fake jfet?

[Davebassman]

Hi,

I want to build a compressor type 1176 stereo and I need to match 4 jfet. To match them I use the method proposed on this website (attached scheme). The jfet's in question are bf245a, I bought them on ebay and I know I did very wrong.

The problem is that comparing my results with those of masonaudio, which also uses bf245a, I see a big difference in VGS between 0V and 0.250V, almost 9V! I think it is not normal at all and that is why I have come to the conclusion that they are fake. What is your opinion? I don't have a lot of experience drawing curves.

https://www.masonaudio.org/diy/comp1176

I am attaching a screenshot of my results and those of masonaudio, so you can see the big difference there is.

I hope someone can give me a hand.

P.S.: if they are false I'm going to regret it, because I just put together a project that uses 14 bf245a.

[Helmholtz]

Are you sure the matching circuit is correct? Certainly is not complete.
Input looks shorted to ground causing a short between gate and source. Where is the pot?
How is Vgs produced and varied?

The descriptions in your link don't show.

[Dave H]

Input is from the wiper of the pot which is part of a potential divider from -10V to ground. Input (Vgs) varies between 0V and -5V

[Davebassman]

I compared a photo of original bf254a and there are diferences. So, this bf254a are fake.

This is the seller.

https://www.ebay.es/itm/304157747953?var=603269161743

[Helmholtz]

Input is from the wiper of the pot which is part of a potential divider from -10V to ground. Input (Vgs) varies between 0V and -5V

Thanks.
So what looks like a ground symbol is actually a wiper and the circuit is self-biasing. Makes sense now.

[Davebassman]

I compared a photo of original bf254a and there are diferences. So, this bf254a are fake.

This is the seller.

https://www.ebay.es/itm/304157747953?var=603269161743

fake bf254a and original one.

[mozz]

Manufactures do change their print/font from time to time,or for different machines or plants. Try to use acetone to see if the prints removes. That being said, from Ebay, the odds of being fake are very high. Diystompboxes has a lot of info on fakes.

[Davebassman]

I compared 10 original devices with my "fake" 10 devices. Same power supply, same procedure... I get a difference of 9V vds with only -0.250V vgs. It's clearer that this devies are not bf254a. They are fet, but not bf254a. It seems a remark fets, I didn't know. If you look at the logo, the laser cut is not precisely.

[Helmholtz]

The things don't behave like real n-channels fets. Id should drop continuously with lower (i.e. more negative) Vgs.

[Mick Bailey]

Nowadays I use a cheap microprocessor component analyser to check transistors to weed out anything suspect. Despite buying 100s off Ebay sellers I haven't really had any problems other than one batch where I checked them and they looked fine but they had been supplied with a different pinout that I failed to spot. This resulted in me spending endless hours trying to trace a subsequent fault after using them. It turned out that there were two different pinouts available for the same device and I received the least common variant.

I've noticed that the new BC107/8/9 transistors have poor printing and look like fakes, but they've come from CPC, Farnell and RS components and are genuine. The main issue I've had is with fake power transistors and ICs bought off the open market.

[J M Fahey]

Thanks.
So what looks like a ground symbol is actually a wiper and the circuit is self-biasing. Makes sense now.

FWIW that is NOT a compressor circuit and is not "incomplete",
Nor does it deal with any *signal*

Read-the-label: FET matching circuit.

It even shows 2 terminals to read Vgs.

[g1]

FWIW that is NOT a compressor circuit and is not "incomplete",
Nor does it deal with any *signal*

I think the problem was the unconventional symbol for the wiper of the 10K pot. Some may have seen it drawn like that before, but I have not. I wasn't really sure what it was either.

[Helmholtz]

I think the problem was the unconventional symbol for the wiper of the 10K pot. Some may have seen it drawn like that before, but I have not. I wasn't really sure what it was either.

Exactly.
I (mis)interpreted the pot wiper as a ground symbol. Remember I asked where the pot is.
With a grounded gate the circuit would make no sense.

[g1]

I wonder if that is a particular schematic software that uses that symbol for pots? I will try to remember it for next time.

[Dave H]

Where I worked that symbol was normally used for Trimpots or pre-set pots i.e not user controls.