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.
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.