# BIAS adjustment for FET style drives using a POT?



## droneshotfpv (May 12, 2022)

Good morning friends, long time no talky, and lots of love! I have been swamped, and decided to take time for myself and get reacquainted with my passions and put work aside. In doing so, I had some ideas I wanted to mess with, and ask the amazing people of this forum if this is possible / get pointed in a direction of how to do it.

So, I love my builds of the M800, Mofeta Preamp, Blue Shoe Gai Pan, etc... All of those pedals have the FET control that needs biased, so I had a thought......
How would I control that BIAS with a POT? Is it just as simple as a resistor to get the initial "ideal value range and limit that amount of voltage up and down so the voltage isn't too much or too little, but allows some fine tune adjustment? 

I know it can be done, but wasn't 100% sure how. Also, if this has been asked / answered, I apologize as that means my search of the forums was a massive failure and I am a n00b.. lol


----------



## temol (May 12, 2022)

In many projects drain reistor is just a trimpot, so you can dial the voltage easily. And you want to go step further and replace trimpot with a traditional potentiometer?


----------



## giovanni (May 12, 2022)

You can do what @temol suggested but that means the pot range won’t be all usable. If you want only usable range, you probably need the trimpot in series with a much smaller value pot so you can set the operating point with the trimpot making sure that the run of the regular pot all has usable range. You’re gonna need to experiment to find the right component values.


----------



## mdc (May 12, 2022)

An easier way to get a similar outcome might be to bias the circuit 'properly' via the trimpots and then install a simple voltage starve pot; then you'd be adjusting voltage across all of the JFETs at the same time rather than having 2 or 3 external bias trims. 

It might sound cool in some circuits, in others it might not do anything at all.


----------



## zgrav (May 12, 2022)

mdc said:


> An easier way to get a similar outcome might be to bias the circuit 'properly' via the trimpots and then install a simple voltage starve pot; then you'd be adjusting voltage across all of the JFETs at the same time rather than having 2 or 3 external bias trims.
> 
> It might sound cool in some circuits, in others it might not do anything at all.


I come down on the side of setting the trim (or external pot) midpoint at optimum so you can both starve the transistor or bump up the voltage a bit it you want less compression/more headroom.  That is what I did with a Barbershop build and it makes sense to my ears when I am adjusting the pot.


----------



## Chuck D. Bones (May 12, 2022)

Different circuits will respond differently to JFET bias adjustment, it will require experimentation to find the sweet spots.  There are some good suggestions in this thread.


----------



## droneshotfpv (May 12, 2022)

Thank you all for the responses! @temol , yes, essentially I want to have a "secondary" adjustment like a regular POT knob (lulz, I said knob) to further find a "sweet spot" that could / would maybe make little tweaks sound better for various other guitars and pickup types. It's more of a "could I do it" type idea than anything else, but it came to me as I was thinking of different ways to tweak.. I guess I am just an old tweaker that was once a twerker... I have video, wanna see? 🙃


----------



## Chuck D. Bones (May 12, 2022)

Ummmm, pass.

Let us know how the bias experiments turn out.  Varying the bias affects the harmonic content and decay characteristics by changing the symmetry of the clipping.  Some bias setting will emphasize the midrange, other settings will increase compression, cause gating, etc.

I have found that a "Fatness" (bass cut) control near the 1st gain stage most effective for tuning a dirt box to different pickups, i.e. single-coil vs. humbuckers.  Examples are Timmy, Hybrid Fuzz Driver, Tube Zone, Mojito, most of the Friedman boxes...


----------



## temol (May 12, 2022)

Couple days ago I was testing a jfet preamp on a breadboard. I took a trimmer + resistor + spdt switch and made switchable drain voltage. This gave me low/hi setting of the drain voltage. But I ended up with fixed value. I know I wouldn't use it in the end. 
For me there are other features I'd incorporate - switchable source capacitor, switchable treble cap in the tone stack and adjustable slope resistor (potentiometer or switched resistor).


----------



## droneshotfpv (May 18, 2022)

temol said:


> Couple days ago I was testing a jfet preamp on a breadboard. I took a trimmer + resistor + spdt switch and made switchable drain voltage. This gave me low/hi setting of the drain voltage. But I ended up with fixed value. I know I wouldn't use it in the end.
> For me there are other features I'd incorporate - switchable source capacitor, switchable treble cap in the tone stack and adjustable slope resistor (potentiometer or switched resistor).


