I/m afraid I must disagree with what's been said here. The 909 (and
707 and 606 and 306 and 405) contains a class-A amplifier which by
definition is conducting 100% of the time, so of course it will get
warm, signal or no signal. That's why there is so much heatsinking. My
306's have been running warm to the touch on the front heatsink for 20
years. *Too warm* or dead channels indicates a fault of course.

In some recording studios in Australia it was the practice to leave
405s (and everything else) on 24 hours a day for months or years on
end, so extended 'on' cycles shouldn't be a problem either.

EJP