audio Research REF 110 and KT 120 tubes


I have a REF 110 with 6550C tube
i'm looking for someone who heard the REF 110 with new tubes KT120
(i know it's possible to change them, Kalvin from ARC confirmed it)

If yes, how the REF 110 sounds ?

I would be interested in the answer as well. Thx

I would also be interested in how they sound in the REF 110. I think that one of the reasons ARC changed tubes is the availability of the SED "C" 6550s. I have heard that the SED "C" 6550s may no longer be in production.
I also read somewhere that replacing the 6550`s with the KT 120`s upped the power by 20 watts per side. Or maybe 20 watts total. Can anyone confirm that?

I was told by ARC that not only was the KT 120 a better sounding tube. That it would increase the power of the Ref 110 to approx 130 wpc. The KT 120 is a drop in replacement in the Ref 110.
Lostbears...ARC told me the same thing. They also said that since they are drop in replacements (i.e., no change in bias settings from when the 6550s are in the circuit), they are being run much more conservatively at the existing bias settings so that the tubes should last longer and theoretically they are expecting less in the way of tube failures. I recently had my Ref 110 retubed by ARC with 6550s so I will probably just wait before retubing again with the KT120s.
I am due for new tubes in my Classic 120's. Wonder if the KT120 would be a drop in as well. It seems that the SED 6550C Winged are really getting scarce. Not too many tube sellers have them anymore and i would need 2 sets of matched Octets. I will call ARC and find out.

Smoffatt, probably not without modification. I believe that the bias pot range may be too small. If that is the case you would not be able to properly bias the tubes. You might need to changed the value of a few resistors. Not that big a deal. I had it done to a Classic 60 years ago. ARC sent me the proper resistors along with a schematic of where to replace than. There might also be clearance issues. The KT 120 tube is much larger than the 6550 tube. They may not clear the cages.
Thanks Lostbears. I know. I had to replace one resistor near the pot when i changed tubes from KT90 to 6550C a few years ago as i did not have enough "play" on the pot to bring the bias down to .65ma. The lowest i could go was approx. .70ma. The Classic 120's operate in triode mode vs the REF110 is Ultralinear. The KT120 may not be suitable for the CL120's. I shall confirm with ARC.
Thanks again.
I spoke to K(C?)alvin the other day w/ regard to my CD3 and I asked about the KT120 for my VT200MKI. He said that first the KT120s are in short supply and they are using all they have for their current production. He did say however, that they are planning to determine which previous amps will allow the KT120 to fit and can be properly biased. He seemed to imply that the "modern" amps would not have bias issues if there is room for the tubes. He also stated there was a roughly 20% increase in power over the 6550. They are working on a new website but his advice was to be patient. If the KT120 will work in my VT200 I'll just suck it up and send it back to ARC and let them get it right.
Great tubes! I'm using them in my DIY PPPdiff mono-blocks. These amps use 4 output tubes in a "forced" class A push pull differential mode (the 2 pp halves are in series, nor parallel like class AB1 and cannot operate out of class A). Operating at 40% of plate dissapation as opposed to 85% with 6550s in these amps is a real benefit. I have experienced one instance where aKT-120 opened and stopped working requiring the replacement of a matched quad. That's better than the time a 6550 arched and turned a tantalum grid-stopper resistor to charcoal. ;~)
The KT-120 actually can run at a maximum plate voltage of 700-800Vdc! This is a very very robust tube. I have fitted a set in one ARC amp in warranty. It is taller, heavier, and the glass is thicker. Also on inspection, the plate of the tube utilizes heat sink fins. This is a very different tube then a 6550 and frankly sounds wonderful. I believe this currently represents the state of the art in tube design. Remember, unlike the U.S., the Russians never stopped making and designing tubes. That being said you will always get a random bad tube. It's the nature of the beast... there only one supplier of the KT-120s? From my conversation with ARC, they said they are not selling the KT-120s on a standalone basis for existing customers to retube their current production amps because they barely have enough supply to meet new amp deliveries that come with the new KT-120s. So it sounds like supply is pretty limited. Did you get yours from ARC or from a third-party tube vendor? Thanks.
Wow I guess my last post never made it on here. Cmalak, the reason I saw the KT-120's is I had to service in warranty a pair of Ref210's and they came with KT-120's. Understand that I can get KT-120's but they would not be ARC KT-120s. ARC runs all their tubes at hard labor for 24 hours to cull out the bad ones and then categorizes them based on their current gain etc. That way when I order tubes from ARC I have to give them the model of the product I'm repairing. Also, if any damage is caused by the ARC tubes within the warranty period, i.e. a screen or plate load resistor failure, it's covered under warranty. This tube represents the current state of the art in Russia tube design. You can email me and I can keep you updated as to when they will be available. Please be patient. Both myself and ARC know that people are chomping at the bit to get these tubes in their amps. This tube is so new they don't even know which amps these tubes will fit in. I just found out today that the KT-120's will not fit in the VT-200 as there's just not enough space. I can tell you that after just working on a Ref 110 the KT-120's will fit, and the factory tells me they work great! Additionally, any ARC amp that uses a 6550 regulator, that tube is not replaced with a KT-120.
Will the KT-120 tube fit in a VT100 Mk II ?
Mabonn.....I believe so.
did someone try KT120 tubes on REF 110 ?
how does it sounds ?

