2SK129A dual N-JFET & 2SK163 single N-JFET specs and replacement

smurfer77

Super Member
I have a Sansui AU-X11 and was interested in subs for some of the unobtainium FETs. However, very little info was available so I decided to do some measurement to share with everyone here. I also ordered some substitutes from Linear Integrated Systems (LIS) who happen to be just across the bay from here in SF, and checked out how they shape up. LIS are the only folks I know of that are currently manufacturing (or rather, having manufactured for them), dual N-JFETs and complementary dual P-JFETS. They have a few neat single JFETs too and are expanding there line-up. I saw Paul Norton and some of the guys (Kevin handles my orders) at Burning Amp 2016 a couple of weeks ago. Here is a video of Paul describing their product line.

I found LIS partly because of a nice Sansui AU-919 thread where @noveltone mentioned the LSK389 as a possible sub for the 2SK129A. This was pointed out to me by Hyperion and some supporting info came from Ronito over in my AU-X11 thread. The LSK389 is readily available and under production by LIS.

However, I wasn't happy with existing info to just go for the LSK389. And there are variants of the LSK389... which one to buy? Others have mentioned various options for sticking two single JFETs together but I don't have a reliable line on any of those mentioned in various threads (including that AU-919 thread)... I wanted to stick to something in production too! Oh, did I mention the LSK389 are monolithic? (the 2sk129A are two singles stuffed into one package) So the matching could be really good!

Here are a couple of pics. The single unit is the 2SK163 (which is a single N-JFET in the Sansui AU-X11).
DTA2Yg5.jpg

BTc7aW1.jpg

Note the pinouts. GPS16 pointed out in my AU-X11 thread that it might be worth double-checking as if the same single transistor packages are stuffed into the can, but flipped around, you might think D and S should be reversed on one side, compared to the other. However, they are interchangeable as we will see later (not that uncommon for small signal JFETs)

In that AU-919 thread @Stene showed a graph for the 2SK129. From that graph I_dss is 2mA (@V_ds = 15 V) and V_gs_off is -.3 to -.4V. I've pulled 4 working (yeah, mine are all working, I don't need new ones, I'm just curious about the substitute for now!) 2SK129A and the characteristics are different. And then over in that AU-919 thread, @noveltone kindly measure for us about 7 to 8 mA for the I_dss of the 2SK129A. It turns out noveltone's I_dss measurements on hits one device, agree well with what I measured on four devices.

2SK129A I_d vs V_ds curves for different V_gs
I don't trust cheap curve tracers anymore so the measurements were made point by point manually, using lab power supplies. This curves are for a single channel of a single device but all four of my 2SK129A are really similar. I_dss (V_ds = 10 V, V_gs = 0 V) was between 7 and 8 mA for all 8 channels of my four 2SK129A.
5iyUSbG.png


X1P8XGZ.jpg


2SK129A I_d vs V_gs (V_ds = 10 V)

V_gs_off was about -0.75 for the single channel plotted below. I.e. V_p = 0.75 V. This was pretty similar for all 8 channels, but you know V_gs_off can be a bit tricky to determine precisely... but you get the idea.
BWLZ9k0.png

CQeDKA8.jpg


2SK129A Drain & Source symmetry
So back to the question of the pinouts and whether D and S are interchangeable on the 2SK129A. Well it turns out that, like some other small signal JFETs, the structure is such that D and S are symmetric and interchangeable. Below I took a single 2SK129A and you can see what happens when I flip around D and S on both channels... not much. As we will see later, LSK389 is also interchangeable. You may also notice from the I_dss that I used a different 2SK129A from the one in the graphs above. Here I_dss is about 7.8mA at V_ds = 10 V. Gate-source voltage here is 0 V of course.

pjM9prk.png

2nqXHFH.jpg



Substitutes

If you go to the LIS website you can grab a spec sheet for the LSC389. V_P os meant to be 0.2 to 2 V. The LSK389C is meant to have I_dss between 10 mA and 20 mA (@10V_ds), but I grabbed some anyway to check them out. I got the TO-71 metal can package.

What I found is that they are not nearly as well matched as in the 2SK129A. But, pretty good.

For V_ds from 1 to 30 V, I_d goes from 12.6mA - 16 mA... but note that the upper V_ds measurements were tricky because the can really heats up and there is significant drift. The temperature response was negative... actually i got 15.93 mA at 20V and 15.2 mA at 30V steady-state... if I take measurements really quickly before it heats up, I can get something else. Case temperature went from 25 C at 1V, to 32 C at 20 V and 50 C at 30 V.... well we are past the 400mW Pd so can't really complain. But yeah, I got the feeling that the dissipation for the C version of the LSK389 is really a bit insufficient.

