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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

Showing 4 responses by bifwynne

bifwynne OP
2,206 posts
 
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??
bifwynne OP
2,206 posts
 
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 OP
2,206 posts
 
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.
bifwynne OP
2,206 posts
 
My mains are Paradigm Signature 8 v2 (beryllium tweeters). They seem to match well with the VS 115.
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