was using while the Power 2 was modded. The 23.5 is known to have the bass balls of a charging rhino. I actually like the bass with the KT 120's every bit as much as the ML. Great deal of fun to have a tube amp that does bass with the best of the ss.

My guess is that we listen to tube stuff because we are drawn to the "more musical" sounds that tubes bring as against the "more accurate" sound of ss. I think in terms of a painting vs a photograph. Some of the golden ears have used the term "organic" which best describes the sound I currently enjoy. The ability to close my eyes and get lost in the experience is what floats my boat. Is it real? Not quite but more real than any other listening experience I've had in 30+ years of trying. At the risk of offending the more conservative folks... For most of us there was an experience where pot and music first came together. I imagine most of us remember it well. The "oh my higher power" moment. Music became alive and we were part of the experience. Real? Who cares because the music was an emotional and personal event that captured our awareness and demanded our attention. The music limited our exposure to the rest of the world. Kind of like being at a concert where the event has a life of its own and we are grateful to ge involved. So in a sense the KT120's plus the amp mods plus the phases of the moon and God knows what else has made it easier to get stoned tot he music without the dope. Many of you will understand emotionally what I have just described. For the rest, go find a friend who has smoked some dope to translate for you.

I really like the KT120. They seem to like and enbrace music. What more is there?

Enjoy the music
I put a set in my VSI60 two days ago and after 12 hrs or so the sound is the best I've heard from ANY Arc amp so far.Tight,deeper bass,wider more defined soundstage.And I thought the response and timbre improved as well.
I think has vastly improved the VSI 60.
I also have VSi60, and I've been curious about trying the new KT tubes on my amp. It sounds like the KT 120 sounds better than 6550 overall. Is there any downside using KT tubes however? I wonder why ARC sticks to the old 6550 tubes instead of KT tubes?
Jylee...they don't stick to the 6550s. All their current production amps are being shipped with the new KT-120s. They barely have enough to meet the needs for their current production amp shipments so they are not yet ready to sell the KT-120s to owners of their amps that have the 6550s in them until they are able to secure an enhanced supply of KT-120s. In any case, if you are interested, call ARC and ask them if they are a drop-in replacement in your VSi60 and if they are at a point where they can now supply them to the broader ARC community.
Bias is set to 65mV. There would be no overheating issues as ARC said that it's ok to use those tubes in that amp. Issues may be with much older product and/or the tube not fitting under the cage. Most output tubes require a break in period and the KT-120 is no exception. Anywhere from 20-50 hrs. should be sufficient. That does not mean let the amp run non-stop for 20-50 hrs! Remember, once the tube is on it's going downhill. Don't waste tube life breaking in the tubes. If people wish to use ARC tubes, (something I recommend), and are in the L.A. area, they can contact me if they wish. I test the tubes from ARC before installing them. I put my number on them so I know when the amp is brought back in, how far the tubes have drifted from new. I provide total RMS power output as well as frequency response. Updates as well.
Post Break-In Update: Probably have 25-30 hours on the KT-120 tubes in my ARC VS-115. So far, no arcs or blow-outs. The sound quality continues to become more mellow and refined -- not rounded or rolled off, just less edgy on the high-end. Imaging is very good, better than with the 6550s. The sound stage is more opened up.

Let's see -- the types of music I've played, vinyl: Carly Simon, No Secrets; James Taylor, JT and Sweet Baby James; George Solti conducting Chicago Symp Orchestra, Beethoven 5th and Tchaikovsy 5th; Stokowski, London Philharmonic, Wagner Selections. CDP: Marilyn Pereaux (sp?), Norah Jones, Jean Yves Thibadeax.

Everything just sounds better than before. As stated, better detail, imaging, soundstage etc. Of particular note is with complex orchestral music, it's much easier to hear the indivudal placement of instruments, less blurring and distortion. With pop-rock, voices are also much clearer, imaging is fantasic.

Bottom line: these tubes sound better in the VS-115 than the old SED Wing C 6550s. My recommendation is to try them when you're ready for your next tube change-out -- but don't forget to check with your amp manufacturer to find out if mods are needed.
cant wait to try them in my rogue m180 monos,I have heard great things !

