It seems to completely fail to notice the huge tolerances on all the FET parameters.
Results will vary an awful lot on the characteristics of the actual device you have. This cn wipe out any confidence a novice constructor has been building. People tend to blame themselves.
To be fair it does on the schematic page
http://www.till.com/articles/GuitarPreamp/
" I should point out that FETs in general suffer from a serious lack of manufacturing consistancy. The FET VGS and IDSS, the parameters that determine the bias point, can be anywhere over a 5-to-1 range and still be within spec. That's pretty awful, but such is life. It's an engineering accomplishment to design a circuit that can function exactly the same over a wide variation of component parameters. But I can't guarantee that in this situation; there's not enough supply voltage headroom.
The upshot is that while this preamp circuit is designed to work with typical J201 FETs, it will not work with all of them and it would be a good idea to try a handful of FETs and throw out any that don't bias correctly. An easy test is to measure the voltage at the drain of Q1 and if it's between 5.0 and 7.0 volts things are fine. "