R.O.G - hand picked jfet importance?

[gregwbush]

I've made a few (more) R.o.G circuits lately...

http://runoffgroove.com/articles.html

Supreaux Deux
Peppermill
Omega
Thor

I'd love to hear from the folk at runoffgroove about this, and of course anyone elses experience

I've noticed/realized/read/etc the Supeaux2 and Thor utilize the 'fetzer valve revisited' topology in selected areas of the circuits; having a trimpot for drain bias, but a fixed source resistor based on the "average" fet's tested by Rog. As many will know, jfets are very inconsistent.

Now what i'm wondering is, since there's a fixed "average" source resistor (1k for j201, 390R for 2n5457)... has it been worth my while taking the time to hand select (using the fet tester http://www.runoffgroove.com/fetzervalve.html), and choosing the closest to "average" transistors i could find from my stash???

My thinking was or is, 'well if i've soldered in a 390R source, then i should pick a tranny that matches this average fet thingy, and adjust the drain with the trimmer to the recommended voltage'.

Or is what i've done maybe overkill??? Will any fet suffice without a big difference?

As a side note, i certainly like the clean side and headroom of supreaux 2. And it might be a placebo effect but i think when i hand picked those "average" transistors, it got sweeter. Thing is, it's tough to tell without A/B ing exact circuits...

So what do you all think???

[B Tremblay]

Now what i'm wondering is, since there's a fixed "average" source resistor (1k for j201, 390R for 2n5457)... has it been worth my while taking the time to hand select (using the fet tester http://www.runoffgroove.com/fetzervalve.html), and choosing the closest to "average" transistors i could find from my stash???

My thinking was or is, 'well if i've soldered in a 390R source, then i should pick a tranny that matches this average fet thingy, and adjust the drain with the trimmer to the recommended voltage'.

Or is what i've done maybe overkill??? Will any fet suffice without a big difference?

I suppose it depends on what your time is worth! 

The intention is for any FET of the specified type to provide reasonably accurate performance, so feel free to use a non-sorted FET.  However, if you do use an "average" spec'ed FET, then you can swap for a different sound by using one at the upper or lower end of the  range of specs.

[diydave]

Maybe this can help: http://www.diydave.be/tools/fetcalc/index_eng.html?
With the fetzervalve fet-tester as startpoint, I went along and made this fet-calculator.
Punch in the measured Vp, Idss and psu-voltage, and you get a list of 10 values for R-drain and R-source.
There's also a graph for which you can choose a certain biaspoint on the Vgs curve, and the appriorate resistors to bias the fet.
Maybe overkill, but it saves me a lot of space on a pcb-board (al those trimpots) and time in twiddeling with different fets and drain-trimpots.

[Earthscum]

http://www.diydave.be/tools/fetcalc/index_eng.html

Maybe overkill, but it saves me a lot of space on a pcb-board (al those trimpots) and time in twiddeling with different fets and drain-trimpots.

I use your calc all the time, I love it. It gives a nice visual reference of the curve, on top of it all.

[gregwbush]

How come calculators and that appear AFTER i do my layout!


.... with trimmers


EDIT... picture waay too big, sorry. what the. pic temp gone
Try again

[Earthscum]

As far as the fets go... they are all over the place, and I think the idea of "what is your time worth" is key. I have tried 2SK117GR's, and personally find them to be excellent at hitting what I prefer for Metal (not tin, rather sledgehammer on an anvil, lol). MPF102's, once "tuned in", give a nice, darker, drive. They don't give out as much gain, probably lowest gain within our useable range in a fet, but they work and some prefer the flavor. J201's sound like, well... guitar sweetness. I don't know how to explain that any better... they just make a guitar sound good, in general. And then I also use the 2N5457's, which seen to sound, to me, possibly a bit darker than MPF's, but with more available gain.

The thing is, the "darker" sounding fets all like to run with more current. 2SK's I have like to have about a 3k3 drain resistor, MPF's seem to run around 1k8, and J201's I have tried like to run anywhere from about 4k7 to 22k. In my experience, the J201's (especially if you have multiple from different batches) would definitely benefit from at least a half-arsed matching, but the others I've used don't seem to vary as much in sound, which seems to be a product of the final gain once you have it "tuned", more than anything.

One thing I do like about Japanese Fets (2SKxx) is they are sorted by grade, so you don't encounter as much variance from device to device.

[gregwbush]

What's my time worth...

Back in high school, a good friend of mine wrote on his folder... "life is a waste of time; time is a waste of life, so let's get wasted all of the time, and have the time of our life"

looking back...
ahhh,

well, it worked at the time

Now....

Now i have time to do this stuff i love....

"The intention is for any FET of the specified type to provide reasonably accurate performance"

"feel free to use a non-sorted FET.  However, if you do use an "average" spec'ed FET, then you can swap for a different sound by using one at the upper or lower end of the  range of specs."


...Well said

How's this... FET's are BETTER than valves... 9v

[Earthscum]

How's this... FET's are BETTER than valves... 9v

Careful... there's a couple 9V tube projects running around here!   

What you said reminded me of a quote from SLC Punk, at the end when Stevo is standing at the steps at the end...

"I mean, there's no future in anarchy; I mean let's face it. But when I was into it, there was never a thought of the future. I mean we were certain the world was gonna end, but when it didn't, I had to do something, so f*** it."

[gregwbush]

Yeah i've seen some low voltage tube stuff... and the charge pump stuff too. Interesting stuff

"One thing I do like about Japanese Fets (2SKxx) is they are sorted by grade, so you don't encounter as much variance from device to device."

Might have to get some and try 'em out 

That anarchy comment is gold btw

[BubbaFet]

I've made a few (more) R.o.G circuits lately...

http://runoffgroove.com/articles.html

Supreaux Deux
Peppermill
Omega
Thor

On a side note... can you give us your thoughts/impressions of these four designs' sounds? What do you like about them? Thanks.

As much as I like JFET circuits, I have yet to build any of the ROG designs.

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