Why are JFETs such a Pain in the Ass?

How many times do we see post in the Troubleshooting forum where the problem turns out to be a JFET?

Like Germanium transistors, JFETs are finicky beasts and buying the right part number does not guarantee success.

First, a very brief primer on how JFETs work.
JFETs are voltage-controlled devices. The drain current depends on Vgs (the voltage between gate & source). The gate current is so close to zero that we can safely assume it is zero. We'll talk about N-channel JFETs (arrow on the gate pointing in) because 99% of the time that's what are used in pedals. When Vgs is zero, the JFET is fully on; the drain current is Idss. When we make Vgs go negative, the drain current is reduced. When Vgs reaches Vp, the drain current is zero. When we using JFETs to amplify signals, Vgs will be somewhere between 0 and Vp.

JFET specs
The two JFET specs we care about are Vp and Idss. Those two number tell the pedal circuit designer all they need to know to set the bias and determine the gain of the JFET. Here's where it gets ugly. The Vp and Idss specs on JFETs are extremely loose. Some more than others. Example: 2N5457 has a Vp spec from 0.5V to 6.0V. The Idss spec is 1mA to 5mA. Parts at the extreme end of those ranges will probably not work in a given pedal. As good as JFET production methods are, there is still a significant variation from lot-to-lot. The manufacturers test and sort the JFETs, but to keep yields up and costs down, the specs are left pretty loose. It is very common for OEMs (Original Equipment Manufacturers), to either pay the JFET manufacturers to cherry-pick JFETs to fit a narrower spec window, or buy lots of extras and do it themselves. We're in the position where we have to do it ourselves. Sometimes, the pedal designer is clever enough to design a circuit that can tolerate the variations in Vp or Idss. Of all the JFET pedals sold on this site, only a few fall into that category. That's why JFET pedals have trimmers in them. But the trimmer can only do so much and a JFET that is near the extremes for Vp or Idss may not work for any trimmer setting.

Why do pedal designers use JFETs if they're so problematic?
JFETs have certain advantages. In some circuits, they can have more gain and/or less noise than bipolar transistors. Their input impedance is very large, so they don't load down pickups or other stages in the pedal. Their biggest appeal to pedal builders is that their transfer function is similar to that of a vacuum tube. They tend to generate lower-order harmonics and overload more gracefully in a properly designed circuit, compared to bipolar transistors. Many of the amp-in-a-box pedals replicate a tube amp's preamp section, with JFETs standing in for vacuum tubes.

What do we do about it?
Well, first you go to college for four years and get a degree in electrical engineering. Ok, so that's not going to work for most people. The alternatives are you find someone who can analyze the circuit and recommend a range for Vp and Idss, or you find someone who has measured a production pedal and try to match their readings. Luckily, there are forums inside and outside this website that can help you with that. Mr. PedalPCB might be persuaded to include JFET spec requirements in the Build Docs for pedals that are picky about such things. You need to own and understand how to use a transistor tester. You can get a perfectly adequate one for about $20US on eBay. The other thing you need to do is you buy quality parts from a reputable vendor*. I've said it before and it bears repeating here: "If you buy transistors on eBay, you need to have the heart of a gambler and the skill to test them." If there was ever a place that demonstrates the adage "If it's too good to be true, it probably is." it's eBay. There are some good semiconductor vendors on eBay, but they are in the minority. The rest are either ignorant or crooks because there are a TON of counterfeit parts sold there.

* A high score on eBay does not a "reputable vendor" make. They might have earned that high score selling umbrellas.

The Bottom Line
Building pedals containing JFETs is not necessarily a paint-by-numbers activity and you need to know that going in. Unless you screen your JFETs prior to assembly, sockets are mandatory. Buy from a reputable vendor and buy at least 3x as many as you will need so you end up with enough that work in your pedal.

