subbing a tranny for a mosfet or jfet?

[marrstians]

is it possible to trade trannys and mosfets and jfets with each other in a circuit? what rules would you need to make them work in the application?

[petemoore]

What circuit are you talking about?
  Ive used all kinda Q's in gain stages. Some re-biasing may be necessary.
 Certain applications like the EA tremolo's LFO make the transistor choice, there are certainly another way to do it, but with a disadvantage, such as requuiring more parts.
 Bipolars IIRC do more voltage gain, Mosfets are better for curren gain apps.

[Boofhead]

There's no simple answers, each circuit requires a specific analysis to see if the change of part completely breaks the circuit or if it's acceptable - the degree of what is acceptable can only be jugded by you and for that specific circuit.  No circuit will be unaffected by the sub.

The problem comes down to whether the sub'd device is biased properly then after that if it behaves sensibly in the given circuit.

If you aren't into pspice circuit simulations then your best bet is to stick the new device in the target circuit and check the bias point.  If it's bad sometimes you can make adjustments.   The next step is to check it sounds OK.

Looking at circuits using different circuits involving different devices will let you make comparisons to whether the biasing is sort of the same.

[R.G.]

I guess a better question is "why?" What result are you trying to achieve by changing between types? Or is this an easter-egging expedition?

[WGTP]

Check out the Multi-Face at Runoffgroove.com.  It does that.

[amz-fx]

Almost any circuit that uses a bipolar transistor can have a mosfet substituted in place for a different, and many times improved, sound.  Usually the resistors providing the gate (base) bias will have to be changed because of the higer turn-on voltage of the mosfet but that is easily done.  Look at the Muffer on my web site...   you can drop a BS170 into that circuit easily.

The fuzzface can be modified to use mosfets, as well.  How about the LBP-1...  sure, that too.  Even an all mosfet Big Muff derivative is easily accomplished.  If anyone is interested, I can post a sample schematic of the MosMuff.

regards, Jack

[petemoore]

Anything that helps me to further understand Mosfet biasing...
 As an added feature [since, if your'e drawing one up anyway] could you include the substitute R values for Mosfet and Bipolar biases on the same page?

[R.G.]

Anything that helps me to further understand Mosfet biasing...

Ah. OK.  I already did a tutorial on that. See MOSFET Boosters, Chapter 1 (http://geofex.com/Article_Folders/mosboost/mosboost.htm
for an in-depth discussion of biasing MOSFETs.

The short course is that MOSFETs are biased like bipolar transistors, except that you have to overcome the Vthreshold of the MOSFET to get things started, much like you have to overcome the turn-on voltage of bipolars. The MOSFET can need as much as 3V to *start* turning on, bipolars are always 0.4 to 0.6V when they start.  But read the article.