HELP! Jfet Help Please - Solved

[BuddytheReow]

Hey all,

I took some time this evening to work on a jfet circuit. See below schematic. My general understanding is that the fet needs half the input voltage to work. The schematic calls for a 100k trim pot but I needed a 2m trimmer to get the input down to 4.5v. The sound is very splatty and doesn’t sound at all like it’s supposed to. The signal dies off almost immediately too. I tried with a j201 and now a 2n5457. The pinout in the pic is GSD read left to right. Can someone please help? There’s gotta be something stupid I’m missing. I took out the 68k resistor and put in a 100n coupling cap. The output cap is also 100n.

 

[fig]

Looks like your drain and source are reversed.

Or did you mean "left to right" in the picture? (the FET is facing the other way)

Okay the pinout is DSG, so in the pic it is GSD...you're good there!

 

[fig]

where is the volume pot?
the wiper of the trimmer goes to a 22n then the vol pot. The 3rd leg...is the one you want to send to the FET.

 

[BuddytheReow]

where is the volume pot?

I don’t think it needs one. I just want the fet to pass signal the way it’s supposed to. I tried a circuit like this months ago and gave up. I’m trying again and hitting the same wall…

 

[fig]

Have you tried fiddling with the source resistance?

 

[BuddytheReow]

When you say fiddle, what kind of range are we talking? I think I have a 1.5k one in there now.

 

[fig]

I would try a 1KΩ and see which way it swings, then take it from there.

 

[BuddytheReow]

Put a 1k in there and no change. I have to strum HARD to get anything through…

 

[fig]

Its not really a boost

 

[BuddytheReow]

Ugh. So frustrating. @Chuck D. Bones am I missing something here?

 

[fig]

What is your expected gain sir?
Edit: I won't add to your frustration...I've built those and they don't always boost. Some signals are attenuated. The math is above me though. Good luck!

 

[BuddytheReow]

What is your expected gain sir?

I don’t understand the question.

 

[BuddytheReow]

What is your expected gain sir?
Edit: I won't add to your frustration...I've built those and they don't always boost. Some signals are attenuated. The math is above me though. Good luck!

@fig i appreciate you trying to help. I’m merely trying to make a simple jfet circuit. Similar to building an lpb-1 when trying out BJTs.

 

[jimilee]

Hey all,

I took some time this evening to work on a jfet circuit. See below schematic. My general understanding is that the fet needs half the input voltage to work. The schematic calls for a 100k trim pot but I needed a 2m trimmer to get the input down to 4.5v. The sound is very splatty and doesn’t sound at all like it’s supposed to. The signal dies off almost immediately too. I tried with a j201 and now a 2n5457. The pinout in the pic is GSD read left to right. Can someone please help? There’s gotta be something stupid I’m missing. I took out the 68k resistor and put in a 100n coupling cap. The output cap is also 100n.


View attachment 21707

According to the ROG article, you need to measure the jfet and adjust your resistors (68k) according to the calculations with the linked calculator. For example, in the linked PCB layout in that article, it’s a 33k. If you want to mess with that value, I’d start there.

 

[lcipher3]

input coupling cap? edit: I saw you already did that...

Try changing R3 to 150 ohms

AMZ - Boosters and Distortion

The effect of gain on distortion in guitar booster pedals.

www.muzique.com

 

[JamieJ]

Have you seen this? http://www.runoffgroove.com/fetzervalve.html

This is the article that image is from. That images is the original fetzer valve.

This is the revised version-


The article talks in detail about JFET biasing.

My next question would be what vp and ldss does that JFET have?

 

[temol]

How about moving 1M resistor after the input cap? Connect 1M between Gate and GND.

 

[BuddytheReow]

Ok all. I stepped away from this late yesterday to regroup. I did take a long read of the runoffgroove article. @temol I think you guessed right! Once I move the 1m resistor to ground things sounded a lot better. I also calculated the drain resistor needed, put in a 20k trimmer and dialed in it. It finally works!

See what happens when you experiment but have no idea what you’re doing?

Now to mess around with jfets for a bit…

Thanks all!