Hi there,yes that's a real good indication of a bad tube.There could be some more bad parts as the tube "connects" the circuitry.You can narrow it down by plugging in one output tube at a time and turning the amp on,if the fuse blows on only one tube and not the other,it's a bad tube,if the fuse blows on either output tube,you've got either 2 bad tubes or 2 bad tubes and power supply probs.Hope this helps a bit,Mel.