It's not so much about being worried; it's more about knowing all potential risk factors ahead of time. So you say if the Mos-Fet fails then it doesn't affect the tube? Because if that is the case then there is nothing to worry about. There was some posts on the net suggesting that there could be a problem should that happen; putting the entire B+ across the tube.
Hey-Hey!!!, I can say I have not encountered that sort of failure. The usual is a gate-source short on the lower MOSFET, and this is prevented by the back-to-back Zener clamps.
When I have had a G-S short with a battery circuit, the short will discharge the batteries through the R-set and ~250k filter resistor. One discovers the problem quickly and a 'trace' gets lifted to open the circuit...:)
Anyway, I don't see much risk, either to equipment or the wallet, and certainly not the operator( that would be US ). cheers, Douglas
Hey-Hey!!!, The battery bias (from output end of R-set to the source) allows a larger R-set value for a given current. The battery voltage plus the bias voltage is used for the V, i is left as the design value, and R is solved for with Lord Ohm's eq'n. One can use up to 4 of the 3,3Volt Li-Ion button cells and not worry about Zener leakage on the G-S clamp.
The batteries are then R-C fileted/decoupled from the gate with a 250k resistor and .015 or bigger capacitor. The Russian teflon is a fine piece in this position, but a small poloystyrene/foil should be quite good as well. cheers, Douglas
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