Microwave Oven Transformers (MOTs) can KILL you because they produce about 2100 volts! They are totally unforgiving. Do not work with them unless you really know what you are doing. I take no responsibility for any information about them found on or through this website.
MOTs salvaged from discarded microwave ovens can provide inexpensive high voltage transformers for your home-brew linear amplifier. There are many videos and articles on the web showing how to knock out the current-limiting shims and removing the 3 volt winding.
Removing these two things allows the 2100 volt winding to be used as efficiently as possible, or it can be removed and a new, low-voltage secondary can be wound in its place. If rewinding, figure about one volt per turn.
If, for example, you want a 13.5 volt DC supply, wind about 20 turns so that you overshoot the AC requirement, and test the finished supply under load. If the AC input to the regulator is more than you want, just take a turn or so off the xfmr secondary. It is to some extent a cut-and-try project.
In my homebrew low voltage supplies, I wound the replacement secondaries with #14 house wire. I put a layer of electrical tape around the frame where the new winding was to be installed. These supplies have run ten to fifteen years with no problems.
In my homebrew high voltage supply, I use two of them with their primary windings connected in series. Having these two coils in series eliminates the need for a “soft start” circuit or other protective circuitry.
The secondaries are also connected in series, which means that the grounded end of the secondaries can remain grounded. The two then look just like a center-tapped “regular” transformer secondary. Thus the transformer frame has no voltage on it.
Using two means that each primary and each secondary is run at half voltage. This in turn means that the transformers run cool, there is no strain on the insulation and they produce the full ~2100 volts. No fan, no heat-sinking and no soft-start needed.
Do try to use two MOTs of approximately the same wattage or else one of the secondaries will try to take more of the load than the other. Since it is at half voltage this probably won’t matter much but it’s best to keep the load as equal as possible on each transformer.
Clever switching of the AC supplied to the primaries also means that the HV supply can provide half voltage to the tube plates when tuning up. Once tuned, go to full voltage. See the power supply drawing for the simple 500/1000 watt Linear amplifier on this website.
Here is a technical reference that might be of some value. It discusses the weakness of MOT secondary winding insulation. Also discussed are ways to overcome the initial AC surge incurred when AC power is first applied. Note that section above on this page also discusses these things.