thunderchief - set values for trimpots?

[vdm]

hey all,
firstly i'd like to know why when we use jfets, they are almost always biased, with trimpots - why is there no simple equation like using a drop-down resistor to take the 9v power and halving it? does each jfet accept voltage differently or something???
and i'd much prefer to use sockets than trimpots, so does anyone have any idea of set biasing resistances for the jfets, or does it vary totally with relation to the jfets???
thanks for the help - and by the way, im going to build the thunderchief but because i love orange amps (and cant afford one) i'm going to put in the comp values from the '72 MKII orange preamp. i was wondering though, why are the cathode bypass caps so big in the orange (250uf) but only very small (0.068uF) in the thunderchief/marshall???
thanks for the help everyone
trent

[spongebob]

I don't think there is an easy way to bias JFETs without using trimpots, mainly because the required resistance varies even between units of the same type. Of course you could replace the trimpots with resistors after you have dialed in the right value.