3 of them are PTV series models, about the size of a yellow pages in a large city. Those were bought on ebay for cheap. All three worked fine, and Spellman was forthcoming not only on the info I needed to fire them up (these don't have a front panel), but on the specific mods that had been done to the ones I have - a lot of these were custom, and in my case I was lucky to get some with specs better than the nominal catalog ratings for that line. All 3 of those are 40kv max, one positive, two negative, and all put out more than the 5ma ratings -- one pair puts out 12.5, which is for sure enough to do real things with.
The fourth one is a brand new SL2KW that is the main supply for the fusor here. This is rated at 50kv and 40 ma. It exceeds that by a comfortable amount. I very rarely do (or can) use full power out of it before things burn up in the tank, of course, but it's nice to have. It answered for once and for all that nagging tickling in the back of one's mind that maybe a little more stuff would help more than proportionally. Now I know, and don't have to worry, guess, or fool around trying to get "more", as this has it already, and I rarely set its current limit above 12 ma.
They were nice enough to cut me a decent price on this guy, after helping me with the ones I didn't even buy from them - now that is a cool way for an outfit to act. They have me as a customer from now on.
I did have one spot of trouble with the new one, but it was my bad all the way. I let a bug (yes, literally an insect is our best guess) get into the thing, it found the worst place to be, and this resulted in a multi KW arc inside. Not really knowing where the trouble was, I disconnected it from the rest of things, and tried again -- arc again above some output voltage. Despite that mal-treatment, the only damage was some carbonization near the HV input to the stack, which was easily fixed. I called them, and they gave me two options -- ship it back, they'd fix it, but it would take awhile to go through the process and re-qualify, or they'd just help me fix it (I'm an adroit technician). We did the latter, and they supported me through the weekend to get it done, it works again fine on the first pass through repair, the problem is really fixed for sure, and no worries again -- it's run many hundreds of hours since that in my normal power supply abusive conditions, never breaking a sweat. As an EE, I would simply point out that the fact all I had to do was repair arc damage speaks volumes about how well the thing is basically designed and put together -- zero semiconductor failures or anything like that -- just arc damage, easy to fix because it was smart enough to shut itself off quick. This stuff is designed and made right.
I will note that the big guy has 8.5 nf of stored charge in the stack, requiring a good ballast to protect other things from arc situations. The smaller guys have less stored charge and maybe don't need that as bad, but for good fusor operation, it's still a good idea. We have ballasts made from the bulk ceramics CliffS has suggested, and of either seriesed 10k 10w wire wounds (a lot of them in either case) or a huge WW 100k 225w resistor that we mainly use -- under forced-air cooling.
I did start out with homebrew, and probably will do more of that. But that's a hobby of its own, so if you just want to get on with it, this is a decent way to get that other part of the elephant stew (let's call this the tiger) so you can proceed to cook, and eat.
Yum, thanks guys
- Spellmans at play
As you can see, I run these in conditions that might be called non-optimal, but they don't seem to mind. In the upper right back is our big ballast (inside the screen, with a ribbed pipe carrying forced air to it), a smaller one made of series-parallel bulk ceramic resistors in pyrex is laying near it, not hooked up, as in this picture, I'm using the ballast L I encapsulated in a pill jar as a ballast for one of the PTV's. The homebrew box with meter is what controls that. I stay behind all that lead and lead glass when running -- this much power can make some pretty fierce X rays. This shielding scheme gets them back to background levels at the operating position.
