How to plot JFET?

[spoontex]

So you have built this, right? then the question is simply : does it work?

To be honest, I don't feel confident to tell you if it would work with your BF545's just by looking at the schematic.

It work! I use every day. It's strange because the bag of this bf545a says clearly NXP, it don't seems fake. I think that only have bad specs, I don't know.

[FSFX]

It's strange because the bag of this bf545a says clearly NXP, it don't seems fake.

I have had some experience with what were meant to be NXP BF862 JFETs. The datasheet shows that the marking code is '2Ap' however it seems that there are also ones marked as '2AW'.
Both markings are correct for the NXP BF862. It seems that the dies were made in a European NXP fab but they were encapsulated in two places, Hong Kong for the '2Ap' and China for the '2AW'.
Anyway, I bought both versions.
The datasheet shows that Vgs(off) has a typical value of -0.8v.
On testing I found that all of the '2Ap' ones measured about -3.6v to -3.8v.
Half of the '2AW' ones measured about -3.8v as well however there were some '2AW' ones that measured around -0.7v to -0.75v which would agree with the datasheet.

So there must be some fake NXP devices around as those values are way off even for out of spec devices.     

[Rob Strand]

It's strange because the bag of this bf545a says clearly NXP, it don't seems fake.

What's the markings on the devices:
According to the datasheet they should be 20p, or 20t, or 20W.

From your test results the devices have:
- low resistance (100 ohm)
- low VP

These aren't characteristics of BF545A's.  They are characteristics of the generic stuff you get off ebay.

[spoontex]

It's strange because the bag of this bf545a says clearly NXP, it don't seems fake.

What's the markings on the devices:
According to the datasheet they should be 20p, or 20t, or 20W.

From your test results the devices have:
- low resistance (100 ohm)
- low VP

These aren't characteristics of BF545A's.  They are characteristics of the generic stuff you get off ebay.

What modern replacement can I use??

[Rob Strand]

What modern replacement can I use??

They look like the real thing.

You might try testing the VP values with a different test jig.  At least that gives you a second opinion.

https://runoffgroove.com/fetzervalve.html

The problem isn't what you buy who you are buying off.

There are legitimate suppliers out there who are trying to shut down fakes and re-marked components. You should be able contact your supplier, find the guy that handles fakes, and explain the issue.   The legit supplies want to weed out their bad suppliers as well.   It's likely they will organize a swap.

For the non-legit suppliers, it's hit and miss.   You might buy another part number and end-up with the exact same problem.  There's many posts on this forum where people have build problems with low VP JFETs.   It's a widespread problem now for JFETs.

[Clint Eastwood]

Onsemi makes the MMBFj309LT1G, wich is an smd version of the J309, wich is equivalent to the BF245a. Mouser Europe has 450.000 of them in stock (!)


Edit: Actually, it is equivalent to the BF245b. 

[POTL]

Honestly though, given the trouble getting hold of the damn things and the huge variability between supposedly-similar parts, why are we bothering with JFETs when we could perhaps be spending the effort researching better alternatives?

Seems to me that JFETs have had their day and we need to look elsewhere. I *know* there's lots of vintage circuits that use them, but they had those parts available in the 1970's and no other options, and now we *don't* have those parts available and we *do* have other options, so why are we trying to do the same thing they did way back then?!?

I completely agree. there are many decent and simple alternatives. VCA and a compressor that works better and doesn't clutter the sound. OTA in feedback mode, like in Engineers thumb. Optical compressors for coloration and natural sound.

[POTL]

Seems to me that JFETs have had their day and we need to look elsewhere.

You could say that about tubes, germanium transistors and vinyl records. Someone always wants that 'less than perfect' sound.

It makes sense. look at how the lamp teacher industry is changing. more and more manufacturers are making compact 20W amplifiers, but with tubes from 50W amplifiers. more and more concentrating musicians take Kemper, Ax Fx, Neural DSP to concerts. germanium transistors, they are becoming less and less common. in my country they were produced until the end of the 90s and they are very cheap, they are still sold in stores (not for guitar diy, but simple radio parts stores). but when they run out there will be new transistors (I bet on MOSFet).

vinyl is popular again. but I'm too used to music services and the ability to instantly change songs and artists. I have a childhood friend who owns a vintage equipment repair business from Eastern Europe. he says that warm and good sound is not vinyl, vinyl is bad, compressed and modern, haha good sound is the bobbin.

[spoontex]

I know, modern times... streaming, kemper and that all stuff that is amazing. But the fact is that I have a device with 14 supicious fet's that it need to change. I've two alternatives for the Dim D build


2sk30, at 0,40€/u
2n5957, smd with adapter 1€/u

For the 1176 build, I buy a matched pair. It will be more economic.

What do you think It's better replacement bf245a for the dim d?

[Clint Eastwood]

Toshiba still make some jfets, the 2sk208 looks like a good , probably even better than the original replacement for BF245a. Look up the datasheet.

[FSFX]

Toshiba still make some jfets  . . .

There are companies like InterFET that specialise in making JFETs. It is their core business as their name implies.

