By default, this will also have to serve as first impressions of my ARC VS115 amp as I have heard neither the KT-120 tubes nor the ARC VS115 prior to purchasing the ARC VS115. First of all, I’m quite familiar with the sound of the venerable 6550 output tube as I have owned more than a few tube amps over the years that used 6550s. Those amps would include the ARC D-76, D-76A, D-70 MKII, D-79, VT-100 MKII, Jadis Defy 7 MKII, Quicksilver V4s, and a pair of monoblock amps I built from scratch some years ago based on the Dyna MKIII circuit.
6550 tubes have a reputation for punchy bass, especially in comparison to the widely used EL-34. If we consider the KT-88 to be a first cousin of the 6550, there is a strong family resemblance in the bass department. The best bass I have heard from a KT-88 tube was when I was using real honest to goodness NOS GEC KT-88 tubes. Those tubes are in a class by themselves and modern day KT-88s don’t compare in sound quality from top to bottom IMO.
So now at this late point in tube audio history, we have a new output tube that was brought to market under the Tung Sol brand, and that would be the KT-120. The KT-120 is a much beefier tube than the 6550/KT-88 family with much greater plate dissipation capability. In a relatively short period of time, both Conrad Johnson and ARC have abandoned the 6550 in favor of the new KT-120. In order to take full advantage of the greater output power potential of the KT-120, new output transformers and power transformers had to be wound. People who are using the KT-120 tube in amps designed for 6550/KT-88 tubes aren’t going to increase their output power over their stock rated power. What you will gain is the sound of the KT-120 tube. My ARC VS115 was designed for the 6550, so I’m not extracting the full capabilities of this tube at least with regards to potential output power.
If 6550s are known for bass punch, KT-120 tubes should be known for bass slam. Used in the ARC VS115, I don’t think I have ever heard a tube amp with this kind of bass punch. Much to my surprise, I actually had to dial down the bass on my four subs, it was simply too much. You would think that coming from the Krell KSA-250 that the opposite would be true. If you play back a recording that has lots of bass encoded in it, the KT-120 is going to play that bass for you. You will hear how well or how poorly the bass was recorded.
For an example of bass that I think is slightly over-cooked, listen to any cut from the Happy Coat album by Shota Osabe. I love this album by the way as I find the music to be soothing and therefore relaxing. I love the sound they captured of the piano. As for the bass, I would classify this as more mid-bass than low bass as you are basically hearing an acoustic bass. If you have any problems with bass muddiness, driver overhang, etc., this recording will bring them all out in a heartbeat. Even though I find the bass to be a bit overblown, it still should sound really good in terms of quality. You may also find that it will set things in your room to rattling as well. The KSA-250 never failed to put a smile on my face when listening to this album and the VS115/KT-120 combo hasn’t either. The KSA-250 excels in the bass/mid-bass department, but the VS115/KT-120 combo held its ground. Also, if you don’t hear lots of bass from this recording, something is amiss with your system.
Compared to my KSA-250, the VS115/KT-120 combo also has more inner detail and you can hear further into the mix than I could with the KSA-250. Maybe some of that was due to the damn hum that I have talked about numerous times in my Krell Saga thread. And yes, the ARC VS115 is super quiet. I absolutely love having a very low noise floor and after years of being way too tolerant of noise and hum issues, I have lost all patience for them now.
My preliminary feeling is that we have a sonic winner with the new KT-120 tube. I think it sets new standards in bass capability when compared to the 6550 and the rest of the KT family. I have heard nothing amiss in the mids or high frequencies which would lead me to question this tube’s capabilities across the audio band. I’m still coming to terms with it, but I like what I hear so far.
As for the VS115, the only nit I’m going to pick for the here and now is the bias scheme which seems to be a recurring theme with many ARC amps. Although the bias scheme for the VS115 amp is way more user friendly than say the ridiculous VT-100 series of amps, it brought back shades of my Jadis Defy 7 MKII. Except for cost, I don’t know why ARC chose to have only one bias pot for each pair of output tubes instead of using two pots so you could set each tube exactly to the correct bias. This was a similar scheme to what the Jadis used, except the Jadis had 3 tubes per bias pot and you were setting an average bias across the three tubes. Now you are setting the average of two tubes. But unlike the Jadis, at least you don’t have to turn a 100 lb amp on its side, remove the bottom plate, and probe its innards with your meter in order to set the bias. The VS115 has all bias jacks on the rear panel. You do have to perform a little fishing with the bias tool that ARC provides as the bias pot is recessed under the top plate opening and the bias pot is very small and so is the bias tool.
That’s it for now.
