Tube blows Amplifier fuse....causes?

I had a interesting experience. I was shipped a pair of 300B tubes(warranty). I put them in my Vac 140 monoblock. Everything was fine the first day and then I got a low transformer hum 2nd day. On the 3rd day after turn on the slo blow fuse blew(inspection showed low voltage). I was able to find the offending tube and replaced it. With the replacement everything is normal.

I sent the tube back to the manufacturer's agent and they say it tests fine. They will replace it though.

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.