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MOT transformer's are designed kind of like HV Neon Tube Transformers. That is to say that they take the center tap most of us would use for power supply and they ground it to the case. They do this to limit how much current the transformer can attempt to put it in a dead short type of situation. It acts to limit current flow past the design limits of the MOT. In a Microwave they run the power output to a decent sized cap then dump it right into the magnatron with no current limiting and other then a diode burried in the cap nothing we would call rectification either very dirty design. To keep the magnatron from drawing more current then they want and burning out the MOT they take the center tap and run it to the transformer case then ground the case to the chasis.

So the above is why we need two of them preferable after their magnetic shunt has been removed in series so you can use cheaper full wave rectifier circuit design.

For th record is you want to have 6.3 volt for filliment after you dock out the shunts their is enough space to take insulated wire and wind two transformer's for 6.3 volts. Usually a 1 turn for 1 volt ratio applies so you can make 7 turns and end up with about 6.6-6.8 volts. You can also do 4 wraps then wire the two filiments winding in series. Not at all hard to get the 7.3-7.8 volts down to 6.3 once you pass it through your rectifer board. A simple resistor could be inserted if you did not want a complicated voltage regulator circuit to get you to 6.3 in an almost idiot proof and bullet proof manner. The diamter of the wire is what controls current so you could use some very large wire since you only need to get 4 turns on each of the 2 MOT's.

I currently have two very nice HV transformer's to use for my project but I also have two MOT's that I will eventually use in a project. I think though that it goes with out saying that if you are going to use MOT's expect to spend more on your caps and diodes to ensure insanely stable, clean,stiff power supply to your rig. The huge sum of money you are saving on the transformer more then offset's the increase in capacitor and diode cost.

For the record anyone that wants to use matching transformer's to make wiring more stable for series work should consider ebay. I have seen brand new MOT's for $11 each and $17 to ship them. That would allow you to use identical transformer's so draw and output when wired in series would be close to identical. I think you will find almost all MOT's will be 2100V under load and about 2500 with no load. The difference from one to the next will be based on the output power of the Magnatron in the unit which will make the current output from one MOT to the next differ if they are from different power output microwaves.

I know I left something out but I think I got most of it.