I'd be happy to model the circuit. I have the Supratex Spice models, so I can model circuits with the DN2540. I'm pretty sure I have a model for the 5687 tube too, but I'll check. If not, I know where to get it; Check here.You might ask OrCAD to send you their demo CD. Click here to go to the request form. It will analyze circuits having up to 50 parts (I think), which is plenty for many tube designs. I'll be happy to make a model of any of these circuits we're talking about, but I think you would enjoy having the demo version of OrCAD/PSpice.