Some Guidance First Pedal JFET

[hyped]

Hi Guys,

Thanks for having me here  it seems like a great forum and lots of great contributions, especially for people new to electronics like myself! I was hoping I could possibly get some advice (sorry in advance for my stupid questions) Now I'm going to tackle the boost projects for newbies as soon as I can get a few parts I need.

In the meantime I was hoping I could leverage some 2sk170GR I have on building a  pedal based on the Tillman design

http://www.till.com/articles/GuitarPreamp/

I had a few questions/areas I was looking for some advice on and was hoping I could get some guidance.

The pedal would only be for tone really, I just thought it would be good to build something like the Tillman to see how JFETS sound in guitar applications and maybe a simple pedal I could use directly into my high z on my sound card when Im playing around at home.

I was wondering, is the 2sk170GR a good candidate? Im only look for a modest gain in line with the Tillman Design.

The 2SK170 I have were marked

RDS: 73ohm
VGS: -0.4
IA: 5.64MA (im assuming this is IDSS)

If so, I was wondering if I can get some advice on what I should set the drain/source resistors at and any other changes to the design I should make??

I have looked at various online calculators/formula's and I have had a few different figures and as Im really new to electronics in general so Im not sure if Im heading in the right direction.

Based on 9V supply the latest calculation I came up with is RD of 3191ohms & RS of 35ohms does this sound right??

If that is close, then from my understanding (sorry if wrong is) the ratio between RD/RS is roughly the gain, in which case it would be 91x or 39db way more than the I suppose 6dbish
I was thinking of!

If I could get some feedback and guidance that would be greatly appreciated. Really glad to be here and look forward to building my first pedal and sharing the outcomes 

Again, sorry if my questions sound unclear or stupid just started my journey

[duck_arse]

hello, hyped, welcome to the forum (we take almost anyone here).

the K170 looks a likely candidate for your purpose. PRR will likely be along with some design numbers shortly, but in the meantime, head on over to runoff groove, and look for the fetzer build guides. there is a lot of why, where, and how in them.

[edit: ] make sure you cop the centre pin is gate, as shown on the datasheet you no doubt have.

[rutabaga bob]

Glad to have you with us!
- Larry

[PRR]

Welcome.

> ratio between RD/RS is roughly the gain

In the Tillman this is almost 6.8K:2.2K, almost 3, but less because the JFET adds to Rs and R4 loads R3. Say 2.

You really want a higher Vgs.

> see how JFETS sound in guitar

Like any device, it depends a LOT on the circuit.

You can try Tillman's values (derived from a cheaper JFET of higher and more-suited-for-this Vgs) but change 2.2K to 1K. But that will increase gain, and may clip the output.

The Tillman is seductive because it has been around longer than many folks have been playing, it looks simple, and with lucky-dip JFETs it often does work OK. As Tillman himself notes, an unlucky choice of J201 won't be a happy preamp. Going to a radically "hotter" JFET makes troubles (clipping or gross distortion) more likely.

[hyped]

hello, hyped, welcome to the forum (we take almost anyone here).

the K170 looks a likely candidate for your purpose. PRR will likely be along with some design numbers shortly, but in the meantime, head on over to runoff groove, and look for the fetzer build guides. there is a lot of why, where, and how in them.

[edit: ] make sure you cop the centre pin is gate, as shown on the datasheet you no doubt have.

Hey duck,

Thanks for the greetings bud appreciated. I'll definitely check out runoff actually came across it in some searching and had a quick look but I'll read some more.

Roger on the gate, got that one although have some around confusing on which is D and which is S due to some bad data sheet drawing out there!

Thx again

[hyped]

Glad to have you with us!
- Larry

Thanks mate



Sent from my 486 IBM WIN 3.1x Desktop...

[hyped]

Welcome.

> ratio between RD/RS is roughly the gain

In the Tillman this is almost 6.8K:2.2K, almost 3, but less because the JFET adds to Rs and R4 loads R3. Say 2.

You really want a higher Vgs.

> see how JFETS sound in guitar

Like any device, it depends a LOT on the circuit.

You can try Tillman's values (derived from a cheaper JFET of higher and more-suited-for-this Vgs) but change 2.2K to 1K. But that will increase gain, and may clip the output.

The Tillman is seductive because it has been around longer than many folks have been playing, it looks simple, and with lucky-dip JFETs it often does work OK. As Tillman himself notes, an unlucky choice of J201 won't be a happy preamp. Going to a radically "hotter" JFET makes troubles (clipping or gross distortion) more likely.

Hey PRR

Thanks for taking the time to reply appreciated.

I did note a lot of people having issues with design by "hoping for best" and seeing I'm still new to this all wanted to seek advice. Seems with the wide tolerances with jfets in general that even the j201 themselves can need tweaking given the variance in specs from one to another

So it seems the 2sk170 probably more suited higher gain ratio /lower signal and maybe pot luck.

If you were to try the circuit do you think it would be better to try the RD6.8k/RS1.1k or keep it closer to the ratio I mentioned and place a voltage divider to after the output of stage?

Either way seems it maybe not worth trying

Thanks all glad to be here and excited to get a few stomps under the belt






Sent from my 486 IBM WIN 3.1x Desktop...

[duck_arse]

I'll type this slowly as I see you are still 486 (really?):

about your D and S confusions - don't. they are as good as 100% interchangable on the jfets (not mosfets) we use in the circuits we use. there are exceptions to this rule (so we don't use them).

[edit :] I meant to put that last night, but forgot.

[hyped]

about your D and S confusions - don't. they are as good as 100% interchangable on the jfets (not mosfets) we use in the circuits we use. there are exceptions to this rule (so we don't use them).

Thanks DA, yeh I heard for most circuits they are interchangeable, which given the confusion on pinouts is a good thing

Haha


Sent from my 486 IBM WIN 3.1x Desktop...