[anotherjim]

History has made the Urie 1176 a very desirable device. If it isn't JFET, it isn't an 1176.
If the JFETs are socketed, you can sort out matching anytime. Put the best one for the compression in for keeps and wait to get a better match for the metering. I cannot understand the concern about the metering. The metering JFET is in a different circuit - it's a source follower and there are two trimmers to set up the response.

[Clint Eastwood]

Toshiba still make some jfets  . . .

There are companies like InterFET that specialise in making JFETs. It is their core business as their name implies.

Yes, but their fets cost 10 times more..

[FSFX]

Yes, but their fets cost 10 times more..

Just like NOS germanium transistors then?

[spoontex]

History has made the Urie 1176 a very desirable device. If it isn't JFET, it isn't an 1176.
If the JFETs are socketed, you can sort out matching anytime. Put the best one for the compression in for keeps and wait to get a better match for the metering. I cannot understand the concern about the metering. The metering JFET is in a different circuit - it's a source follower and there are two trimmers to set up the response.

I need to match for a stereo 1176.

[amptramp]

There is always the PWM phaser that depends on duty-cycle modulating analog switches to mimic JFET's operated in the variable resistance mode:



It uses a triangle wave generator operating above audio frequencies to set the switching on-off point of analog switches.  This is modulated by the LFO to lengthen or shorten the the ON time.  The advantage?  The JFET's are replaced by CD4066 devices and there is no need to select devices.  The disadvantage is the high-frequency oscillator means that if you want to operate from AC power, you have to pass electromagnetic compatibility testing to be able to sell it.

[FSFX]

Yes, but their fets cost 10 times more..

If cost is the only concern then if the OP wants to messages me with their address I will post off a strip of 20 genuine ON Semi MMBFJ113 JFETs (marking code H_59) to them for free.
A sample of them I tested showed Vgs(off) = -1.06v. Idss = 8.56mA and Ron = 49 ohms.

There, despite all the flack I keep getting here, I can't be fairer than that.

EDIT: I also have TO-92 THT versions if they are preferable to using the SMD ones.

[anotherjim]

History has made the Urie 1176 a very desirable device. If it isn't JFET, it isn't an 1176.
If the JFETs are socketed, you can sort out matching anytime. Put the best one for the compression in for keeps and wait to get a better match for the metering. I cannot understand the concern about the metering. The metering JFET is in a different circuit - it's a source follower and there are two trimmers to set up the response.

I need to match for a stereo 1176.

Ah, Stereo! I'd find making just one channel a major project. Although if you have PCB already worked out, it isn't so bad.

Have to say I don't understand JFET types. Some are marketed as switches and some as signal amplifiers. Are any specifically made to be voltage-controlled resistors? I would think the amplifier types were closest but I think the 2SK30A has been used in all three roles.

[FSFX]

Have to say I don't understand JFET types. Some are marketed as switches and some as signal amplifiers.

Have you read any of these ON Semi documents?

https://www.onsemi.jp/download/application-notes/pdf/an-6609.pdf

[FSFX]

Are any specifically made to be voltage-controlled resistors?

The answer is yes.  Check out Linear Systems - here is an extract from a document of theirs -

Linear Systems has expanded its family of JFET Voltage-Controlled Resistors (VCRs) for reduced-power circuits over IC-based devices.
Two new additions to the JFET Voltage-Controlled Resistor Family include the LS26VNS, a Single N-Channel JFET VCR, and the LS26VPS, a Single P-Channel JFET VCR. Both the N and P-channel versions are made from the same die geometry, making them complimentary.
They are in addition to the existing VCR11N, Dual N-Channel.

The advantages of using VCRs for voltage-controlled amplifier or voltage-controlled gain systems include:

- VCR-based devices operate with lower voltage and power consumption, enabling battery-powered systems to last significantly longer.
- VCR JFETs can be used with low power op amps to provide a low power solution for voltage-controlled gain systems.
- With low voltage op amps such as the LMV358 or TLC272, VCR JFETs are used to perform voltage-controlled gain at extremely low supply voltages such as +/- 2 volts, + 5 volts and even down to a single 3.3 volt power supply.
- Circuits can be designed for very low power consumption when low voltage, low power op amps are included with the JFET VCRs. One example would be to include using an LSK389A as a VCR for these low voltage circuits.
- The matched pair JFET VCRs (VCR11N or LSK389A) can provide two identical channels of voltage-controlled gain where the control voltage tracks for both channels. This can be used in a stereo audio signal attenuator.
- Typical active voltage-controlled amplifiers drain more current and require higher minimum power supply voltages needing at least +/- 4 volts for a specialized voltage control amplifier integrated circuit.
- P-Channel JFETs including a J177 can be designed as a VCR circuit with a single a power supply. For example, the P-Channel JFET VCR circuit is ideal with positive supply voltages since it's control voltage to the gate is also a positive voltage.

There is a Vishay application note regarding the use of FETs as VCRs.

https://www.vishay.com/docs/70598/70598.pdf