JFET Auto-Biasing

[aethertransit]

So has anyone else figured out how to auto-bias JFETs for use in Fetzer Valve type circuits? By auto-biasing I mean leveling the playing field for all JFETs by using simple fixed value parts, thus eliminating the need for transistor sorting, trimmers, or additional active devices. I'm thinking that from a manufacturing standpoint this would save an enormous amount of time and money.

[R.G.]

That is a remarkable understatement.

Pretty much the actual electronics industry ( instead of the new generation of thousand-blooming-flowers effects makers ) dumped the JFET long ago, largely on the basis that you can work and work and work and make things better, but you still have to mess with them, and there are generally other ways to do the same thing that do not involve relying on the huge spread of JFETs.

From your post, I'm guessing that you want to save that enormous amount of time and money when you're manufacturing Fetzer valve type circuits.

I wish I could help you, but like so many other situations where a particular innovation would save an enormous amount of time and money, if one was known, it would probably be everywhere by now. Since it's not ubiquitous, or even rumored, it probably does not exist. Communications are simply too easy today.

[aethertransit]

Hey R.G. I'm a huge fan and before I go on, I just wanted to say thanks for everything that you've contributed to the music industry over the years. So my original question was if anyone ELSE had figured this out yet, as I developed a method to accomplish this about a year ago while doing R&D for a major music electronics manufacturer. In typical corporate fashion, the manufacturer got word of the idea and wanted to claim it for themselves. Needless to say I told them where to cram it, and now they're down one engineer. At this point I'm using this biasing method in custom designs that I build for various guitar players throughout the industry, and almost unbelievably I haven't encountered a single problem. 100% success rate. I posted this question out of morbid curiosity, and honestly I'm not sure how to proceed from here. Thank God I'm also a guitar player.

[Thecomedian]

im not going to claim much working knowledge about this stuff, but fets are, or at least have been, used in integrated circuits for the benefits they provide. That is, they work alongside BJTs.

If you can expand the fetzer valve circuit to include a BJT to "auto-bias" the FET, wouldn't you be halfway there? Transistors are resistors who's value changes, so if you work out the math for how you want the FET to behave, within certain ranges, it could be as simple as one transistor to help control the FET's behavior.

[R.G.]

I developed a method to accomplish this about a year ago while doing R&D for a major music electronics manufacturer.

Well, then, congratulations. There's a first for everything.

It's not constant current source biasing, is it, analogous to emitter biasing of bipolars?

[Thecomedian]

Hey R.G. I'm a huge fan and before I go on, I just wanted to say thanks for everything that you've contributed to the music industry over the years. So my original question was if anyone ELSE had figured this out yet, as I developed a method to accomplish this about a year ago while doing R&D for a major music electronics manufacturer. In typical corporate fashion, the manufacturer got word of the idea and wanted to claim it for themselves. Needless to say I told them where to cram it, and now they're down one engineer. At this point I'm using this biasing method in custom designs that I build for various guitar players throughout the industry, and almost unbelievably I haven't encountered a single problem. 100% success rate. I posted this question out of morbid curiosity, and honestly I'm not sure how to proceed from here. Thank God I'm also a guitar player.

much as it sounds unfair, that would be why a person does private R&D if they want to claim the idea as wholly theirs. If you work for a company, the company is a part of that idea creation by extension, right?

[aethertransit]

Hey Comedian, thanks for the input on this. Yes it's been proven that would work, as would methods such as the constant current or "Mu Amp" design, but I was talking about keeping it simple and not using any additional active components. Just so you guys know, the idea originally stemmed from the need for a new preamp design that didn't involve regurgitating the same old diode clipping designs or lifeless digital emulation. MOSFETs were first used in a manner similar to a certain well known modular design, but the signal characteristics weren't what we were after. JFETs were the only option, but the production time and cost couldn't be justified.

[aethertransit]

It's not constant current source biasing, is it, analogous to emitter biasing of bipolars?

No.

[aethertransit]

much as it sounds unfair, that would be why a person does private R&D if they want to claim the idea as wholly theirs. If you work for a company, the company is a part of that idea creation by extension, right?

That my friend is a long story. 

[Thecomedian]

I'll get back to you in 6 months after I've thoroughly exhausted my vintage and 201 Jfet supply in builds  

[aethertransit]

I'll get back to you in 6 months after I've thoroughly exhausted my vintage and 201 Jfet supply in builds  

Yeah, right?  And I still don't know why all of the transistor manufacturers decided to stop producing all the decent through-hole FETs. I've spent hours on the phone with Fairchild and Toshiba with no luck. The only thing that they could tell me was that they have no plans of making any replacements.

[Joe]

The best way to minimize the variance is to use high source/drain resistor values, also increasing the gain. J201 gain is so freaking low that it's actually an improvement. And some positive bias helps instead of just a single 1M to ground.

Otherwise if I were making JFET pedals, I would just test and bin the stupid things according to the correct drain resistor value.

[Gus]

combination bias?
http://www.vishay.com/docs/70595/70595.pdf

I saw a schematic of a Fender amp with a jfet preamp in a book that looks like it did not need parts selection because of the way it was designed.

[aethertransit]

The best way to minimize the variance is to use high source/drain resistor values, also increasing the gain. J201 gain is so freaking low that it's actually an improvement. And some positive bias helps instead of just a single 1M to ground.

Otherwise if I were making JFET pedals, I would just test and bin the stupid things according to the correct drain resistor value.

Right, similar to booster designs such as the Stratoblaster or EP Preamp. The problem involved with that approach rears it's ugly head when you try to cascade multiple stages. The idea was to keep the gain characteristics as close to being valve-like as possible while keeping the parts count down as well.

[aethertransit]

combination bias?
http://www.vishay.com/docs/70595/70595.pdf

I saw a schematic of a Fender amp with a jfet preamp in a book that looks like it did not need parts selection because of the way it was designed.

I've studied that particular application note in the past but the method I'm talking about isn't represented there. That's interesting about the Fender design, I've worked for Fender and I don't recall ever seeing anything like it.

[Joe]

I don't get the point of this thread at all.

[aethertransit]

I don't get the point of this thread at all.

What's not to get Joe?

[gritz]

I don't get the point of this thread at all.

The point is something like "trimmers are evil." Having to individually tweak every circuit you produce costs time and money. Far better to be able to just assemble something and know that it will work.

[aethertransit]

I don't get the point of this thread at all.

The point is something like "trimmers are evil." Having to individually tweak every circuit you produce costs time and money. Far better to be able to just assemble something and know that it will work.

Yes exactly! Trimmers ARE evil, and a huge pain in the ass. And so is going through thousands of active devices looking for a few that will get the job done.

[Thecomedian]

jfets work in boutique circuits because the cost is offset by the low supply of the pedal causing greater end price, whereas the supply of mass production units is too high, so each individual one isn't worth manually tweaking. the irony of mass production is that it invariably makes itself obsolete, since you'll eventually flood the market, and then you cant sell anything, let alone lower ppu that mass production originally allows. (unless you design your product to fail/break after X years!).

Dumble amps and boutique pedals don't have that same problem.