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Why do Magnetron Tubes Fail?

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The magnetron tube is the heart that beats the energy into your microwave system. Most new mag tubes have a life expectancy of three to five thousand hours with pro-rated warranties available.

Rebuilt magnetron tubes are becoming increasingly harder to get due to the short supply of mag tube cores that are viable. There are a finite number of microwave systems and over the years the tubes for the transmitters have been subjected to numerous rebuilds.  After a while, usually 3-4 times being rebuilt, these mag tube cores are no longer able to be repaired.

This lack of worthwhile tube cores has pushed the need for new magnetron tubes to step up production to meet the demand. And as with many supply chain issue affecting our current economy, raw materials used in the construction of these tubes are scarce or being delayed causing backlogs of orders.

Even though all machine parts will need to be replaced or repaired at some point, it’s important to know what causes your magnetron tube to fail and how to possibly prevent it or at least, extend the life of it that will curtail expensive replacements and delays in production.

Here are some Red Flags to watch for.
  1. Circulator is defective, possibly Ferrite material in the circulator becomes cracked. If your transmitter runs excessively hot this can cause the circulator to run out of tune. The Ferrite material will become cracked due to arcing happening from the output of the transmitter, possibly the waveguide or the rotary feed.
  2.  The lower polepiece inside the electromagnet becomes rusted due to water or condensation collecting around it. If the lower polepiece is not seated properly against the transition, causing a gap, it will allow microwave leakage between the joints which causes arcing.  This arcing can then melt the plastic sleeve which causes the tube to run hot. This in turn can then melt the plastic sleeve which will overheat the tube. If the mag tube runs hot for too long, it can cause it to mode which can crack the dome.
  3.  The mag tube is improperly installed. As simple as this seems, often untrained or unfamiliar staff will install a mag tube incorrectly both in placement and connection. Poor seating of the tube gasket can lead to arcing An improper filament setup will cause the tube to mode, which as described in line 2, can cause the dome to crack. Badly installed filament connectors.  It is important that the connector be lined up with the edges of the tube cap.  If this is not done right, it is pushed all the way down the tube. Without proper air cooling will again cause the tube to run hot.  This has the domino effect of creating arcing, moding, and other serious conditions. Loose or poor filament connector adjustments will create a hot tube scenario.
  4.  Burned or arced up rheostat. Then rheostat controls the filament adjustment during the power up stage. If the rheostat coupling gets loose, the filament will not be adjusted properly during operation, causing the tube to mode.
  5.  Improper tube warm-up or cool-down.  If the tube is not allowed to cool-down for the required period of 3-5 minutes, this will damage the tube.