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
@Chuck D. Bones how low does the Vp need to be? Smd J201s have been fairly consistent in the ballpark of 0.7 for me, so maybe that’s an option?

Depends on the pedal, the Calamity needs JFETs with Vp at the very bottom-end of the range, around 0.2V. I've seen a schematic on the 'net of the King of the Britains with measured voltages indicated; the Vp on those parts was in the 0.4V range.
 

phi1

Well-known member
Ah gotcha, yeah I was asking about the KOTB for HamishR, but my post wasn’t clear. So maybe not ideal for that.
 

Chuck D. Bones

Circuit Wizard
I think if you tweak the drain resistor on the 1st FET and the source resistors on the others, you can get there. Use your lowest Vp FET for the 1st one. Don't bother trying to dial in the 4th FET, you get what you get when the 3rd one is set. Here's the annotated schematic, I'm assuming it's correct. Hit those drain voltages within ±10% and you should be good.

Menatone king of the britains.jpg
 

benny_profane

Well-known member
I have a question about source resistor selection, but I think I'll hold off until you post specifically about biasing. (Placeholder so I don't forget.)
 

benny_profane

Well-known member
Regarding tube emulation circuits, most of the JFETs seem to be self-biased through the establishment of a Q-point by applying the voltage dropped across the source resistor to the gate. This isn't the only JFET biasing method, but it seems to be the one least dependent on the drain resistor. Why is the trimmer placed on the drain in that case? And why do different circuits using the same JFET device and topology have such large source resistor value differences?
 

manfesto

Well-known member
You need to own and understand how to use a transistor tester. You can get a perfectly adequate one for about $20US on eBay.
Off-topic, I've seen those cheapie transistor testers on eBay and Amazon, but near as I can tell none of the cheapies measure leakage (which would come in very handy for testing germaniums).

Is there anything cheaper than the DCA55 you know of that measures leakage (besides that Geofex circuit on a breadboard)?
 

Chuck D. Bones

Circuit Wizard
Off-topic, I've seen those cheapie transistor testers on eBay and Amazon, but near as I can tell none of the cheapies measure leakage (which would come in very handy for testing germaniums).

Not true (well, the off-topic part is true). Here is an MP38A Soviet NPN Germanium under test.

MP38A test.jpg
hFE is the DC current gain.
6.7mA is the emitter current during the hFE and Vbe tests.
Vbe is the base-emitter voltage.
ICEO is the collector to emitter current with the base lead open. This is usually the definition of leakage.
ICES is the collector to emitter current with the base lead shorted to the emitter.
The in-circuit leakage is somewhere between the two.
I have compared the hFE results on this tester with the results I get on a curve tracer and I can assure you that this tester is accurate and it takes leakage into account when measuring and reporting hFE.
 

manfesto

Well-known member
Not true (well, the off-topic part is true). Here is an MP38A Soviet NPN Germanium under test.

View attachment 8442
hFE is the DC current gain.
6.7mA is the emitter current during the hFE and Vbe tests.
Vbe is the base-emitter voltage.
ICEO is the collector to emitter current with the base lead open. This is usually the definition of leakage.
ICES is the collector to emitter current with the base lead shorted to the emitter.
The in-circuit leakage is somewhere between the two.
I have compared the hFE results on this tester with the results I get on a curve tracer and I can assure you that this tester is accurate and it takes leakage into account when measuring and reporting hFE.
Awesome, thanks! Will pick one up!
 

Chuck D. Bones

Circuit Wizard
Got mine on eBay, not everything there is crap. I bought it assembled, except for clear plastic case. Putting that together is like building a house of cards. Hint: painter's tape. You'll need a POSITIVE CENTER 9VDC power source.
 

manfesto

Well-known member
Got mine on eBay, not everything there is crap. I bought it assembled, except for clear plastic case. Putting that together is like building a house of cards. Hint: painter's tape. You'll need a POSITIVE CENTER 9VDC power source.
Ordered the unassembled version to save a few bucks, thanks for the heads up on the power source! Ordered it with a battery adapter because I wasn’t sure, should’ve checked back in the forum before purchasing haha

For anyone who’s looking like I am, make sure when you search eBay you specifically search “GM328” to get the same model transistor tester Chuck has that tests leakage (ICEO and ICES); there are other cheap testers on eBay (most branded “mega328” it looks like) that *only* show hFE and Vf (they have green monochromatic screens and will be a little cheaper)

EDIT: found a pic, this is the one you *don’t* want. The GM328 will be worth the extra $5.
 

