audio Research REF 110 and KT 120 tubes

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.