J201 substitute? Oh no, not again...

[lrgaraujo]

That's right. To some degree it's possible to correct frequency response by changing resistor or capacitor value along the way or maybe even adding/removing R or C. Unfortunately, sometimes an improvement in one place breaks something else. Also, when making corrections it's necessary to observe both - frequency response and sinewave shape.

I've just tried to match 2SK117GR circuit with J201.
After some tweaking I managed to get from this:
View attachment 40246View attachment 40247
to this:
View attachment 40248View attachment 40249

I love this! Would you mind sharing your spice models for the fets, Temol? I have a lot of 2sk117-gr, so it would be nice to be able to recreate some J201 stages

 

[Chuck D. Bones]

Well the smallbear generic J201's arrived this morning..... I'm pretty good at testing tubes, but have no idea what to look for in testing J201's.....
View attachment 40614

Actually, those number all look good. Idss = 1.1mA is in spec.
The next two numbers indicate the drain current at some arbitrary gate voltage, Again, looks ok.
Last two number say: when the drain current is zero, the gate voltage is 881mV. That's the definition of Vp. Also in spec. The gate voltage is actually negative; the minus sign is implied.

Bottom line: that J201 looks real.

 

[RetiredUnit1]

Actually, those number all look good. Idss = 1.1mA is in spec.
The next two numbers indicate the drain current at some arbitrary gate voltage, Again, looks ok.
Last two number say: when the drain current is zero, the gate voltage is 881mV. That's the definition of Vp. Also in spec. The gate voltage is actually negative; the minus sign is implied.

Bottom line: that J201 looks real.

Great, and these were the Generic ones for .67 each! Thanks. I have to many projects going right now to even attempt building a tester. Big ones, painting the house, fixing the roof on my shop, and a JCM800 2203 Superlead 100 (oh, and a Champ. That's right, I'm building a 5w and a 100w at the same time, lol)

 

[temol]

I love this! Would you mind sharing your spice models for the fets, Temol? I have a lot of 2sk117-gr, so it would be nice to be able to recreate some J201 stages

A bit of explanation - in most cases I use the same model for all transistors in the circuit (of course, if there is one transistor model in the schematic). So in the simulation all transistors are with the same Vp and Idss, which in reality would require matching transistors.
If we want a fairly faithful simulation of the circuit, with transistors with specific parameters, we need to modify the ltspice models and have a separate model for each transistor.
I do that sometimes, but with some simplification when it comes to modeling parameters. Vp - a simple matter, just measure and enter the value in the model (Vto). BETA is responsible for the second parameter - Idss (in fact, not only BETA, but for my needs changing BETA is enough). There is no 1:1 measurement ratio here so I try different BETA values until readings match.
What readings? Simply, in ltspice, I measure the Vp and Idds of the transistor, with the same measurement circuit as in reality. I change BETA until readings more or less agree with the real transistor.
At the beginning, you always need some transistor model as a starting point anyway. If I don't have it, I'm looking online. This was the case with 2SK117. I found the model, simulated the measurement of Vp, Idss, compared with the real transistor and started modifications.

Here's the original model I found.
.model 2SK117GR NJF (Vto=-0.42 Beta=30m Lambda=5m Is=10f Isr=100f Vk=100 Alpha=10u Rd=15 Rs=15 CGD=9p CGS=15p Pb=0.55 M=0.33 mfg=TOSHIBA)

and after modification

.model 2SK117GR NJF(Vto=-0.706 Beta=13.95m Lambda=5m Is=10f Isr=100f Vk=100 Alpha=10u Rd=15 Rs=15 CGD=9p CGS=15p Pb=0.55 M=0.33 mfg=TOSHIBA)

And one of my J201 models.
.MODEL J201 NJF(VTO=-0.718 BETA=1.031M LAMBDA=2M IS=114.5F RD=1 RS=1 CGD=4.667P CGS=2.992P M=.2271 PB=.5 FC=.5 VTOTC=-2.5M BETATCE=-.5 KF=604.2E-18)

Here you can find some math behind Idss in ltspice

JFET model generator

JFET spice model generator It might be not the most suited place to post it here, but since this is my favorite section in diya and we are exclusively using FETs for amplifications in our projects so I decided to share something that I developed during the weekend and I hope it will be helpful...

www.diyaudio.com

 

[lrgaraujo]

A bit of explanation

Thanks a lot, Temol! That's a nice explanation!