there is always detail in datasheet, saying how much voltage for gate going above source is needed to make BigBadaBoom ; that's usually in range of 20-ish Volts
use 9V1 , as you can see in plenty of Papa's schmts and you're good - bigger enough than working Ugs, while lower enough than Dodo voltage
practically , everything between 7V5-16V, what you have in drawer
use 9V1 , as you can see in plenty of Papa's schmts and you're good - bigger enough than working Ugs, while lower enough than Dodo voltage
practically , everything between 7V5-16V, what you have in drawer
Here is the cap plot of the Semisouth SJEP120R100
Ciss also given at DS 1000V
Difficult to estimate the values at 20 Vds
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1W is enough
in fact , 0W5 are customary used there
in source follower arrangement , I would always look first to use part with greater xconductance, after that looking at capacitances , and think of linearity in last place
ppl are looking for laterals thinking about better linearity in most cases, but here that's not so critical (follower thingy working in that manner that source is always trying hard to follow whichever way gate is traveling)
The MOSFET gate glass WILL blow-up at 40 Volts.
The datasheet says about "20V" to cover for bad days or careless users.
We NEVER need even 10V of gate drive.
Generally not even 5V, for post-1990 MOSFETs, in audio circuits (we do not need 50 Amps).
Power? What are you putting into the input jack? Few preamps can deliver over 0.1 Watts (8V in 600 Ohms). The Mofo wants more voltage but that often means less current. And the diode will only conduct half the time.
6.8V 0.5 Watts is a happy value. If you have a bag of 10V 1W, that's perfect also. (I would not want to use 5W: they leak more, and are more expensive for no benefit.)
Thanks a lot for your explanation! 
I am using the IRFP250 as suggested in the original MoFo document and I am running it at 2,5A / 24V power supply (using Hammond 193V choke). I will try with the 10V 1W that I already have at home, then.
By the way, I am driving it with a slightly modified version of the Bride of Zen in order to have a max of "clean" 24Vpp to drive the mosfet...
I am using the IRFP250 as suggested in the original MoFo document and I am running it at 2,5A / 24V power supply (using Hammond 193V choke). I will try with the 10V 1W that I already have at home, then.
By the way, I am driving it with a slightly modified version of the Bride of Zen in order to have a max of "clean" 24Vpp to drive the mosfet...
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No. I'm sorry that's a dumb idea AC-wise. No idea where I was going with that. Disregard.
I tried to add Gate-Source zener (10, 18 or 20 volt is the same) but it still happens that MoFo starts absorbing more than 10 amps if I drive it with a signal while load is disconnected. Any suggestion to avoid it? I am just thinking of a "fast fuse" on Vcc to immediately shut down MoFo if it starts absorbing more than 4-5 A. It is not a very elegant solution, I guess...