For what it's worth, I dropped ARC supplied KT 120s in my Ref 110 and can say with assurance that they provide noticeably more power, dimensionality and transparency. The caveat is that I did this bit of listening with a temporary pre (in place since I sold my Ref 5) as I await Ref 40 to be delivered next week. The pre is by a small manufacturer who I would rather not name until I have concluded my listening with the Ref 40. I am confident that not more than a handful of Agoners own this pre.
In any event, there is a stridency in the lower treble upper mids that I did not hear before, possibly because my 6550s , with less transparency, did not reveal as a preamp or source (cd) problem.I suspect it is not the fault of the new tubes. I will report back after Ref 40 break-in which may take some time.
If it works out as I believe it will, I will be able to wholeheartedly recommend the kt 120s.
thanks for your review
so the KT120 can be mount ont the old REF 110
that's a good new
I had some informations of Kalvin from ARC:

i have a REF 110 from 2008 (230V - 50Hz)
i would like to know if i can put KT 120 tubes instead of my 6550 tubes


"Yes you can. They are a direct replacement.

Leonard Gustafson
Customer Service"

Thanks Kelvin

inside the new REF 110 it's the same as the old (like mine 2008)
the only difference is the tubes KT 120 ?

How the ref 110 does Sounds with the KT 120 ?


"Hello Patrice,

The KT 120 has more dynamics and better bass than the 6550. The amplifier also has more a little more headroom before it gets compressed.

Best Regards,

Kalvin Dahl
Customer Support"

Thanks Kalvin

inside the new REF 110 it's the same as the old (like mine 2008) ?
the only difference on the new REF110 is the tubes KT 120 inside ?

"Hello Patrice,

Yes. The only difference is the new KT120 tubes."
BTW I just retubed a VS60 with KT-120's. Awesome. In case someone was wondering if you can put KT-120's in a VS60. You can.
How about putting one of these KT 120 tubes in the Ref 3 power supply location? I would think that would make for an improvemnt in an already great preamp. They will be available sometime this year, correct?
Tlg...ARC is NOT using the KT-120 in any power supply in any of their products. 6550 only...I have KT-120's
As a follow up, the problem was, as I suspected, with the temporary pre. The KT-120s are not strident in the lower treble or upper mids. Rather it is an upgrade that can be heartily recommended all around.
I just put 2 matched pairs in my VS55 and biased them to 65 and they sound great much better bass. Over all seemed to impart a greater sense of power. I had been using NOS tungsol 6550 and my first impressions are that the KT 120's sound as good in the highs and midrange and much better bass with my dynaudio excite 16 speakers only about 10 hours so far I would think that they need bait more time to break in
I just put 2 matched pairs in my VS55 and biased them to 65 and they sound great much better bass. Over all seemed to impart a greater sense of power. I had been using NOS tungsol 6550 and my first impressions are that the KT 120's sound as good in the highs and midrange and much better bass with my dynaudio excite 16 speakers only about 10 hours so far I would think that they need bait more time to break in
Just adding this list to help out anyone who may stumble upon this thread.

Credit for this goes to A'gon member Hifigeek1. He posted this on another thread.

04-19-12: Hifigeek1

The following is a list of ARC amps that are authorized to use the KT-120:

VS55, VSi55, VS60, VSi60, VS110, VS115, REF110, VM220. I hope people find this useful as there seems to be a lot of confusion about it.
@Jedinite24 thank you for the kind words. I neglected to add: REF210 requires field or factory mod. REF610 requires factory mod. May the force be with you!