ARC VT100 with KT-120 tubes ?

Bifwyinne:
Thanks for being a good sport!

More plate voltage is not very important in the context of most tube amps, because the plate voltage is fixed by design. However, the ability to take more plate current is an advantage, for the following reasons:

The Svetlana 6550C has the following limiting values (only ONE limit can be exceeded at a time, and only momentarily, for example, during power up):
-680V plate voltage
-plate dissipation: 35 Watts as triode, 42 Watts as tetrode

For best sound / least distortion, ARC tends to run their tubes at about 450 Volts B+ and 65 ma current, which is aproximately 30 Watts, pretty close to the tube's limiting value.

The KT120 has the following limiting values:
-650 plate voltage connected as a triode, 850V as a tetrode
-Plate dissipation: 60 Watts

Running a 60W tube at 30W dissipation is much easier on the tube that running a 35-42W tube at 30W dissipation.

So, instead of plate voltage, power dissipation is the main KT-120 advantage.
A 60W tube running at 30 W dissipation will be loafing along and will probably last much longer than any 6550 tube, provided that the power transformer can take the extra heater current and that ARC makes the (depending on amplifier model) necessary bias supply changes.

I agree with you, this tube has great potential but is not a simple drop-in replacement.
Enjoy the music!

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.