[Amps] Filament life

[Rob & Terri Sherwood]

I was in commercial broadcasting for about 25 years, and filament life 
was an issue.  What puzzles me, however, is Alpha's recommendation to 
turn off their amps if one is not going to use it for two hours.   If I 
was going to be operating on and off during a Saturday, for example, my 
inclination would be to leave it on until I was finished operating for 
the day.   Any idea why Alpha says what they do?