This is the photo from my watsonseblog dated about Sep 18, 2011. I don’t have the text. I may have it saved somewhere, if I can find it.
The toroid core was an ICH-ZJ43615TC from Surplus Sales. They most likely have sold out, but you can find the datasheet here.
Viewing the photo, one can count the primary and feedback windings. The secondary HV winding could probably be counted, but it really makes no difference; just add turns until the desired voltage is reached. Other than that, the rest of the circuit is just a regular JT. The rectifier was a BAV21 250V high speed diode, but the peak to peak voltage was probably higher than 250V. I should’ve used a UF4007 or put two or three BAV21s in series.
I see the cold end of the HV secondary is tied to the primary and collector for added voltage. I could have used a voltage doubler with two diodes and two filter caps. I had to put three 63v capacitors in series to get enough voltage. It was probably close to exceeding those, too. If you use some larger HV caps. remember that the voltage can be hazardous or lethal, so be careful.