Good idea but I do not think I can get 1 Amp.
As others have asked, 1 amp through what impedance? 1 amp (rms) at 50
ohms is only 50 watts. If you think you can't get 1 amp induced in a
metal ring in a microwave oven, you've probably never tried it (and
then tried to remove the ring with your bare fingers).
A magnetron is a good way to generate a lot of microwave power
cheaply, but it's possible you have other needs, too, that you haven't
told us about. Does the frequency matter? How about frequency
stability? How accurately do you want to control the 1 amp? Does the
load change over time (e.g. as it heats up)? What, exactly, is the
load? (I can fairly easily get you one amp out of my signal generator
that can output only +20dBm, if it's through a load of my choosing.
Using a quarter wave long transmission line resonant at the frequency
of interest, made from copper pipe, I can transform the 50 ohm output
to a very low impedance. 1 amp at 0.1 ohms is only 0.1 watts, or
+20dBm.)
Cheers,
Tom