Ramifications of a Mosfet instead of a Jfet in a ROG design?

[darwindeathcat]

I was just perusing Syndromet's transistor pinout encyclopedia that he just posted on anther thread, and I noticed that the Jfet's used in most of the ROG circuits (mpf102/j201) have the same pinout as all the MosFets he listed. I know the main differenced between the BJT type of transistors preclude them from being used when a FET type is called for, but what is the difference between Jfet and Mosfet in terms of functionality? Could one stick, say a BS170 mosfet in the place of a mpf102 in a circuit? What would be the result of such a swap (ie. smoke and flames, or just different sound, or nothing?).
  The reason is that I have a bunch of BS170's I got cheap in a bulk purchase, but I only have one mpf102 left in my parts box... I already finished the project that required the BS170's (the solid state tube sounding preamp. See the newbie forum for that thread), and want to start fooling with some of the ROG designs for my multichannel combo amp (see the "preamp for lm3886" thread in this forum)...
  Any sage advice would be greatly appreciated!

[J M Fahey]

They´re similar but the BIG difference lies in the bias: FETs require typically around -2 V on their gates; so they can "self bias" (like tubes) by just tying their gate resistors to ground (0 volts) and using resistors from source to ground. Study your circuits. MosFets require Positive bias (around 2 or 3 volts), so basically you should tie the gate resistors to around + 4 volts supply (Use a preset to adjust it)

[rowdy_riemer]

Well, Enhancement type MOSFETs are biased that way. And, of course, most MOSFET's on the market are enhancement type including the BS170, but depletion mode MOSFETs are biased the same a JFETs.