6550 replacements for upgraded CD7

Is it OK to use any 6550 equivalent (KT88, KT90, etc) in an upgraded CD7 power supply?
Call Calvin at ARC. I am a little puzzled by your Q. I used to own the CD-7 with upgraded power supply. The upgrade involved replacing two of the three 6H30 PS tubes with one 5881 tube. I am puzzled because I'm simply not aware that one can drop-in a 6550 or KT-88 tube in exchange for the factory installed 5881 tube. Be careful.
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.
Correct Gary. Bottom line point. I would not drop in a 6550 tube in the upgraded CD-7 unless checking with the factory. It may simply be the wrong tube.
Hi, just to align the discussion with my other thread, the use of the 6550 was real life tried by some EU audiophiles and for them it has been working flawlessly.
Nevertheless, not wanting to breach any ARC policy, I'll have a word with Calvin on this topic before I proceed.

On a slightly different note, as this tube goes in the power supply section, how important is it, sound-wise, to go for some fancy NOS tubes other than modern, cheaper, models?


I discussed this with Leonard a number of years ago when I sent my CD7 in for the upgrade. The upgrade at that time was free if I remember correctly. I was told that the reason for the power supply being upgraded was that there were problems with some of the 6H30 tubes aging differently. This could cause reliability issues as well as other problems in the power supply. That is why the 2 6H30 tubes were replaced with a single 5881. The reason that the 5881 tube was chosen instead of a 6550 like in the CD8 was space. There was concerns that there would not be sufficient clearance for ventilation using a 6550. This could cause heat issues as well as the possibility of shortened tube life.

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.