I_dss for the LSK389C at 10 V was for the 4 samples:

(I_dss1, Idss2)

16.0mA, 15.09mA

16.2mA, 15.1mA?

12.84mA, 13.04mA

13.10mA, 13.25mA

which gives Idss1/Idss2 within spec (10% variation), but not as good as the 2SK129A that I have at hand. I have a "?" beside one of the I_dss values because it kept jumping around between two values, almost like it was bistable somehow... it was not my setup. I have more detailed data that could be used to make curves if anyone wants, since curves are not given in the spec sheet.

Now, the LSK389B though, is what we are really interested in. So on tho that next with some curves (which are not in the spec sheet). (sorry, I meant to finish this post including the single channel JFET data tonight, but I have an early start... so will resume this thread soon... but don't worry, it all works out and we have a nice sub for both!).
 
for your distrust of component checkers: I have some (although old NOS) 2-pin FET current sources. (CR530 if I remember)
If I hook one up to my DCA75 component checker, it deteriorates every push of the button, about 6 pushes and it is totally defective.....

The Huntron tracker is even capable of deteriorating small signal bjt transistors.
 
2SK129A Transconductance and more on pinch-off voltage

I derived a bit more data from that which I posted above for the 2SK129A. Basically I calculated transconductance from the derivative current with respect to gate-source voltage. And I used the approximate equation Ids/Idss = (1-Vgs/Vgs_off)^2 and rearranged to get an equation for Vgs_off. Finally you can see an average of V_gs_off is -0.69 V. I.e. Vp ~ 0.68 V. The previous value of V_gs_off of -0.75 V, I got from the graph by eye. Not a big deal either way.
A3vvoXJ.jpg

RoEpddY.png
 
LSK389B dual N-JFET according to spec sheet has Idss in the range of 6-12, which is more like what we are looking for in terms of a sub for 2Sk129A. Again, I ordered the TO-71 package since it will fit nicely (but note pinout differences).

I ordered a few of these and here is data from a typical unit, but note that FETs vary more than you would expect (but these LIS jobs seem pretty well matched straight out of the packet).
6tOKGCw.jpg

i2CK1Wt.png


With it's lower Gm and Idss than the LSK389C, the LSK389B runs at lower power, and the dissipation issue I saw on the C version is not really an issue on the B version. You can run this up to 300mW (as I did with V_ds = 30V), and the package only gets up to body temperature. But still, I did notice temperature drift, more than on the 2sk129A, and had to take care in making measurements to not move too quickly from high power to lower power, otherwise I had to wait a while for things to settle down. But I'm perfectly comfortable with running this thing at full steam, especially with typical V_ds of ~10V, whereas the C version I wouldn't run at it's rated power... it's too hot for me and temperature started to affect parameters too much for my liking.

Here I tested interchangeability of D and S. "Unit 1" means a single dual fet device where #1 and #2 are the two channels on the single package.Fairly symmetric.
ZtwGSCr.jpg


Here is the dependence of current on gate voltage for V_ds = 10 V. So we are looking at a very similar V_gs_off = -Vp, as for the 2SK129A.
16RJKBp.png


And from all of that we get the transconductance and V_gs_off. Vp is about 0.61 V. Again, note that Vp is rough and can vary a bit, but you get the idea. And transconductance is ~20 at close to full open, similar to the 2SK129A. (see previous post for calculation details).
hfBC3aa.jpg

mGuUYyk.png


So in summary we have:

2SK129A: Idss ~7.5mA (Vds=10V,Vgs=0V), Gm ~ 18 (Vds=10V,Vgs=0V), Vp ~ 0.68 V (Vds=10v)

LSK389B: Idss ~7.8mA (Vds=10V,Vgs=0V), Gm ~ 21 (Vds=10V,Vgs=0V), Vp ~ 0.62 V (Vds=10v)

*Keep in mind that FETs parameters vary from sample to sample, much more than BJTs, so you may want to test/match for yourself when you order some, but it looks like we have a good, and current production, sub for the 2SK129A which is used in differential input stages in numerous vintage models. Again, if you want other specs let me know... i've looked into capacitance and frequency response too, but just for now leave it at saying the critical capacitance parameters on the LSK devices (drain-gate) are super low as far as I can tell.

More to come on the single N-JFET in the Sansui AU-X11 (2SK163) protection circuit of the driver board (which doesn't appear on the Sansui AU-X1). I know this is a bit niche, but I will review specs of a bunch of devices that might still be interesting for other amps.
 
Excellent analysis - thank you for putting in the time and effort to produce the above data and summary - truly appreciated
 
I'm always knocked out by the work and research I see some of you guys do, and selflessly share with the rest of us.

Smurfer77, that was an amazing bit of research. Thank you so much for sharing!
 
