audio Research REF 110 and KT 120 tubes

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...