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.
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.