Neal.
They are called Sonotubes and available from local concrete supply companies. Just call any concrete contractor in your phone book and ask him where he buys his Sonotubes. They are thick cardboard and work very well. I used one with a coil with a 9 ft spark and it's still being used daily in a museum for the past 21 years. Be sure to coat outside and inside with 3-4 coats glyptal of Dolph's AC-43 for best results. Prior to winding. Dr. Resonance
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