So in summary we have:

2SK129A: Idss ~7.5mA (Vds=10V,Vgs=0V), Gm ~ 18 (Vds=10V,Vgs=0V), Vp ~ 0.68 V (Vds=10v)

LSK389B: Idss ~7.8mA (Vds=10V,Vgs=0V), Gm ~ 21 (Vds=10V,Vgs=0V), Vp ~ 0.62 V (Vds=10v)

*Keep in mind that FETs parameters vary from sample to sample, much more than BJTs, so you may want to test/match for yourself when you order some, but it looks like we have a good, and current production, sub for the 2SK129A which is used in differential input stages in numerous vintage models. Again, if you want other specs let me know... i've looked into capacitance and frequency response too, but just for now leave it at saying the critical capacitance parameters on the LSK devices (drain-gate) are super low as far as I can tell.

More to come on the single N-JFET in the Sansui AU-X11 (2SK163) protection circuit of the driver board (which doesn't appear on the Sansui AU-X1). I know this is a bit niche, but I will review specs of a bunch of devices that might still be interesting for other amps.

Thanks for all this work and input Smurf ... I'll dive soon into a 919 I bought recently and even if (due to COVID) I'm far from home, I do some homework in order to prepare for the needed recaping this beast needs !
there measures will allow me to assess if my SK129A needs to be changed and what by !!
I found some genuine dual mounted sets but they are proposed at a prohibitive price of 50USD the pair sold in their host can !... It's a bit excessive don't you think ?
 
Hi there,
Hopefully you will not need to replace the 2sk129 dual fets, so please study the case well to determine what is the problem inyour unit. If you find that the dual fets are bad, and the lsk129B is difficult to get, then you may want to consider getting several 2sk117GR and match them. B+D has them in stock but you may need to buy a dozen to build two mathec pairs. The 2sk146 is a bit more difficult to replace, the interfet IFN146 in mouser is out of stock but comming soon.
As far as I learned from this forum, there were relatively few instances where dual fets required replacement. Good luck with your diagnosis of the AU-919.
 
Hi there,
Hopefully you will not need to replace [...] Good luck with your diagnosis of the AU-919.
Thanks Joseph for the reply.
As I said, I'm abroad and kinda stuck in Middle East so not yet facing my newly acquired 919.
I'm just warming up and impatiently starting to build a project in Mouser for a recapping session. But at least I'll be prepare should something appears to be faulty in these components .

Now and that is a way far off from the subject, some specialists of this amp are underlining another potential issue with the varistors labelled SV03, MV12 and MV103.
I digged a bit and found out basically this :
MV 12 > 2 serial 1N4148
MV103 = SV03 > 3 serial 1N4150
Usage of Series types multi-chip silicon diodes as in HERE can do the job with values almost as close as the genuine product.
Have you any input on this subject ?

Sorry for digressing from the original subject ...
 
Now and that is a way far off from the subject, some specialists of this amp are underlining another potential issue with the varistors labelled SV03, MV12 and MV103.
I digged a bit and found out basically this :
MV 12 > 2 serial 1N4148
MV103 = SV03 > 3 serial 1N4150
Usage of Series types multi-chip silicon diodes as in HERE can do the job with values almost as close as the genuine product.
Have you any input on this subject ?

Sorry for digressing from the original subject ...

https://audiokarma.org/forums/index.php?threads/replacing-the-stv-3h-and-4h-diodes.279530/
https://www.audiokarma.org/forums/index.php?threads/sv03-sv04-diode-replacement.756284/
 
facing my newly acquired 919.
I'm just warming up and impatiently starting to build a project in Mouser for a recapping session. But at least I'll be prepare should something appears to be faulty in these components .

Hello, and welcome to Audiokarma.
Considering that you will work on a new unit, you will greatly benefit if you start a new thread about your AU-919. The members of this forum are very enthusiastic, and many are very knowledgeable, so start your thread, and stay tuned. I am no expert but l once you start your thread i will point out a number of threads i have compiled about that particular model and answer anything i can.
Jose
 
Hello, and welcome to Audiokarma.
Considering that you will work on a new unit, you will greatly benefit if you start a new thread about your AU-919. The members of this forum are very enthusiastic, and many are very knowledgeable, so start your thread, and stay tuned. I am no expert but l once you start your thread i will point out a number of threads i have compiled about that particular model and answer anything i can.
Jose
Thanks for reply and noted your advice that I will follow in order to raise my concerns as I build up the replacement list of components to be changed or that may need to be
I have already followed posts on the 919 and even implemented a post opened by LeeStereo concerning the 555A (He seems to have lots of experience in Sansui amps, although controversal views on restoration when seen by purists to whom I showed some revisions of the caps values proposed... this is another subject.) Post of my work on my 555A can be seen HERE.
That close the parenthesis. Lets meet in the one I'll open later.
Cheers, Yves
 
Back
Top Bottom