J201 vs MMBFJ201

[bsas]

So, I just built a bunch of "J201" for me using SMC adapters and the MMBFJ201 part. It works, but, the measurements are so weird (and consistent).

For example, if I measure (using the "Greatly Improved JFET Matcher") an NOS J201 I have, I get an average of I=0.45mA @ Vg=0.35V, but the SMC one gives me I=0.15mA @ Vg=0.12V. Far lower!

Trying to use them in some pedals just don't work. I tried the Tayda EP booster PCB for example and it basically don't boost at all. The NOS works fine (I always socket JFETs :D).

The weird thing is that I have some sets of SMCs to replace those "hard to find" and ALL them give me those far lower results then the NOS counterparts, consistently: MMBF5484, MMBF5485, MMBF5486, MMBF5457 and the MMBFJ201 in question. All original from Fairchild and from Mouser.

So, am I doing something wrong while measuring or soldering those SMCs, or this behavior is expected? I see a lot of guys using those SMC adapter to make hard to find transistors like the J201, 2n5457, etc. But, if the SMC counterpart is so different, what can I do?

Thanks all! :D

P.S.: Besides the "Greatly Improved JFET Matcher" I also use this fancy "component tester" kit I built, and the results are very consistent between the kit and the matcher.

[induction]

bsas wrote

For example, if I measure (using the "Greatly Improved JFET Matcher") an NOS J201 I have, I get an average of I=0.45mA @ Vg=0.35V, but the SMC one gives me I=0.15mA @ Vg=0.12V. Far lower!

Questions:
Is that the Greatly Improved JFET Matcher with the op-amp, or version II with no op-amp?
Are you giving us the direct readings, or have you done the multiplication in advance?

[bsas]

The version II with no op-amps. I am doing the multiplications.

But, the weird thing for me is that those measurements are confirmed by a "transistor tester" that I build too. Here is tester:

Yosoo GM328 LCD Display Transistor Tester ESR Meter Cymometer Square Wave Generator
http://www.amazon.com/Yosoo-Display-Transistor-Cymometer-Generator/dp/B00OOQC2E8?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage

Love that thing by the way, super handy :D

[induction]

According to Fairchild's J201 datasheet, which represents both the through-hole and SMD versions, Vgsoff should be between 0.3 V and 1.5 V, and Idss should be between 0.2 mA and 1.0 mA. So, assuming you're measuring them correctly, your NOS's are in spec, and your SMD's are not. Since you get similar measurements with your transistor tester, I suspect the measurements are correct.

Whether this is reflective of a larger trend in SMD jfets in general, I couldn't say.

[Beaker]

I can't really say what's going on here - all I know is mine seem to work fine, but I have not measured them, so don't know what they read.

One thing comes to mind though - Muadzin mentioned in one post about SMD JFETs that they need a much larger trimmer value to bias them correctly. I think he said 100k instead of the usual 10k. I have been using 25k trimmers on mine and not had a problem so far.

It might not make any difference at all to your build, but perhaps worth trying?

EDIT: just thinking out loud here, but maybe it's something similar to the Russian Ge transistors, that measure "too low" to work in some fuzzes, but sound great. Or am I talking bollocks?

[bsas]

This is the problem I am facing: I was trying to do the Tayda EP Booster PCB, but basically it wasn't boosting at all! When I used the NOS it just worked fine!

That is the moment I measured it to verify and I saw the weird measurement.

Also, I was trying to do the EchoPlex pre-amp PCB from Aion (Ares Preamp) and I wasn't able to find good SMD 2N5484 and 2N5485 that got even close to the standard measurements. I am getting afraid that when I solder them to the adapter board, they get affected somehow.