Next Time: Biasing JFETs
 

cooder

Well-known member
Chuck you're amazing, thanks for that and you have a way with words too. Like your humour "Well, first you go to college for four years and get a degree in electrical engineering."
Very enlightening and yes we do need someone (wink wink....) to enlighten us which circuits on this site need special attention, what measurements are required and how to keep sane. Having that in the build docs aside from the schematics (cough cough...) would be really good.
So I'll ask someone (that is you...) "someone who can analyze the circuit and recommend a range for Vp and Idss" what would I need in for example Catalinbread DLS MK3 and 5F6 (Covert and Tweed Man) and Formula 55 (Drive 55)? How about the Fairfiled Barbershop (Chop Shop)?
Cheers man. You should write a book. Well you're actually doing that here kinda.
 

Barry

Well-known member
Chuck you're amazing, thanks for that and you have a way with words too. Like your humour "Well, first you go to college for four years and get a degree in electrical engineering."
Very enlightening and yes we do need someone (wink wink....) to enlighten us which circuits on this site need special attention, what measurements are required and how to keep sane. Having that in the build docs aside from the schematics (cough cough...) would be really good.
So I'll ask someone (that is you...) "someone who can analyze the circuit and recommend a range for Vp and Idss" what would I need in for example Catalinbread DLS MK3 and 5F6 (Covert and Tweed Man) and Formula 55 (Drive 55)? How about the Fairfiled Barbershop (Chop Shop)?
Cheers man. You should write a book. Well you're actually doing that here kinda.
Really enjoying the articles Chuck's posting, many thanks to him for taking the time to share some of his infinite wisdom
 

Danbieranowski

Well-known member
I really believe that buying anything on ebay is just a bad idea all-around. There are so many good reputable vendors that deserve your money and can provide genuine parts (SmallBear, DigiKey, stompboxparts, even big boys like Tayda and Mouser, etc etc). I've purchased dodgy ICs before (clearly salvaged) and it was a good reminder to me to just spend the extra few bucks and support a good business while you're at it.

Additionally, you're a king as usual Chuck. Thank you for breaking this stuff down in human terms for us. I haven't had time to dig into pedal projects lately, but I still read through each of these posts and try to soak up as much info as I can.
 

cooder

Well-known member
Hit us with your rhythm stick Mr. Chuck, bang in some algebra. Looking forward to it, my pidgeon brains hurts already... ;)
 

phi1

Well-known member
Great article Chuck! Ever since my first misadventure buying J201s from eBay... I started buying exclusively smd jfets and using adapter boards.

I built the simple (non-IC) version tester from this page. I’m not sure if this method produces perfect accuracy (Chuck could probably weigh in), but for my purposes of finding consistency it seems to work great. I’ve read the background articles from runoffgroove that were also interesting.

since I was using exclusively smd, and some boards have spots to solder smd directly, I designed and ordered this pcb with through hole socket and smd pads, following that tagboard circuit. Gotta hold the jfet down on the pads with tweezers which is annoying, but I’m getting better at it haha. I printed some info and some target values. These are MY approximate averages from several smd parts I’d measured. So don’t take these numbers as text book truth. I have extras of these boards if anyone is interested (I won’t charge, maybe just shipping).

my experience has been pleasant in that most land pretty close to the numbers printed (within 0.1-0.2 or so). This includes smd jfets from tayda, pedalpcb, and diyguitarpedals. I’ve only had a few that were way off from this (though I think they were still technically within the spec).

I’m really curious if others have had similar experience with smd, and what method you all use for measuring.
 

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HamishR

Well-known member
For whatever reason I have had good results in builds which specify 2N5457, 2N5458 or 2N5952. It's those bloody J201s which drive me crazy. Some wonderful pedals, such as many by Menatone, use J201s (as Chuck suggests, in place of tubes) and they are so fussy about biasing it takes the fun out of building them. The King of the Britains is a great overdrive despite the ungrammatical name. The 6-knob version is outstanding but difficult to build because of the J201s. I have a ton of J201s bought from Small Bear but until I have a streamlined process for using them they stay in their little bags.
 

cooder

Well-known member
Yes it would be great to see what other methods and devices peeps use and recommend for jfet testing and also the fiddly smd breed ones that I also like to use. Thanks phi1 for your input there!
 

