"Killer" Screen Driven Push-Pull Amp Initial Queries

[wrenchone]

With the triode/Jfet input solution in mind, I'm going to spend some time tracing load lines to make sure that the Jfets will survive even under extreme overload/overdrive (happens all the time...) I may need to add clamps on the Jfets to protect them. The Jfet/triode combination is not the most cascodey of cascode solutions, as the Jfet drains will wander around some with drive extremes. but I'm more interested in protecting the Jfets than I am providing an ideal cascode environment. If I use Jfets like the PN4393 and J309/310 with low miller capacitance, this will not be a big concern. With the 2SK170s, I'd have to look into it a little more.

[wrenchone]

I've been wobbling back and forth as to what output iron to use, then an opportunity came by that settled things. An Ebay seller offered output transformers meant for a 160-200W guitar amp. I nabbed a pair at a price lower than a comparable Edcor offering. Initial measurements indicate that they are 7k:8 units, with a primary inductance of a whopping 600 H, measured with a sine wave input of 100V, 300 Hz. They weigh in at 11 lb apiece. I think I will have low end extension if I want it. I suspect these were intended for use with selected tubes at 700V B+ or so to get the stated output rating. I'll need around 650V B+ to reach my stated goal of 100W per channel. With this turns ratio, any sweep tube worth its salt will laugh at the voltage and current.