The transformer is a 36 volt and although it is not happy about being driven by a square wave it works and provided a good range of output. Thru trial and error found this one to be best. Transformers work both ways so you can use the primary as the secondary and they don’t know the difference. The trick is to not exceed the insulation value.  Cheap Radio Shack transformers tended to get hot or buzz.

Good job Ray!

The transformer is the secret to these things, and many just won't work efficiently (hot) or put enough back EMF into the FETs that they get hot or fail from overvoltage.    Mallory built bazillions of mechanical vibrators and vibrapacks and literally wrote the book on how to design power supplies using them - you can find it online:   http://www.tubebooks.org/books/mallory_vibe.pdf

There's a whole chapter on transformer design (and more) - but of course you can't buy one nowadays.  However you can scrounge them - or experiment with what you have and maybe find one that has the desired magnetic characteristics.   Vibrators typically ran around 100 - 120 Hz.    I haven't tried a 400 Hz transformer yet but it might be a good way to go, and a bit smaler and lighter as well.

73, Bob W9RAN