Question concerning MOSFET/JFET usage, and a possible biasing method

[joelap]

Hey guys.  I'm going to begin trying to build my own overdrive pedal on a breadboard to see what happens.  But before I do that, I have a couple of questions.

1) MOSFET/JFET, can they be used interchangibly?  I understand the differences in their construction, but I'm just wondering if one can be used while the other is called for.

2) In regards to the biasing method in many of the amp sim pedals like Dr. Boogie, 100k trimpots are used to bias the drains at 4.5V.  Would it not be possible to use a 4.5V Zener to ground, and then take the voltage from that point for all of the drains?  Conversely, woulding your simple "vbias" voltage divider on pedals like the proco rat/tubescreamer work as well also?  Or is the trimpot method the only way because the resistance plays a part in the gain equation?

3) Compared to, say, the AMZ mosfet booster or Tim Escobedo's Duende which stay clean for a large dB gain, what makes one stage of the Dr. Boogie distort?  Is it as simple as just amplifying the signal so it exceeds the supply rails?  What makes one stage clip where another stage will stay very clean?  Are there any common techniques that force a FET/BJT into distortion (like diodes in the feedback loop?  Everyone knows the tube screamer method of amplifying the signal, and the higher it is amnplified the more that gets clipped.  I'm more concerned with components like FETs.

Thanks for any clarification.

[newbie builder]

JFETs and MOSFETs can't be used interchangeably- one is a depletion mode device that needs a negative input voltage (JFET), the other is an enhancement mode device.

[joelap]

OK, thanks that helps.  Now what about the biasing thing with the 4.5V Zener?  Anyone know?

[spudulike]

Biassing all jfets in eg a ROG emulator to Vcc/2 is not the way to go. All jfets have tolerances and a batch will test out to different values. The Vcc/2 (ie 4.5V) is a starting point, and you trim by ear to get the best sound to your ears. If it was as simple as you suggest then all drains would be taken to a rail-splitter and left at that.
The whole point of the trimpot is to adjust to taste, and accomodate a range of jfets. I dont think any of my jfet builds were set to exactly 4.5V, and I know for a fact that one has bias points up to 6V.....

Mosfets are a different beast, and bias differently.

[petemoore]

  the zener is used for mosfets for protection, for mosfets that don't have diode protection.
  It limits G/S voltage, some mosfets are sensative and can blow, must have been humid [low static], I've gotten away without the protection zeners using 2n7000's and IRF510's etc. by handling only the body when installing in socket.
  I hear they might blow even after installation...