Yep... you got a bad tube. I've got a number of BAD tubes from ebay sellers stating them as new. (I suspect the tubes are factory seconds.) The tubes from certain ebay sellers measure poorer than the ARC supplied tubes with 2000+ hours on them. I only now purchase from ARC, Tube Depot or Upscale audio. I've had multiple EH 6992 tubes blow after 30 or 40 hours, from a seller on ebay; I inserted the original ARC Sovtek 6992's (with 1000's of hours on them 2 years ago.) and everything is great.
KT88 vs 6550 tubes
I recently got a quad of EAT kt88 tubes for my Mcintosh MC275. Those are supposedly revered as one of the best. However, less than 10 hours one of them had a spark of fireworks in it and shorted the fuse. I heard KT88s are in general not as reliable as 6550. Is this true?
I thought also that KT88 and 6550 are interchangeable in the MC275 without any additional bias adjustment. The original 6550 tubes in it has been working for a long time without issue.
I thought also that KT88 and 6550 are interchangeable in the MC275 without any additional bias adjustment. The original 6550 tubes in it has been working for a long time without issue.