Off and on, I have been messing with making a little transformer. My first attempt was last winter. They never seem to really work very well. Is there any sort of rule regarding the amp-turns needed around a core with a particular permeability? My earliest attempts used a straight rod. More recently, I have been winding around toroidial cores. Typically, the greater of my windings has a hundred or more turns. Strangely, one of my early "straight rod" attempts has actually provided to have best performance, thus far.