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
 

Chuck D. Bones

Circuit Wizard
I would be extremely interested in what you make of the King of the Britains.
I just breadboarded KotB using PF5102 FETs. Got the bias dialed-in by tweaking the source resistors. Lotta gain there. On headphones it's pretty harsh, even with GAIN, TREBLE and MASTER dialed way down. Tried running it at 18V; it got louder but the tone didn't change much. I'll recheck my wiring and plug it into an amp & speakers tomorrow, see if it sounds any better.
 

Chuck D. Bones

Circuit Wizard
OK, today I spent some time with KotB. Played my Chibson Res Paur thru it and straight into a tube amp. Sounds pretty good, but not exceptional (to me anyway). The Marshall tone stack doesn't do all that much, no surprise there. Having two Gain controls is a plus. I have two other Marshall emulator pedals: DLS mk 3 (Covert) and ROG Thunderbird. To me the KotB is similar to the Covert, but the Thunderbird is superior to the other two IMHO. One thing that the Covert has over the KotB is its mu-amps respond well to increased power supply voltage, whereas the KotB sounds the same at higher voltage. I'll play thru it some more, but I doubt it will get beyond the breadboard stage. I made a few minor mods. Here is the as-built:

King of the Britons cb mod v1.1.png

I replaced 5 of the J201s with PF5102s and one (Q4) with a 2N5089. Most of the changes were to adjust the bias for the PF5102s. D1 was added to correct Q1's bias. I added R5 because there is no need to dial the GAIN all the way down to zero. R9 was increased to 4.7K to correct Q2's bias. R13 was increased to 2.7K to correct Q3's bias. R14 was 100K, which to me was way too high and could have been a typo, so I changed it to 10K. Q4 was originally a source follower. I tried an emitter follower instead and saved a JFET. Out of necessity (not enough A1M pots), I scaled the impedances of the tone stack down by approx 5x. Rescaling the tone stack might cause some small tonal differences when Q4 is overdriven hard. Where R18 is, there used to be two 1K resistors with R17 connected in the middle. To get Q5's bias right, I moved R17 to the bottom of the pair and replaced the two 1Ks with a 2.2K. R23 was increased to 750Ω to dial-in Q6's bias. The VOLUME pot was decreased to A100K and C17 was increased accordingly. Did I build an exact replica of the KotB? No. Is it pretty damned close? Yes.
 
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Chuck D. Bones

Circuit Wizard
Just for S&G, I cannibalized some pots from another breadboard and assembled the tone stack in accordance with the original schematic. Sounds the same. It's a decent pedal, but it's not for me. I definitely like the Covert & Thunderbird better. Next up... the EQD Monarch, with the bias issues corrected.
 

djmiyta

Well-known member
😁
😁
😁 Thank you Chuck D Bones I have a much better perspective now very cool way of explaining that quantum string theory Jfet mystery I had going on that you were unaware of
1 question is this or will this work for the info needed obviously I'm still not fully grasping all the electronic lingo but I snapped a pic of my tester sadly I dont understand any of the info it provides maybe someone could explain ? Does what you said about the jfets include mmbfj201 for instance?I mean should all jfets be auditioned per circuit even SMT ones that are in spec aside from the tolerant circuits? And would the PAL 800 be considered a tolerant circuit?
 

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djmiyta

Well-known member
OK, today I spent some time with KotB. Played my Chibson Res Paur thru it and straight into a tube amp. Sounds pretty good, but not exceptional (to me anyway). The Marshall tone stack doesn't do all that much, no surprise there. Having two Gain controls is a plus. I have two other Marshall emulator pedals: DLS mk 3 (Covert) and ROG Thunderbird. To me the KotB is similar to the Covert, but the Thunderbird is superior to the other two IMHO. One thing that the Covert has over the KotB is its mu-amps respond well to increased power supply voltage, whereas the KotB sounds the same at higher voltage. I'll play thru it some more, but I doubt it will get beyond the breadboard stage. I made a few minor mods. Here is the as-built:

View attachment 8600

I replaced 5 of the J201s with PF5102s and one (Q4) with a 2N5089. Most of the changes were to adjust the bias for the PF5102s. D1 was added to correct Q1's bias. I added R5 because there is no need to dial the GAIN all the way down to zero. R9 was increased to 4.7K to correct Q2's bias. R13 was increased to 2.7K to correct Q3's bias. R14 was 100K, which to me was way too high and could have been a typo, so I changed it to 10K. Q4 was originally a source follower. I tried an emitter follower instead and saved a JFET. Out of necessity (not enough A1M pots), I scaled the impedances of the tone stack down by approx 5x. Rescaling the tone stack might cause some small tonal differences when Q4 is overdriven hard. Where R18 is, there used to be two 1K resistors with R17 connected in the middle. To get Q5's bias right, I moved R17 to the bottom of the pair and replaced the two 1Ks with a 2.2K. R23 was increased to 750Ω to dial-in Q6's bias. The VOLUME pot was decreased to A100K and C17 was increased accordingly. Did I build an exact replica of the KotB? No. Is it pretty damned close? Yes.
Interesting I pulled mine out of the meh box last I built the I believe 1st version 3 knobs 2 toggles when it’s paired it’s nice but yeah overall back to meh pile I’ve only heard great things of ROG designs and yet to build one
 

Chuck D. Bones

Circuit Wizard
Unfortunately, those testers do not measure Idss or Vp. In a few other threads, I described how to measure Idss and Vp using a DMM. The bottom line is very few circuits will tolerate JFETs over their entire spec range. The PAL800 is tolerant to the extent that the trimmers have a large adjustment range. Still, no guarantees every in-spec J201 will work. The pros have to test and select their JFETs if they want every pedal to work.
 
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