No, it isn't from Pete Millet's site but thanks for reminding me.I saw several space charge circuits here and there, and this was the simplest one. They were running it with 12VAC on the heaters, but I think I'll try it with DC, same as B+. Seems like the heaters will attract electrons from the cathode even more than the anode does, but I guess that's just part of the whole space charge thing.