from a comment I added to this YouTube video
https://youtu.be/GCh2LVHUavQ
The main difference between a power transformer and an audio transformer is the way the E and I laminations of the core are arranged. The power transformer has an E on the left and I on the right of the first layer, then an I on the left and an E on the right of the second layer and so on, so alternating layers have E or I on the left. This makes the core almost like a solid block and the transformer is optimized for 60 or 50 Hz.
The audio transformer has all E’s bundled together, and all I’s bundled together, and they are assembled and held together with a piece of thick paper between the E’s and I’s. This air gap created by the paper optimizes the transformer for a wide band of audio frequencies.
You should try to take one of the power transformers apart. The first E and I laminations is very hard to get out, but after that it’s easier. A rubber mallet helps a lot but don’t break the windings. Then you can put it back together with the paper and now you have a wider band transformer. But the way you have been doing it works, for noncritical use.
The big MOT has too much core loss. Save it to make a spot welder. 🙂
Great video.