Here is a simple Jacob’s Ladder design that uses the transformer from a florescent tube. I fed it with 25V by putting a 20V supply in series with a 5V supply (its too bad the 40V one is out of commission!). I added a Gabriel electrode at the bottom to help start the spark. This work well to start the spark, however tended to stay stuck in one position. To remedy this I pointed an old cooling fan at it to get the air to move. There are a couple of pictures here.
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