Adapting a JFET circuit to be BJT-friendly

[11-90-an]

Here's the circuit/s I'm trying to adapt... The Fetzer Valve found on the ROG website...



or



(Of course, I'm doing this for the availability of BJTs here and lack of FETs, but when I get dem FETs I'm gonna build it "properly")

Here are the stuff I know:
- When overdriven BJTs sound very different from overdriven JFETs
- I can't just drop a BJT in replacement of the FET directly, as in the circuit the JFET can self-bias with Vgs (or something like that)
- This circuit has no voltage at the gate, so Vg = 0... but BJTs need the voltage base at least 0.6~0.7v above the emitter (or something like that)
- No feedback from drain to gate, so BJT variant probably shouldn't have collector base feedback too... am I wrong?

Questions:
- Best way to make the circuit output JFET-esque overdrive sounds? (Even if it's complicated, I still do want to know, to feed my curiouity... )
- Best way bias BJT for this application? I'm guessing something like this...

- gain is calculated as gain = Vc / Vb?

[iainpunk]

you can't polish a turd.

there is no decent way to get BJT to sound like Jfet. but in general, i don't think it matters. the bjt sound is still quite nice and asymmetric.

if you leave out the clipping diodes, it sounds quite good for a transistor gain stage.

cheers, Iain

[antonis]

"Adapting" doesn't stand when dealing both for FETs and BJTs..
Common Source and Common Emitter amps are inherently different configurations..

You can take advandage of higher CE amp gain (due to higher BJT transconductance) in the expence of more complicated bias/bootstrap configuration (to maintain high input impedance)..

>- gain is calculated as gain = Vc / Vb?<

The above may be rather confusing..
Better consider Gain as R_L*I_C/0.025 ..