Black 65 JFET biasing

[fixxe]

I was wondering how I can sort out the right JFETs for this pedal. I want to use the 5.6k drain resistors like the original. Do I just pop them in and then measure? And if so, do the voltages of the JFETs interact with one-another?
Or must I build a vero testing-board and test them there?

[copachino]

I was wondering how I can sort out the right JFETs for this pedal. I want to use the 5.6k drain resistors like the original. Do I just pop them in and then measure? And if so, do the voltages of the JFETs interact with one-another?
Or must I build a vero testing-board and test them there?

the best thing its build the board, then socket the transistors, and then, try fets until you get 4.5V aprox on each try first on 1st stage when you get the 1st good, leave connected the transistor and move to the next and do the same for all

[midwayfair]

Chi_boy's build document has the testing procedure. You need to match the JFETs on their own first, and then if you really MUST use exactly the same resistors as the original, you need to find matched pairs that bias correctly at 5.6K. However, I can tell you it's not going to matter too much and you're going to need a giant pile of J201s to get enough pairs.

You can rough sort the FETs into pairs if the board is socketted, but the bias point of a FET on its own will not be the same as a pair, so you have to do them one at a time before trying each pair.

[fixxe]

Yes! That was the build document I was looking for, thanks!

[garfo]

Hey there, I did the black 65 without matching the fets and it worked great for me. Anyway, by the timeI made that pedal I wasn't sure on how to test the jfets, but now I've found out that someone has a ready to etch board and fully documented pdf to make a jfet matcher pcb. If you want PM me and I'll send you the project. I'll be building one for myself. It's very simple, just 3 10k resistors, 1 10uf cap and a TL071.

[Muadzin]

I use the Jfet matcher that came with the Nomnom. Methinks Madbean should offer it as a separate build just for cases like this. It's easy to measure J201's with that. Giant piles of J201's for testing have become expensive though, and reputable sellers that sell them in such large numbers are both rare and a dying breed. Unless you want to use Chromosphere SMD conversion boards.

Whoever designs these boards that require biased matching JFETS should really incorporate trimpots into them. Using resistors really makes it a pain in the ass. I can understand that pro builders like Wampler don't use them as they got probably shitloads of J201's in stock and since trimpots cost more then resistors try to keep costs down, but for a cloned pedal I'd consider the extra cost for trimpots money well spent.

I bought 30 J201's, discovered several matching groups and found it far, far easier to socket the draining resistors to correctly bias them then to hope that I would a matching group of J201's that would bias correctly to the resistors. Resistors are after all dirt cheap, J201's are not.

[Tremster]

Whoever designs these boards that require biased matching JFETS should really incorporate trimpots into them.

+100

[midwayfair]

Whoever designs these boards that require biased matching JFETS should really incorporate trimpots into them. Using resistors really makes it a pain in the ass. I can understand that pro builders like Wampler don't use them as they got probably shitloads of J201's in stock and since trimpots cost more then resistors try to keep costs down, but for a cloned pedal I'd consider the extra cost for trimpots money well spent.

It's not just cost. For one thing, trimpots can be noisy. For another, they take up space on the PCB and sometimes the point of a project is to do it in a certain size enclosure that might be impossible with the trimpots there.

Also, FETs get matched in these pedals for reasons that go beyond "will this bias at the same voltage?" It needs to bias at the same voltage with the same drain/source resistance while producing the same amount of gain the pedal is designed around. You could have a trimpot on the whole drain assembly, but you're still stuck matching the two FETs. (If you pair them up and one draws more current than the other, it won't function the same way.) If you just match two FETs but they bias +/- a quarter volt or so off from the fixed resistor, then you're not going to produce the amount of gain needed to make the pedal sound right. It's the same reason you use J201s instead of 2N5457s or MPF102s, because the J201 is the FET called for in
this design.

Did you miss where the OP said he wanted to use the same drain resistor as the original? Maybe that indicates that he's trying to build an actual clone.

If you don't care about doing the circuit right, then you could just throw any old thing in there, use a single J201 instead of matched pairs (you'll get about 66% the amount of gain, is that close enough?) and put the FETs on a breadboard first to figure out the closest fixed drain resistance.

Besides, you can solder two legs of a trimpot into a PCB for fixed resistors. The reverse is also true.

[Muadzin]

It's not just cost. For one thing, trimpots can be noisy. For another, they take up space on the PCB and sometimes the point of a project is to do it in a certain size enclosure that might be impossible with the trimpots there.

Also, FETs get matched in these pedals for reasons that go beyond "will this bias at the same voltage?" It needs to bias at the same voltage with the same drain/source resistance while producing the same amount of gain the pedal is designed around. You could have a trimpot on the whole drain assembly, but you're still stuck matching the two FETs. (If you pair them up and one draws more current than the other, it won't function the same way.) If you just match two FETs but they bias +/- a quarter volt or so off from the fixed resistor, then you're not going to produce the amount of gain needed to make the pedal sound right. It's the same reason you use J201s instead of 2N5457s or MPF102s, because the J201 is the FET called for in
this design.

