Hi All - I bought a pack of 10 "sj74"'s recently from China, with a limited expectation of getting real parts. I've a Mini Aleph-J I'm halfway through building and getting appropriate jfets is proving a sticking point.
I did a quick test on a few of them (ala Borbely's jets articles - attached) and came up with the following results... Note I used a virtual ground provided by a pair of 1K5 resistors across a 20V supply, as opposed to dual supplies of 10V
#1 Vp= -1.20V, Vgs @2mA = -335mV, Vgs @ 5mA = 106mV
#2 vp= -1.05V, Vgs @2mA = -294mV, Vgs @ 5mA = 158mV
#3 Vp= -1.55V, Vgs @2mA = -398mV, Vgs @ 5mA = 42.7mV
First time I've done this, so if there's any glaring errors, please mention it... other than the fact that as I understand it these aren't sj74's as I understand how to interpret the results.
So, my question to you all - what are they? And can I actually still use them in a mini AJ?
Addendum - I have a pile of J174's headed my way as a potential substitute... pending my learning to use them in that particular circuit!
I did a quick test on a few of them (ala Borbely's jets articles - attached) and came up with the following results... Note I used a virtual ground provided by a pair of 1K5 resistors across a 20V supply, as opposed to dual supplies of 10V
#1 Vp= -1.20V, Vgs @2mA = -335mV, Vgs @ 5mA = 106mV
#2 vp= -1.05V, Vgs @2mA = -294mV, Vgs @ 5mA = 158mV
#3 Vp= -1.55V, Vgs @2mA = -398mV, Vgs @ 5mA = 42.7mV
First time I've done this, so if there's any glaring errors, please mention it... other than the fact that as I understand it these aren't sj74's as I understand how to interpret the results.
So, my question to you all - what are they? And can I actually still use them in a mini AJ?
Addendum - I have a pile of J174's headed my way as a potential substitute... pending my learning to use them in that particular circuit!
That is a nice read, Thanks for the post!!
I just got some LSK170C's that I want to use for an Alpeh X and that will help me more of how to understand what I am getting into with JFET's.
I will probably the the ACA (Amp Camp Amplifier) First with them.
Here is a simple little circuit that I discovered just recently that will help find the parameters of both N and P type JFET's.
Simple circuit lets you characterize JFETs | EDN
Enjoy !!
jer
I just got some LSK170C's that I want to use for an Alpeh X and that will help me more of how to understand what I am getting into with JFET's.
I will probably the the ACA (Amp Camp Amplifier) First with them.
Here is a simple little circuit that I discovered just recently that will help find the parameters of both N and P type JFET's.
Simple circuit lets you characterize JFETs | EDN
Enjoy !!
jer
positive Vgs indicates you have increased the bias too far.
If those results are correct then the Idss is far lower than 5mA.
From there you can then see that for low Idss devices the Vp is far too high for a high gm device.
Measure the Idss and using the Vp compare to a typical 2sj74 to see if you have a high gm device.
You can use Borbely's equation to determine gm.
Idss's
@AndrewT - I did actually work out the idss's for these parts previously, and they range from 3.9mA to 5.1mA over the set of ten. I'll get the idss for these three and do the math on getting the gm.
@geraldfryjnr - Thank you for the link to that circuit - I'll have a read shortly. I'll attach the other Borbely article to this post for you should you not have found it already.
I guess the next question is "what are they then?"
... just got the idss's #1 is 4.42mA, #2 is 3.96mA, #3 is 4.90mA.
@AndrewT - I did actually work out the idss's for these parts previously, and they range from 3.9mA to 5.1mA over the set of ten. I'll get the idss for these three and do the math on getting the gm.
@geraldfryjnr - Thank you for the link to that circuit - I'll have a read shortly. I'll attach the other Borbely article to this post for you should you not have found it already.
I guess the next question is "what are they then?"
... just got the idss's #1 is 4.42mA, #2 is 3.96mA, #3 is 4.90mA.
Just had a quick reread of the Borbely article and he defines Vp as 0.1uA. I believe I had about 11uA during that test so I presume my data might be inaccurate. I reckon I might call it a night and give it another whirl tomorrow morning.
@AndrewT - Thanks for the confirmation. I'm going to redo my rather jury rigged test setup tomorrow and retest the lot I think and then go through the math of it all properly.
@AndrewT - Thanks for the confirmation. I'm going to redo my rather jury rigged test setup tomorrow and retest the lot I think and then go through the math of it all properly.
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I guess he did.
@ Aspringv: I'm curious how you made the 2 mA en 5mA CCS for the test jig?
Are your 2SJ74's GR types?
Can you post a picture? Maybe they are the same fakes as I have...
