MPF102's and Fetzer Valve questions

[Earthscum]

Hey all. I have a couple questions concerning Fetzers.

1) I've built a couple FV's according to (of course) Runoffgroove.com, and they appear to work, although I haven't managed to get anything but a straight signal through them. I understand that the Rs and Rd are starting points to get you within the proper d and s voltages. Say, for example, that the calculator says to use a 100k Rd and a 10k Rs (just for ease of argument), but I want to actually get GAIN out of it. If I went with the same ratio, using (again, simply for ease of argument) 10k Rd and 1k Rs, I would be pushing the same voltage to S and D, but it would give more gain, correct? Basically, that's my first question (I'm just starting to understand, not just know, how to bias stuff). Every Fetzer I've made so far has no gain at all, but I gotta admit that putting it right after a distortion gain stage does smooth things out alot (at least with Bass guitfiddle). To get gain, can I rebias and keep those same voltages, or am I better off using a different FET, like the j201? Subquestion: Say I use a J201 in first gain stage to drive a MPF102, would this give me a little more reactance and headroom, but still retain a nice distortion?

Next:
2) Those damn ratshack MPF102's. Someone else in town was hogging alot of them... now we're fighting over all the mosfets in town, lol. Anyways, ROG's site claims that:
Out of twenty-four (24) MPF102
average Vp: -2.34V (spanning from -1.76 to -3.11)
average Idss: 5.65mA (spanning 3.91mA to 7.15mA)
average Rs: 344 ohms
average Rd: 688 ohms
average Gain: 1 times or 0 dB

Every one I've obtained has been measuring approx. Vp=3.01V  Idss=0.89mA (0.089V). I even built a couple different testers, and even went as far as to specially select the resistors, and still only varied off my original measurements by about 2/100ths. Is this a problem with newer MPF102's? Did they meeeasure wrong, possibly? Has anyone else had MPF102's that actually hit within what ROG measured? I normally wouldn't bug so much about it, but that Idss is WAAAAY off from their findings.

[DSV]

The Vp seems OK to me, and is within the ROG tolerances (apart from the - sign, which depends on how you hook our voltmeter). The Idss seems indeed very low ...have you tried with other scales of the multimeter? Mine tends to underestimate the values, and this behavior is especially apparent for low values on a given scale (ex. for the 2V scale, when values are under 200mV; for 100mV signals, the error can be as high as 50%, compared to the measurements on the 200mV scale).

[Earthscum]

the volt meter thing is something I was kinda wondering about as well... at 500 I get nothing, at 200 I get nothing, at 20 I get .1, and at 2000m I get the actual reading. I am using a Mac Tools EM310 digital meter. It seems to work good... on a 9V battery I read 9V with no load, and 8.6 with a load. it appears to be able to tell me the forward voltage drops on my diodes (in the diode test mode, of course). I DO know that the ohm meter works from hand selecting resistors. The volt meter seems to work ok... when I did my other fetzer valves, the resistances calculated gave me voltages that were within a couple ten mV's of what it was supposed to be.

[Earthscum]

Well, Apparently my meter is fine... I just found out what's going on with the MPF102's.

I was looking into a comparison of the beloved J201's vs MPF102's, and came across this little project:

http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm

On THIS PAGE, they kinda dig into the gain issue.

So, I guess all my MPF102's are going to be used for buffers (instead of tossing them in a box and never touching them again). I'm really disappointed, lol... but not surprised. Guess I get to take a bike ride across town and get me some J201's. Hopefully. The only shop we have in Fort Collins carries National Semiconductor, which isn't too bad, but there's a couple things I can't get there and have to do subs for... and they are kind of expensive. So far, what I've bought there and RS, I could've got through Small Bear and saved about $100. So much for trying to keep my spending as local as possible, eh?

[stm]

Every one I've obtained has been measuring approx. Vp=3.01V  Idss=0.89mA (0.089V). I even built a couple different testers, and even went as far as to specially select the resistors, and still only varied off my original measurements by about 2/100ths. Is this a problem with newer MPF102's? Did they meeeasure wrong, possibly? Has anyone else had MPF102's that actually hit within what ROG measured? I normally wouldn't bug so much about it, but that Idss is WAAAAY off from their findings.

Your Vp reading is perfectly normal.  In fact, I wouldn't be surprised if someone found a specimen as high as 4 or 4.5V.
The Idss reading is clearly wrong.  It should rather be 8.9mA instead of 0.89mA.  There is no way an MPF102 has such a low Idss reading.  I tend to think that in your JFET tester you are using a 10 ohm resistor instead of a 100 ohm resistor.  This would explain the low voltage reading when attempting to measure Idss.  I suggest you measure your 100 ohm resistor to make sure it is 100 ohm.

Next, the MPF102 will have near unity gain when fed from a 9V battery.  You can get actual voltage amplification if you use 18V instead.  You can verify this with the online calculator.

Another possibility is to add a large source bypass capacitor (around 10uF).  This will raise gain by about 6 dB when using the suggested Rs and Rd.  In this case the harmonic content will deviate from the Fetzer valve target, and will be higher and mostly 2nd harmonic
.
Finally, for even more gain, you can try using a large drain resistor, like 100k, and then tune the source resistor so the drain voltage biases properly.  You need a large source bypass capacitor in this case.  This will give even more gain, at the expense of a great deal of 2nd harmonic distortion.  The Omega applies this principle.

[Earthscum]

Hey, thanks for the info. I have tried a couple setups so far, and still getting less than unity until I used a source cap (tried 10u-470u, about 10 or so different caps).

And I doublechecked my tester, and sure enough... I have a freakin 10R instead of a 100R. I don't know how that slipped by me so many times (brown-black-black vs Brwn-Blk-Brwn) grrr... thanks for catching that one. I bet I get a bit better results. I should still be able to use my tester, right? I just have to take the reading x100 instead of x10, or should I just unsolder and put in a 100R?

I've read about the source cap (that's why I tried the cap trick from that article I linked above). I was wondering if there may be some other way, besides running a pedal with 2 batts, to get gain out of these things?

an idea: basically I just want toe clipping characteristics of the Fetzer valve. I tried building a pair (given the readings I was trying, I don't know how they worked at all, lol), and then used a booster circuit to drive them into clipping, but I could still hear the tinny bjt sound. Would a mosfet booster be a closer approximation to the JFET sound? I haven't got a chance to play with MOSFETS yet... they are scarce around here... someone keeps buying them all as soon as they get in. Grrrr..... I haven't even been able to get my hands on an IRF510 from Ratshack. That's because someone stole what they had, so computer inventory says they have some in stock and won't allow them to order more until some manager comes down, blah blah blah.