Thermionic Valve behaviour

[YT2095]

I`ve done alot of work with thermionic valves (vacuum tubes) in my Lab.

 

I`m wondering a few things about an interesting phenomenon I happened to notice a few times.

 

when a magnet is placed by, over or around a valve it seems to shut down? the cathote`s still lit but it function ceases. now that part I can partly onderstand as the electrons are being dragged away from or not reaching the plate.

but at the same time it doesn`t emit RF energy as a magnetron does? and yet it`s exectly the same principal, and when checked for X-Rays it even seems to damp them too?

 

any ideas?