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cooder

Well-known member
Thanks for all this and the hint of the GM328 tester, seems better than my cheapie I got and great to know that it seems to agree with Chcks curvetesting, which I'm sure will be a benchmark.
So I'll be looking forward to see the Chuck version of the King of britons including correct spelling...
So where would I get a good batch of the PF5102 jfets then that you keep popping up? Mouser doesn't seem to have them in stock, did you buy them all up, Chuck?
 

Devoureddeth

Active member
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.
Do you imagine jfets Vp could br the issue with people trying to recreate the EQD Monarch? Schematic
 

Chuck D. Bones

Circuit Wizard
No.

That schematic contains two glaring errors that by all rights should prevent any board built that way from working. There is also at least one typo. Other schematics I've seen of this board contain the same errors. JFETs need to have a stable voltage, even if it's zero volts, present at the gate terminal in order to set the operating point (bias). The gates on Q2 and Q4 have no DC path to anywhere, therefore their bias point is unknown. The only way a board built this way could work is if C7 and C14 are leaky or there is enough flux residue on the board to absorb some moisture and provide a DC current path across the surface of the board. The right way to correct the problem is:
  1. Replace C14 with a jumper.
  2. Replace C7 with a jumper.
The typo is the value of R19. 47K is too low and it loads the GAIN pot. 470K would be a more appropriate value. I could even make a case for deleting R19 and replacing C15 with a jumper.

For sure, getting the right Vp is important. Q2 and Q4 both need a fairly low Vp. Q4's Vp determines Q3 and Q4's drain current. Q2 and Q3's Vp set the headroom. I'd shoot for a Vp around 1V on all three JFETs. Again, part number is not important, Vp and Idss are. J201 will not work for Q4.

I really must finish that article on biasing JFETs!

The idea of replicating the Orange Amp sound is intriguing, but there is a lot of room for improvement to this implementation. MOSFETs are noisier than JFETs or BJTs, so using one as the first stage is not something I would do. JFETs have the same transfer function as MOSFETs, have more gain at these drain currents and are quieter.
 

Devoureddeth

Active member
No.

That schematic contains two glaring errors that by all rights should prevent any board built that way from working. There is also at least one typo. Other schematics I've seen of this board contain the same errors. JFETs need to have a stable voltage, even if it's zero volts, present at the gate terminal in order to set the operating point (bias). The gates on Q2 and Q4 have no DC path to anywhere, therefore their bias point is unknown. The only way a board built this way could work is if C7 and C14 are leaky or there is enough flux residue on the board to absorb some moisture and provide a DC current path across the surface of the board. The right way to correct the problem is:
  1. Replace C14 with a jumper.
  2. Replace C7 with a jumper.
The typo is the value of R19. 47K is too low and it loads the GAIN pot. 470K would be a more appropriate value. I could even make a case for deleting R19 and replacing C15 with a jumper.

For sure, getting the right Vp is important. Q2 and Q4 both need a fairly low Vp. Q4's Vp determines Q3 and Q4's drain current. Q2 and Q3's Vp set the headroom. I'd shoot for a Vp around 1V on all three JFETs. Again, part number is not important, Vp and Idss are. J201 will not work for Q4.

I really must finish that article on biasing JFETs!

The idea of replicating the Orange Amp sound is intriguing, but there is a lot of room for improvement to this implementation. MOSFETs are noisier than JFETs or BJTs, so using one as the first stage is not something I would do. JFETs have the same transfer function as MOSFETs, have more gain at these drain currents and are quieter.
Awesome Chuck, love the information but I am still trying to wrap my head around mu-amps. Hopefully I will get time over Christmas break to breadboard some changes. Maybe this could be another Chuck PCB. I know people have been wanting a version of this for sometime.
 
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