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.