SOLVED - Can't Bias JFET in the Jack Orman FET Output Stage

[Fabeii]

Hi all guys!

I've built this nice little circuit but I'm experiencing some problems with it. It sounds nice, but it's not behaving as it should.



I've designed it on Eagle and ordered some PCBs, so I've tested it on pcb and on the breadboard to be sure, with the same results.

I'm using a pot instead of the 4k7 resistor, no signal, 9volt from regulated supply, DMM probe on the fet Drain.

I just can't get the voltage to 4.5v using a J113 (MPF102 replacement) and I even tried a 1Meg pot but I could reach only 6v.
I managed to get it right with a 2N5457 but no luck with J113s (and J112, J111 just because I had them in my stock).

What do you think? What would you do?

Thanks a lot!

[amz-fx]

The variations in jfets, even within the same part number, will determine what values it takes to get the bias right.

1. I would use the 2N5457
2. If you have to use the transistor (MPF102) that you have, try a 1N4001 diode instead of the LED.
3. The bias does not have to be 4.5v, it will be more like 6v to 6.5v when using the LED.

Best regards, Jack

[Fabeii]

Oh my, the man itself!! Thanks a lot for all your work and for sharing it so willingly!!

1. Yes, that's what I was doing but I just wanted to know if I was doing something wrong.
2. Going to try asap and report, maybe someone would find it useful
3. That's very good to know, because my hears were telling me that but on the Shaka Braddah Notes I read:

9/25/98 - New Mod by Jack Orman. Here's a red heart for the Shaka Braddah! This mod will give the Shaka Braddah more harmonics and fullness with the expense of the big boost. Two components need to be changed on the FET booster at the end of the circuit. The mod is noted in the schematic. Basically the 6.8K resistor is changed to 4.7K and the 27K resistor is changed to a RED LED with the cathode going to ground. To see where this idea came from, look at Jack Orman's FET output circuit.

9/26/98 - More modification of the output stage. In order to get a good boost going again, you need to lower the value of the resistor going from 9V to the drain of the FET (MPF102). Adjust the value until you get 4.5 volts or so. I used a 1K resistor but I don't know what effect the type of LED has on the biasing of the circuit. To check the voltage, set your multimeter to DC and connect the black probe to ground and the red probe to the drain of the FET, try different resistor values until you get the gain you want.

9/28/98 - More on the biasing resistors. I have found that the MPF102 FETs I got from JameCo are way different in terms of biasing! To get the same 4.6V at the drain, I have to use a 680 ohm resistor. With a Radio Shack MPF102, I use a 2.2K resistor. Quite a big difference. Basically you need to measure the voltage with your multimeter and sub resistors until you get what you want at the drain.

And that's why I was aiming so hard at the 4.5v! Thanks again!

[amz-fx]

The variations in the MPF102 that were being used at the time definitely has some impact on how the circuit needed to be adjusted. Recent MFP102 jfets seem to be all over the place in their Vp rating so it is not surprising that you found it hard to adjust.

regards, Jack

[Fabeii]

The variations in the MPF102 that were being used at the time definitely has some impact on how the circuit needed to be adjusted. Recent MFP102 jfets seem to be all over the place in their Vp rating so it is not surprising that you found it hard to adjust.

regards, Jack

The 1N4001 solved my problem.
I could have stick with the 2N5457 but the J113 is the only JFET left that is readily available for me and that doesn't cost that much, so if I want to make more of these little boards and have some sort of consistency it's the best choice. It's even cheaper than the SOT23 ones that I usually use (MMBFJ201)!

Anyway, thanks a lot!! I think this thread can now be considered as solved!