On Thu, 25 Sep 2008 10:22:15 -0500, AZ Nomad
wrote:

usually the power supply will give out long before the magnetron
has degraded enough to be noticeable.

But he says his has lasted a long time and the power output does
decline with use.

Because the magnetron is a vacuum tube with a hot filament cathode the
emissive layers of the cathode degrade slowly with time. The result
is weakened emission and diminished power of the tube,

One reference of many --
http://aginfo.psu.edu/news/1999/8/microwave.html

"the electrical element that converts electrical energy into microwave
energy, will not maintain the same power levels over time. 'If a new
microwave oven boils a cup of water in one minute, after five or 10
years it may take 90 seconds'