ARC VT100 with KT-120 tubes ?

Well the bias setting is the same as with the 6550. I can tell you it's at the very least an improvement in power output. They sound very good. No down side. I've only heard them in a pair of Ref210's.

Karma16. The KT-120 draws a bit more heater current per tube. There are possibly other variables with the KT-120 that I am unaware of... The Ref210's are mono blocks and are rated at 210w across 8 ohms with 6550's @ 120Vac line and that's what I measured. With KT-120's approx. 250w across 8 ohms with the same line voltage. I verified this on my service bench using a Dale 250W 8 ohm non-inductive wirewound resistor. Bias was the same as was plate and screen voltages. You can contact Audio Research and they will give you the before and after wattage outputs for 6550 vs. KT-120, just specify the amp in question. I fix their products in warranty.

That is correct. You can not use a KT-120 for the regulator tube in ARC amplifiers. ARC is still in the process of determining power output of their amps with the 6550 vs. the KT-120. In some ARC products the KT-120 will not even fit. The KT-120 represents the latest design from the Russians. The tube is capable of running up to a maximum plate voltage of at least 700Vdc. The hope is that tube life will be extended.

Karma16..Have a nice day.

Bifwynne, I will be getting in a VS-115 with KT-120's for service. I will let you know what the power output is once the unit has been gone through.

Both the Ref210 and Ref610 require changes to one of the regulated power supplies in those two amps. I will probably be able to update the Ref210 at my office, the Ref610 on the other hand requires a more extensive update and therefore Ref610's will need to go back to the factory.

I think they will work great and given the amp runs the outputs in triode it should sound fantastic!!!

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.

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.

Farobari I try and tell my ARC customers that all the time. ARC tubes are more expensive but well worth it, especially if you want to get your amp up and running quickly. I also recommend factory trained technicians do this because you are dealing with high voltages in the amp and one wrong move and the results are not pretty. With ARC tubes, I don't have to sit there and let the tube bias settle in for a week. The bias is usually stable after an hour since they had been burned in for 48 hrs. I can run it for an a day, set the bias, do a frequency response and a power output test and return it back to the customer in short order. I had to recently re-tube a C/J amp and purchased tubes from a reliable vendor. Two minutes after I installed the tubes, wouldn't you know one of the EL-34 started running away. And these were supposedly tested tubes that I paid extra for. The point is you really can't trust many tube vendors out there. Granted, it's possible the tube got damaged in shipment. I find that with ARC products, installing non-ARC tubes in their products is like putting low octane gas in a Ferrari.