Did you miss where the OP said he wanted to use the same drain resistor as the original? Maybe that indicates that he's trying to build an actual clone.

If you don't care about doing the circuit right, then you could just throw any old thing in there, use a single J201 instead of matched pairs (you'll get about 66% the amount of gain, is that close enough?) and put the FETs on a breadboard first to figure out the closest fixed drain resistance.

Besides, you can solder two legs of a trimpot into a PCB for fixed resistors. The reverse is also true.

I got the part of matching FET's, in fact didn't I say boards that require biased matching JFETS? I've build a Pinnacle and Plexi Drive with matched FET's. The FET matcher that comes with the Nonnom board was a god sent in that regard. For the Pinnacle I had to socket resistors until I found the right ones. The Plexy Drive was a Rullywow board and it was designed with trimpots. Man, that made the biasing so much more easy.

I get the point about trimpots maybe being noisy, and that may be a valid point if you're Brian Wampler, but me being me and dirtpedals being noisy anyway that's a tradeoff I'm willing to make. As for enclosure size, a cermet trimpot is not that much bigger then a single resistor in length and 3 of them next to each other . And unless you're going for a 1590A box I doubt having a couple of trimpots will take up that much more space that it won't fit into a 1590B. The Pinnacle PCB required only two drain resistors that had to be biased correctly and the Plexi Drive PCB was even smaller and still fitted 3 trimpots.

And no, I did not miss the part where wanted to use the exact same drain resistor as the original. The point I was trying to make was that its easier to change resistor values to suit a group of matched JFETS then to find a group of JFETS that will suit the resistors. Because resistors cost nothing whereas a giant pile of JFETS will cost a lot. And as long as the JFETS are matched and then correctly biased it should not change the sound one damn bit. Or am I wrong in that regard?

And yes, you can always solder two legs of a trimpot into a PCB. It will be harder though and require some creativity to get them in place if the resistors you are trying to replace are in between rows of tightly packed resistors. The reverse however is always easy. So why skimp on that?

[wgc]

I've started avoiding trim pots for basing jfets because the noise they add is not the kind of noise I want.

If you find them so appealing, I can appreciate that, but it sounds like a great reason to start working on your own layouts.

[midwayfair]

And as long as the JFETS are matched and then correctly biased it should not change the sound one damn bit. Or am I wrong in that regard?

Yes, you ARE wrong in that regard. A JFET that biases to 4.5V with only 3.9K resistance will have less gain than one that biases at 5.6K. One that biases at 10K will have significantly more. The Black 65 was designed to use paired J201s with identical gain levels at a particular bias point. The bias point matters because it defines how the wave is clipped. The gain level matters because it defines how much it will clip the next stage.

If it doesn't matter to you to build it exactly like it was intended, then use your breadboard to find pairs and their drain resistors.

I do agree that it's cost prohibitive for many people to buy enough JFETs to find suitable matched pairs for this effect, but the whole point of the PCB was to be able to build a clone of the pedal; it's not going to be designed to take shortcuts or skimp on the details.

[selfdestroyer]

This maybe a stupid question but I always wondered. Do large companies get parts made in the spec they need from companies like Fairchild? Can they order 10,000 units of the same spec, with in reason? or.. are large companies constantly having to re-bias circuits before shipping?

Cody

[jubal81]

+1 with what Jon's been saying.


I've build more than a few of these types of drives and I'm at the point where, to me, getting it in the ball park is good enough and an exact clone doesn't really concern me. Actually, my favorite Black 65 I did was VERY low gain compared to stock. Sounded awesome to me.


At this point, the best route is to buy a batch of 100 mmbf201 (SMD) at the $0.14 price break and a pile of adapter boards. Being from the same batch, though, they might even be too close in tolerance to one another to get the exact ones you need.


If you absolutely HAVE TO HAVE a perfect clone, buy a genuine unit second hand.
If you're building it for someone else, tell them you modified the design a bit for a 'more complex gain structure to emphasize certain harmonics' or something like that.

[pickdropper]

This maybe a stupid question but I always wondered. Do large companies get parts made in the spec they need from companies like Fairchild? Can they order 10,000 units of the same spec, with in reason? or.. are large companies constantly having to re-bias circuits before shipping?

Cody

It depends.  The FET manufacturing process has pretty wide limits.  I've never dealt with FET manufactures, but I have dealt with other component manufacturers that will bin them for you for a nominal fee.

Binning is a time honored practice done by either the component manufacturer or the end equipment manufacturer.  My guess is that the pedal companies are doing the binning, but I don't know that for certain.

[selfdestroyer]

This maybe a stupid question but I always wondered. Do large companies get parts made in the spec they need from companies like Fairchild? Can they order 10,000 units of the same spec, with in reason? or.. are large companies constantly having to re-bias circuits before shipping?

Cody

It depends.  The FET manufacturing process has pretty wide limits.  I've never dealt with FET manufactures, but I have dealt with other component manufacturers that will bin them for you for a nominal fee.

Binning is a time honored practice done by either the component manufacturer or the end equipment manufacturer.  My guess is that the pedal companies are doing the binning, but I don't know that for certain.

That makes sense. Thanks for the info.

Cody