ARC VT100 with KT-120 tubes ?

Casouza there's more to it. The construction of the KT-120 is far more robust than the 6550. Taller plate with heat sink fin structure to allow for more efficient heat dissipation, as well as a thicker glass envelope. The KT-120 also has getter flashing on the top and on the side. I do agree with you, it is NOT a drop in replacement.

Thansk Hifigeei and Casouza for the tech information. Hifigeek, care to share what's wrong with the VS115 you're working on??

Lyric Hi Fi in Manhattan indicted recently that Audio Research had issued a bulleting listing the ARC products that are compatible with the Tung Sol KT120 tubes. I was happy to learn that the VT100 MKII was included. I ordered a matched octet and installed them about a week ago. After initial break in of about 20 hours I am starting to get the full measure of the performance enhancement of the VT100/KT120 combo.

Admittedly, replaced 4+ year old power tubes with new ones will lead to improvements across the board. However there are specific areas of improvement that are due to the KT120s.

From the outset it was clear that the dynamic range was enhanced. Not so much in terms of sheer loudness: The gradations from "p" to "pp" and "ppp" were now more clearly defined. While quiet passages seem even quieter.

There was a greater sense of control, flow and ease to the music

There was greater refinement and smoothness. Highs were more extended and clear

The sound stage was deeper and wider

Bass had more control and power.

After about 20-25 hours the sound became noticeably more powerful and robust

Last night I had a surreal experience while playing Bizet Carmen Opera (EMI) with Maria Callas. It was breathtaking. The power, majesty and sound staging were incredible. Maria and orchestra were in the room with me. The climaxes and "ffff" were without strain or stress. The VT100 MKII had never sounded this powerful and suave before.

There have been no down sides so far. Surprisingly, the amp seems to be running cooler than before (which is contrary to what I would have expected). Presumably, the amp was coasting along for the ride.

I would highly recommend these tubes to VT100 owners

It is possible that the KT-120's are running cooler because the plate of the tube is much larger. Therefore, there is a greater area for heat dissipation and less chance of a hot spot. There are also heat sinks on the plate structure as well, making cooling more efficient. The KT-120 represents the latest state of the art in power tube design.

I bought 6922 input tubes and 6550C output tubes on the internet, but they did not meet the standard and it was impossible for my tech to get the settings right and the bias up to the correct value. He recommended that I got the “authorized” tubes from ARC and I ordered a complete set of new tubes and got the KT120 instead of the 6550C from ARC (or actually from Audionord.se, the Nordic distributer).

This time the tech had no problem getting the setting right and the bias correct and I got the VT100mkII back in no time.

The change in sound was dramatic. Before the sound was easy, soft and pleasant, the amplifier seemed to work fine and I was happy. But now it is much more open, revealing and tight. I listen mainly to acoustic jazz and the double bass is much better defined, the sound from the plucking, the string and body is more coherent as from one instrument (as it is in reality). Drums and especially cymbals sound natural and again the attack and ringing of the cymbal is coherent. The recording of the piano is clearly revealed, but still sound neutral and musical, not analytic.

I find that one of the greatest strengths of ARC gear, is the “recreation” of the human voice. And the voice of Ella Fitzgerald or Diana Krall shines through the VT100mkII with the new KT120 output tube.

Before I could read a book or work on my laptop while listening to music, but now it is difficult to concentrate and, as I am writing this, I have to stop and listen all the time, even to music I have heard a hundred times.

I can second what gmorris writes and I can recommend use of the KT120 in the VT100. And I will absolutely recommend buying the “authorized” tubes from ARC, it can save you and/or your tech a lot of time and trouble.