I ordered 10 and 20 from different Dutch sources and they appear to be from the same fakes distributor.
They also have a gm of about 8 to 9 mS.
Strange enough when I put them in a F5 test amp they seems to work fine....
Still wondering which P-channel JFET they used to produce the fakes.
My BL's range from 3 -16 mA Idss, that says enough. Also when testing for Idss the real JFETS slowly rise in Idss when heating up. These fakes decline when heating up.
Walter
Testing methods
I'm about to redo my testing and do it a bit more methodically than yesterdays efforts, and get a conclusive answer on Gm and hopefully have all the clues for what they actually are.
@AndrewT I did indeed do the Vp test as detailed by Borbely, I'll be redoing this morning. The jig I've set up is as per Borbely...ish. It's on a protoboard, and isn't particularly neat, but is reliable.
@WalterW. I used a J310 and a 2k trimmer to make a variable current source, and did a run with it set to 2mA and then a run at 5mA (Target current is within .1 of the target mA value. I also did a few sims of the setup so I could sanity check my results. Attached below. They are labelled as BL's and I found a matching photo elsewhere online somewhere on a Czech site that seemed to be discussing fakes. I'll try and get a decent photo later.
@bear I'll go looking for that thread later on - Thanks!
To be honest this is now something I'm doing for the sake of learning a bit more about Jfets, and because who doesn't enjoy playing detective?
I'm about to redo my testing and do it a bit more methodically than yesterdays efforts, and get a conclusive answer on Gm and hopefully have all the clues for what they actually are.
@AndrewT I did indeed do the Vp test as detailed by Borbely, I'll be redoing this morning. The jig I've set up is as per Borbely...ish. It's on a protoboard, and isn't particularly neat, but is reliable.
@WalterW. I used a J310 and a 2k trimmer to make a variable current source, and did a run with it set to 2mA and then a run at 5mA (Target current is within .1 of the target mA value. I also did a few sims of the setup so I could sanity check my results. Attached below. They are labelled as BL's and I found a matching photo elsewhere online somewhere on a Czech site that seemed to be discussing fakes. I'll try and get a decent photo later.
@bear I'll go looking for that thread later on - Thanks!
To be honest this is now something I'm doing for the sake of learning a bit more about Jfets, and because who doesn't enjoy playing detective?
Results
And here you go.
Jfet Vp Idss Gm Vgs@2mA
1> 1.21 4.22 6.98 -335
2> 1.06 3.86 7.28 -249
3> 1.57 4.72 6.01 -399
4> 1.41 4.46 6.33 -372
5> 1.17 4.42 7.56 -380
6> 1.13 4.39 7.77 -340
7> 1.28 3.96 6.19 -299
8> 1.00 3.62 7.24 -236
9> 1.40 4.46 6.37 -373
10> 1.06 3.72 7.02 -277
Gm is a calculated value in this chart.
The seller has offered to take em back and refund me (and pay for postage also) but if these are a useful parts for something then it's probably not worth the hassle. There's actually some nice matches in there!
Attached are pics of data and jig.
So what is the minimum Gm for a jfet for a Mini AJ? Probably best to start a new thread for that question
And here you go.
Jfet Vp Idss Gm Vgs@2mA
1> 1.21 4.22 6.98 -335
2> 1.06 3.86 7.28 -249
3> 1.57 4.72 6.01 -399
4> 1.41 4.46 6.33 -372
5> 1.17 4.42 7.56 -380
6> 1.13 4.39 7.77 -340
7> 1.28 3.96 6.19 -299
8> 1.00 3.62 7.24 -236
9> 1.40 4.46 6.37 -373
10> 1.06 3.72 7.02 -277
Gm is a calculated value in this chart.
The seller has offered to take em back and refund me (and pay for postage also) but if these are a useful parts for something then it's probably not worth the hassle. There's actually some nice matches in there!
Attached are pics of data and jig.
So what is the minimum Gm for a jfet for a Mini AJ? Probably best to start a new thread for that question
Attachments
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Wow.
So the fake's i have have an even lower Gm than these two sample curves (thanks BenY - really quite helpful to see this visually).
Correct me if I'm wrong, but if you paralleled (for discussions sake) fake sample #1 and fake sample #2 the curve would be #1+#2? And this is the workaround to create a pseudo high Gm Jfet?
So the fake's i have have an even lower Gm than these two sample curves (thanks BenY - really quite helpful to see this visually).
Correct me if I'm wrong, but if you paralleled (for discussions sake) fake sample #1 and fake sample #2 the curve would be #1+#2? And this is the workaround to create a pseudo high Gm Jfet?
Ahum, err, if you get Linear Audio Vol 6 it comes with a free LSK170B and an LSJ74B. Gets you started!
jan
jan
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