Okay, maybe I need to explain why I did what I did. My ideas may be off, so please let me know if I am missing something (and what).
Zener connection at the gate:
I put the Zenter directly at the gate (not before the the gate resistors) because I was thinking to use the gate resistor as a "fuse". It the gate voltage goes too much negative, the Zener will start to conduct, and the current will flow through the Zener. If the voltage applied to the gate resistor becomes very negative (aproaching the -70 V), the current will increase and blow up the gate resistor, thereby disconnecting and protecting the gate.
Also, Nelson seems to connect the Zener directly to the gate, not before the gate resistor (example:
https://www.firstwatt.com/pdf/art_diy_sony_vfet_pt2.pdf).
+/-70 V for the buffer:
As mentioned in the
thread/post mentioned earlier, the buffer FET (STF3LN80K5) has very low input capacitance (1 pF or so)
if it has 45 V or more across drain/source. To maintain the low input capacitance even under full AC swing (25 Vpk or so), we need 45 V + 25 V = 70 V.