Hello Bifwynne,

Wonder what speaker you are using with the VS115(KT120).
My mains are Paradigm Signature 8 v2 (beryllium tweeters). They seem to match well with the VS 115.
Just adding this list to help out anyone who may stumble upon this thread.

Credit for this goes to A'gon member Hifigeek1. He posted this on another thread.

04-19-12: Hifigeek1

The following is a list of ARC amps that are authorized to use the KT-120:

VS55, VSi55, VS60, VSi60, VS110, VS115, REF110, VM220. I hope people find this useful as there seems to be a lot of confusion about it.
Looking to get in touch with HifiGeek1 want to get some KT-120's but wanna test them out first.

give me an email at

johngilmour attttttttttttattttt rocketmail dottty com
It sounds pretty amazing. I wonder if I could put hte KT-150 tube in and expect more. The Midrange is magical.

So I went and met Kevin Deal- nice guy, opiionated- but with good reason.

I bought replacement 6550s to replace my non winged 6550s. Went for the cryro treatment.

OK on leaving Kevin said I should try running them at a lower bias.

Now, As my tubes aged I found the highest I could run the lowest measuring tube was 57milliamps. So i set all the tubes there- and to my surprise the amp still sounded really really good.

So I was still excited to get better Cryoed SED 6550c wing tubes and run the bias back up to 65milliamps.

Kevin asked what I was running at before and I said 57 milliamps which I thought was low, he said "No actually that should sound pretty good".

When I got the new tubes from Kevin - Kevin said I should run them at 57 Milliamps. And at 65 milliamps I would just burn them out sooner and that hte amp sounds better at lower bias.

Of course...the doubting idiot in me thought... "How would Kevin know, he is not an ARC dealer....maybe his tubes can not handle 65milliamps...and besides...I want the best screw it if the life might be shorter."

So I got them biased the amp to 57 milliamps and it sounded really amazing. After about 20 hours- my main transformer was buzzing from an unrealted issue. I sent the amp back to ARC.

It came back with the tubes biased at 65milliamps. After warm up... it really did not sound as fluid and amazing as before.

So rebiased to 56 milliamps....and it sounded sorta uncontrolled... I thought...well then 57 milliamps likely isn't the sweet spot. So I tried 58.4 milliamps... too controlled, then 57.4 milliamps...still too controlled- WTF???- , then back to 57 milliamps and all was blissful again. Wow.... how can such a tiny adjustment make that sort of difference? I guess I should have tried 56.5milliamps... but frankly I was heated up over this tube amp- and it sounded so good I did not feel a reason to tweek any further.

How Kevin remembers all this stuff is a mystery to me... I won't bother wasting MY time or his time doubting him again. I think he knows his stuff and if he isn't sure ...he doesn't bullshit you...he will just say he doesn't know.

I'm not sure I should change anything because with my Chapman T-9MKII and this GNSC modded ARC VS-110 amp and GNSC Modded Wadia 860x it just sounds ridicoulously real with quite a few recordings... but alas..I had to give up my MIT cable and I have some DH Labs Q-10 stuff which is looking to be promising...getting it back from a cryro in a few days.

Why continue?? ... well I know that the better the system gets you end up with even more recordings that sound absolutley real. right now it sounds like 5-10% of what I play sounds real, I would like to up that to around 20%...whcih probably is about the limit of recorded good material.

So other possible upgrades are the Bob Carver Black beauty modded 305's- possibly The top of the line synergistics cable, and an EAR preamp.

But the tubes seem like something easy to try. Does anyone have a set of KT-150 they might want to try out in my system if I paid a 2 hour rental fee for your hassle? Seems silly to just toss a few hundred buck out just to try them.
My vsi 60 came with the new kt120 . Sounded harsh at first then mellowed after 25 hrs, I'm using revel performa 3o and naim cd 5xs with nordost heimdall 2 and kimber 8 tc , voodo wave power cord . I want to try gold pin tube , has anyone heard this to compare with the standard tube ? Cheers sti
If you check your owners manual you will find that the tubes should be biased to 65mV. The amp will sound it's best with the tubes running at 65mV not 57mV.