One of the members of our weekend group on seventy five meters owns an SB-220, it works well and is lightly modified as he put it. The first weekend in January he had been doing some cleaning had every peace of gear in it's place save for the SB-220. One evening while in QSO while the ampli9fier was sitting idle he gave the amplifier a gentle push back to it's normal resting place and at that moment. BANG!! He shut down the amplifier and proceeded to troubleshoot it the next day. Besides the fact the grid shut was blown off the board one of the tubes had developed a filament to grid short. This brings up the question; how many of those tube failures are caused by moving or even bumping the amplifier while the filaments are hot? never know but given the conversations that have been raised on the subject I had to pass this along as just one more data-point in the discussion. I've always taken it for granted that you don't move an amplifier with hot directly heated cathode tubes in it and I have always rolled my eyes when I hear of people in the old days running those Heath compact KW amplifiers mobile.