The magnetron is supposed to read nearly zero ohms across the two terminals. You are measuring the resistance of the filiament/heater/cathode of the tube. Activity in the magnetron occurs when the heated electrons are emitted from the cathode, pass through the magnetic field, swirl about, bounce in and out of indentations (vanes) in the tube, and finally land on the plate (antenna). The resistance from the cathode to the plate should be infinite. If not, you probably have a bad tube. Or, if there is continuity between the heater and the case, then you have a shorted tube.
What eventually happens to most vacuum tubes is that the cathode (filiament in this case) becomes coated with a material which inhibits the launching of electrons into the tube. When the flow becomes too low, oscillation ceases, and you have no microwave output. Sometimes they reach the threshold, but do no cross over quickly. Then you have a magnetron that is intermittent for a while.
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