6550 replacements for upgraded CD7

I think he's going to tell you not to use a 6550 because it's supposed to be a 5881/6L6. That tube is used as a pass element in a voltage regulator circuit and should not be changed. @Bifwinne I believe the update was replacing 2 6H30's with one 5881 which is an industrial compact version of a 6L6 tube. Those tubes are identical and can be substituted but I think ARC uses the compact version for space requirements. The replacement/update may have had something to do with tube reliability ie. with 2 6H30's you are 2 times more likely to have one fail vs. one 5881 and they used 5881's with great success in the SP-10 preamp with no issues.

Since the 5881 is a pass element in a regulated power supply, I doubt changing it would offer much sonic improvement. You can always try an NOS, but it must fit properly and allow for proper ventilation, or problems may arise.