Two are fine! As long as they are the same brand and biased the same!
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It's EXTREMELY unlikely you will damage anything by putting KT120s in that amp, as there have been zero reports of any issues from switching to 120s which, in my and many other's opinion, are much better sounding than any 6550. You can use KT150s also by the way…no reported issues anywhere…go for it…live on the edge!
Thank you. If I change from 6550 to all KT 120, is sound quality will be better than 6550?
Just got off the phone with ARC tech at ARC. He said KT120 will improve sound quality but it will shorten life of transformer.You won’t improve the sound going from a Kt-88 of good quality to a KT-120. The KT-120, I expect, will not be lasting long in the face of the KT-150; the big complaint against the KT-120 is increased distortion. This is the same problem that was faced by the KT-90, 99 and 100, of which production is severely curtailed if no gone entirely due to the distortion issue.
JJ is making some excellent KT-88s (which are a direct replacement/ higher quality version of a 6550).
In the case of shortening the transformer life, if the above quote is correct, it will shorten the life of the transformer to that of hours rather than years! IOW don’t do it!