My ST120 has been working flawlessly for about 18 months with great sound and no tube problems. Being incapable of leaving well-enough alone, I wanted to try a matched set of KT-120s (from a reliable source) to replace the Tungsol KT-88s in my amp. I installed the KT-120s and then let the amp warm up (while listening to Solar by Miles Davis) so I could bias the tubes. The sound was fine. After about 5 minutes I heard a rumbling, then I turned the volume all the way down to bias the amp. The heaters were on but there was no bias voltage (I didn't notice a rectifier flash). Not being able to get bias voltage, I replaced the rectifier, then went to bias again and I saw fireworks in the rectifier tube, then the fuse blew. I put in the first rectifier, hoping that it was still good, and the fuse blew. After being down two rectifiers and two fuses I decided to put things back the way they were, so I re-installed the original KT-88s, put in my last spare rectifier and another fuse. Everything was fine and I reset the bias to 0.55v.
I'm glad that the ST120 amp is a nice, solid design and nothing under the hood blew up in spite of my abuse (in this case not referring to my music tastes).
So before I sacrifice any more rectifier tubes, is the best procedure to install the new KT-120s on a cold amp and immediately bias to 0.55v? Or could it just be that one of the KT-120s is bad?