jubal81

Well-known member
For whatever reason I have had good results in builds which specify 2N5457, 2N5458 or 2N5952. It's those bloody J201s which drive me crazy. Some wonderful pedals, such as many by Menatone, use J201s (as Chuck suggests, in place of tubes) and they are so fussy about biasing it takes the fun out of building them. The King of the Britains is a great overdrive despite the ungrammatical name. The 6-knob version is outstanding but difficult to build because of the J201s. I have a ton of J201s bought from Small Bear but until I have a streamlined process for using them they stay in their little bags.
People will always prefer the J201 because it's the part number originally used in all those designs. I have a hunch it became popular because its |vp| and IDSS lead to source and drain resistor values that can be closer to the values you commonly see in typical guitar tube preamps when used as common-source amplifiers.

There are a good number of other JFETs available with typical |vp| values less than 1 volt that can handle a lot more current - often designed with audio or RF signals in mind - that I've made some great sounding gain circuits with. In fact, I just ordered a new Aion PCB for the DLS MK2 that I'm going to 'hotrod' with RF JFETs.

Personally, I've never heard any 'magic' out of any particular JFET part number, and in my experience they all break up similarly and work great as long as they're set up to operate correctly. This also includes my personal experiments with the ROG 'Fetzer Valve' setup, which I never heard any particular 'magic' from.

I'm a certified JFET junkie. They make great audio amplifiers and give great sounding distortion. But like Chuck says, you design for the actual values, not the part numbers, to get them working right.
 

Chuck D. Bones

Circuit Wizard
Great article Chuck! Ever since my first misadventure buying J201s from eBay... I started buying exclusively smd jfets and using adapter boards.

I built the simple (non-IC) version tester from this page. I’m not sure if this method produces perfect accuracy (Chuck could probably weigh in), but for my purposes of finding consistency it seems to work great. I’ve read the background articles from runoffgroove that were also interesting.

That method is very good, it's as accurate as your DMM. I have used that method numerous times. I plug stuff into a protoboard to make the Idss & Vp measurements, but it's the exact same method.
 

Chuck D. Bones

Circuit Wizard
I agree with Jubal81. JFETs with low Vp tend to have higher gain, but at the expense of lower headroom because you can't drive the gate beyond 0V or Vp without getting much nastier distortion. With a little finesse, we can have any value source or drain resistance we want; we don't have to pick a JFET with a particular Vp. About the only restriction we have w.r.t. Vp is it has to be less (a lot less) than the power supply voltage.

HamishR, you have been lucky so far, or you're building pedals that are tolerant of Vp variation.

The Vp range of the PF5102 overlaps the J201 and with careful selection, PF5102s can be used in most pedals that call for J201.
 

Chuck D. Bones

Circuit Wizard
The King of the Britains is a great overdrive despite the ungrammatical name. The 6-knob version is outstanding but difficult to build because of the J201s.

I just looked into that pedal and yes, it depends on the low Vp of the J201. Not all legit J201s will work on that board; their Vp needs to be near the low end of the spec. I think I can see a way to make it a little more FET-friendly and still retain the sound. Basically, it comes down to adjusting the source resistors to get it into the ballpark and then fine-tuning it with the drain resistors. I ran a sim with PF5102s and some resistor changes... looks like it works, only problem is the overall gain goes up by about 20dB. Oh wait, that's not a bad thing! Another project for the breadboard.
 

Chas Grant

Well-known member
@Chuck D. Bones I would like to say Thank You! The knowledge you have been sharing has been invaluable! Especially on the JFET's! Your explanations are easy to understand and quite pertinent to what we are using them for and what we are trying to accomplish with them. Nothing is requiring you to share this knowledge, yet you choose to do so. This choice is helping all of us to gain more knowledge in a "Hobby" that we are all passionate about! So again, Thank You!
 

HamishR

Well-known member
I would be extremely interested in what you make of the King of the Britains. I'll just say it once - it should be called King of the Britons. The current name is ungrammatical and plain wrong. :mad:
 
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