The microwave over transformer can handle 700Watts of power or more. It is therefore a good alternative as a cheap high power transformer, if proper modifications are done:
As a simple rule, when winding the new secondary, a copper wire with a section of 1mm^2, can handle a current of 3Amps continuous use, or 5Amps max for short use. 2.5mm^2 section wire, can therefore handle 7.5A for continuous use, or 12.5A for short use.
Before starting to wind the secondary, we need to know how many turns we need for a given output voltage. In my case I needed 3 separated secondaries, one for 6.3V, one for 12.6V and one for 24.3V. The first two are for powering filaments of high power vacuum tubes . To determine the voltage/turn ratio, wind one turn and measure the voltage using an AC multimeter, or for better precision, wind 5 turns, measure the voltage and divide by 5. Yes, the output voltage is proportional to the number of turns based on the voltage/turn ratio. In my case the ratio was 0.9V/turn.
For 6.3V I needed 7 turns, for 12.6V the double, and for 24.3V 27 turns. Keep in mind that the metal core of the MOT cannot be dismantled, so you’ll need to wind the transformer as it is, using very long pieces of wire, inserted carefully around the core. Not easy, but do-able. I used PVC insulated wire (not the best choice but it was all I got), 1.5mm^2 for 6.3V and 12.6V and 2.5mm^2 for the 24.3V.
So my 3 secondaries can output the following voltages:
6.3V at 4.5A max (7.5A for short term use)
12.6V at 4.5A max (7.5A for short term use)
24.3V at 7.5A max (12.5A for short term use)
So I ended up with a rewound mot, capable of outputting 28.35+56.7+182.25=267.3 Watts. A little downgrade but it’s fine since it suits the purpose.
I’ve built a nice wooden case, a rectifier and a tripler, and you can select the AC output to be used for these two: