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RE: MOT to NST "extra transformer"
The ARRL Handbook has a very practical section on re-winding transformers,
once a popular practice among hams. A quick review:
1) Get a transformer rated for the kVA you want, and test it.
2) Disassemble the core. (detailed instructions included)
3) Unwind unwanted secondary winding, noting number of turns.
4) Divide # turns by the voltage of that secondary, get turns per volt.
5) Wind your own secondary using same ratio.
6) Reassemble.
Step 4 is where the "extra transformer" could run into trouble.
You can wind the correct turns ratio and still not get proper
operation of the core. Using the original primary avoids
this problem entirely, up to the maximum power handling
capability of the core.
Of course you can rewind a MOT secondary for more turns.
The ARRL handbook gives good rules of thumb for wire gauge,
core properties, insulation, etc,. Most of their audience
wants only a couple of kVA at a few kV, but its a very good
reference anyway.
Will