I was playing with this idea last night, and remembered what you said here. I wonder if I took it a step further, and do like you said, and use a switch after finding 2 (or 3 if using a 3 way haha) settings I like , say for single coil and for humbucker, and then just have it selectable that way as a fixed value. 

Lots of fun with these ideas! again, I appreciate everyone's input!


----------



## Greg (May 24, 2022)

I’ve been going through a similar ordeal. I purchased a used Wampler Plexi-Drive Deluxe and have not been having good results with either the internal overdrive or an external one, specifically at the 12th fret with a humbucker in the neck (Duncan 59). Before shaving off some bass by reducing the input capacitor I was having trouble with anything before the pedal that has a buffer. I tried another guitar with a DiMarzio PAF Pro in the neck position. It’s a medium output humbucker and the same problem all over again but now even going straight into the pedal without the internal overdrive.

Since I have the problem at any setting of the gain control, the first stage is definitely part of the problem.  The first stage is a J201 with a 1k source resistor (un-bypassed), the gate referenced to ground with 1M and a pot at the source so that the source is biased at 4.7V with a 9V supply. I’ve trweaked the source resistor as well as the gate voltage using an op-amp configured as a voltage follower. None of that was effective. Seeing as though the two subsequent gain stages have the same configuration without inter-stage attenuation, I have a hard time believing they aren’t also problematic.

Today I put an opamp buffer feeding a typical Marshall treble peaker before the first gain stage. It has helped but I still can’t boost with an overdrive.

Before all this experimentation I had purchased a second pedal just to see if it exhibited the same behavior before sending it back. It did.

I’m prepared to work on the op-amp input stage and add soft LED clipping in order to get more dirt and move the gain control so that it’s between that and the first JFET stage.

Does anyone have any advice? I love the concept of this pedal as well as the crunch and it is dead quiet, so I don’t want to throw in the towel.


----------



## Chuck D. Bones (May 24, 2022)

The problem might be that the first stage JFET is being driven to the point that the gate is forward-biased during part of the positive half-cycle.  J201s have a very low Vp, which makes it all too easy for this to happen.  What size drain resistor do you have?

One solution is to use a JFET with a higher Vp.  You'll have to adjust the source and drain resistors if you do this.  The best way to approach this is to breadboard it first. Swapping out parts on a PCB runs the risk of damaging the board if you repeatedly solder and unsolder parts.


----------



## Greg (May 24, 2022)

I was using 6.8k shunted with 1.5k in series with a cap that created a 100Hz corner. Maxing the drain pot brought the voltage at the source somewhere around 6V, though using an 18V supply didn’t fix anything. Like I mentioned, I added a positive voltage to the gate in an attempt to overcome any issues with the voltage swing falling below pinch-off.


----------



## Greg (May 25, 2022)

I guess my question is how are people comfiguring jfet gain stages that can handle higher than vintage-output neck humbuckers, let alone mid-focused signal boost like a tube screamer with the level maxed and the drive at somewhere around 10 o’clock?


----------



## temol (May 25, 2022)

Almost all my jfet preamps have 2N5457 or 2N5458 in the first stage. First stage just amplifies the signal, no clipping.


----------



## Chuck D. Bones (May 25, 2022)

First stage will clip if driven by a TS with LEVEL at 10.  I would look at what Brian Wampler did with the Velvet Fuzz.  He has boosters in front of a (slightly modified) Plexi-Drive.  He deliberately attenuates the booster output so as to not hit the 1st JFET stage too hard.  Driving the gate-source junction into conduction will generate some very harsh harmonics.


----------



## Greg (May 25, 2022)

That’s what I was afraid of. In the deluxe version Brian configured the TS circuit with a fair amount of gain and the boost control affects the output. I’ve lowered the gain significantly; to equivalent of about 9 o’clock on my TS9. I know I’m not alone in preferring that an OD actually overdrive the next stage of a crunchy amp rather than just add a layer of distortion.

I would say the quality of the distortion I’m experiencing is more of a squish like a fuzz pedal with the note folding over on itself and taking a while to recover before the note has clarity.

Now I’ve reduced the boost level below the threshold of the unwanted behavior. Because I’ve cut back the gain of the boost circuit the tone is thinner despite the added compression, which isn't surprising.

My preferences out of the way I really appreciate the feedback and am happy to learn that this is normal behavior.


----------

