No J-fet luck

[jrc4558]

It's amazing, it truly is.

I tried about 15 different DIY overdrive designs implementing J-fets as gain stages.

NONE worked.

Is there a magic word that has to be said before powering up a j-fet cirquit?

[Joe Davisson]

Different JFETs have different pinouts... And all bias differently. Which ones are you using in particular?

[brett]

Tried the "Odie" at runoffgroove.com ?  It's a corker!!  Blissful overdrive, searing tube-like distortion, ....  

[ErikMiller]

After years of dealing with nothing but silicon transistors, about 50% of the time I hook up JFET's as if the gate were the center pin, even when I know it's not.

I mean, it sort of makes sense, doesn't it? The base on most silicons is the center pin, and the gate is analogous to the base....

Right? I'm not crazy, right? :-)

[Paul Marossy]

When I first built my BSIAB, I had lots problems. It wouldn't function properly. I put it away for nearly a year and pulled it out the other day, pulled out the JFETs and put sockets in. I just randomly picked five of them out the envelope and popped in the sockets, and bingo! It sounds great! JFETs are a pain, though. They can be quite variable in their specs., within the same device number. This can lead to some unexpected results...

RG has a good write up on that at GEO.

[Tim Escobedo]

The most immediate things that come to mind are pinouts and biasing.

There are no conventions regarding pinout, so you must know which pin is which.

And biasing can be rather tricky. As said, JFETs are much more prone to device variability. I generally suggest trimmers be used on drains or sources so that biasing could be dialed in. There's no gurarantee that a circuit using a J201 will work with your J201 without some bias tweaking.

Other than those two provisos, I find they are  pretty easy to use.

[Paul Marossy]

Yeah, using trimpots is a lifesaver!

[b_rogers]

i just tried to build the grace and big daddy on runoff groove and i cant get either of those to work. the drive and vol pots work, but no distortion... jfets dont work in my multiface either. oh well, maybe a trimmer on source and drain will make it work?

[jrc4558]

Hi, Joe.

The ones I have an easy access to are 2N5457's, MPF102, BF245

They all work excellent in a buffer mode, but truly suck as a gain stage.


To all. Even when I use a trim pot to adjust the fet into operating range there's so much noise that I justgive up on them.

Don't get me wrong here, I built a Zombie chorus on a pad-per-hole AND it didn't tick!!! So I know how to do it. Maybe the fets are just bad.

Who would you guys recommend as a supplier?

[smoguzbenjamin]

Hey constantin, Do you have a pad-per-hole layout of your zombie perhaps? I've been thinking of building it 'cos it'll save me having to dial in chorus sounds on my flanger... But t seems complicated to build on perf....  :roll:

[Peter Snowberg]

These days I buy most of my parts from http://www.mouser.com but http://www.digikey.com sometimes has parts that Mouser doesn't.

For capacitors and resistors, I have been quite happy with http://www.circuitspecialists.com

Take care,
-Peter

[petemoore]

Dally rant...
 Seems everyone would be happy if the signal ALWAYS went to the middle pin...on transistors anyway...
 Makes me wonder if something about the construction of FET's makes the gate naturally come off one side...?

[jrc4558]

So? Any ideas?

I really want to build an Odie from runoffgroove. Tried a few times - no luck. J'fet's tried - 2N5457, 2N5458, BF245, J201, MPF102

[javacody]

I doubt its the pinouts if you tried the 2n5457 or the MPF102 as their drains and sources are interchangable (not sure about the others). Obviously you are doing the same thing wrong in every one of these circuits otherwise you would have gotten one of them to work.

Can you try one of Jack Orman's JFET buffers and see if you can get that to work? Then report back to us?

Edit: Nevermind, I see you've already tried in buffer mode. I've been playing around with JFET buffer's and placing a 100k pot on the drain with a 10k trimmer on the source along with a 22uF cap shunted to ground (this affects tone, play with the value), the cap increases the gain. The higher the resistance on the drain, the higher the gain will be from the JFET. You should be able to get any of those JFETs to distort so badly that your ears bleed.  

As a buffer, I settled on 4k7 on the drain, left the 10k trimmer on the source, along with the 22uF cap shunted to ground. 4k7 was the lowest value resistor I could use and keep any kind of clean with the buffer. Removing the cap really changes this though. You might want to build a simple JFET buffer on perf this way and experiment a bit.

Now, I'm just a begginer and don't truly understand how JFET's work, I'm just relating what I found in my experiments on a MPF102 and a 2n5457.