I have KT88 Gold lion reissued, I used it on my Pre-owned Mc275 for almost 6 months, so far so good. May be it is an isolated incidient as MC275 runs very high voltage which blow my JJ KT88. Check whether your KT66 is a geniune KT66 reissue, and also check the bias of your amp to see whether the amp has changed the bias. What amp you are using on the KT66 ?
