6550 replacements for upgraded CD7

Is it OK to use any 6550 equivalent (KT88, KT90, etc) in an upgraded CD7 power supply?
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.
Quick after the upgrade of the powersupply I put in a 6550 Tungsol black
plate . Wow it still works for years without any problem . After buying the CD 7 I replaced all the small tubes by 6H30P-DR versions .Still in use , also a big improvement .A problem prices for the Tungsol and DR are very high . 

Glad to read that you had good results in switching the 5881 for a Tungsol 6550.  That said, I am surprised you did not damage the CD7.  

Did you check with ARC as to whether the 6550 is an appropriate tube roll for the 5881?  FWIW, I used to own the CD7 and since moved up to the CD8, which uses a 6550 in its power supply. It never occurred to me that the 5881 was interchangeable with the 6550.

Just askin' …...
hello bifwynne , no I did not ask ARC , I followed the advice of a university graduated engineer who is doing business in the Netherlands
with his own brand of CD players , amplifiers and other highend products . The 6550 NOS tungsol was installed in 2009 , and is still working . BTW I installed this tube also in my REF 3 SE , I made my REF 3 a home SE version by the capacitor upgrade . All upgrades and mods in my PH 5 ,
REF 3 and CD 7 have made a big improvement , so I did not have to buy new units . I like ARC very much but in my opinion and the Dutch designer the units could be  much better by some improvements.