I have a Defy 7 Mk IV, and have had some experience with bad tubes and blown fuses. The cause of my problems were low quality tubes I purchased new in 2003 labled as Svetlana, wich were developing grid current after only a few weeks of normal use. My best advice is to be absolutely sure to replace the fuses with the correct value and type, and replace your tubes with quality new ones, do not try to reuse them after adjusting bias. I damaged my amp due to impatience and ignorance. (It was my only amp at the time, and it failed on a Friday night. Naturally I rushed to get it back in service with some unpleasant results.) I had the amp repaired and throughly inspected/tested by a qualified technician and in the end it was just bad tubes. The "Tungsol" 6550s currently in production sound good and have proven to be the most reliable of all I have tried, I'm getting about a year and half to two years out of my power tubes with Sonus Faber Electa Amators. I've had the amp for nine years now, and it still plays music...but I keep a solid state amp handy, just in case.
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