I sent the tube back to the manufacturer's agent and they say it tests fine. They will replace it though.
Did they test it on a tester or in a real circut that may push it to the limit? Testers rarely test at different maximum loads.
Is the bad tube pulling to much current? I have never had a tube cause a fuse to blow or produce a transformer hum(lucky). Can somebody tell me what's happening, My guess would be the filament is drawing too much current.
I would guess the tube not the filament. When filaments draw too much current they normally burn out causing a dead tube.