Out of my Vulcan Mind!

[jafo]

I built one stage of the jfet Vulcan a couple of days ago, albeit with suboptimal subs (MPF 102, wrong sized caps), and I have to say I'm impressed. I think I'll include diode compression in the next dozen or so discrete builds -- it really does help the feel. Many thanks and much good karma to Joe Davisson!

I have a Q or 2. First, why are the source and drain resistors so much higher than in other designs? I'm still really hazy on the theory, but does that allow for more juice to flow out of the drain (as opposed to ground or back to the battery) than, say, 470 and 2k? Also, most designs seem to use a 4:1 or 5:4 drain:source ratio, and this is a 3:1; does that have something to do with the specific Q point?

Anyway, happy Nigel Tufnel day, all (while it still lasts).

[jafo]

Well, the dates still go to 11 here, at least (in fact, it's 11:11 right now).

[PRR]

> > why are the source and drain resistors so much higher than in other designs?

Designer's Option.

Low-value resistors can put more Power to a load but give less Gain.

> most designs seem to use a 4:1 or 5:4 drain:source ratio, and this is a 3:1

Not that simple. You want to look at the ratio of drain resistor voltage drop to JFET cut-off voltage Vgs(off), AND whether you want the JFET run nearly full-on or nearly full-off.

These ratios may be nearly consistent for all 9V pedals but not if designing for 15V, 24V, 40V.

Also different for different JFETs. And since there is no trim and no selection guide, we hope that the biasing is figured to "work well" with any worst-case J201 JFET.

Also note the 10Meg Diode 1Meg network at the Gate. This raises the gate voltage so the biasing is not solely about the JFET. The Gate will sit higher than ground, perhaps at +1.3V. The source resistor must be larger to keep the same current with the higher gate voltage. And in fact this 1.3V partly "swamps" the 0.3V to 1.5V variation among J201s.

Missed the cutoff for 11/11/11.

[jafo]

Cool, thanks.