Biasing JFETs

[playedincanada]

Alright, so.
I understand the concept of biasing the JFETs and why it's done.
I also understand that there are some circuits that have had trimmer pots added so that they can indeed be biased correctly.

3 Questions-->

First is, with your multimeter, what setting are you putting it to and where are you putting the leads to properly read the voltage.
This is of course with the pedal plugged into power? In this case it would be the Argentum 1212.

Second, with a new version of a circuit board like the Chop Shop, how am I using the provided "test pads" above the trimmer to read the voltage?

Third, with a circuit like the Whipped Cream, why is there no trimmer pots to bias those JFETS, or does it not matter in this particular circuit?


I know, sounds like something someone should easily know, but after scouring the forums and searching (maybe I didn't use the right words?), I would like to be able to wrap my head around this as I move forward into building more circuits with JFETS.

I do know that you want them to measure around half of what power is provided for the circuit, so for most they would want to measure around 4.5, I have seen others say around 6 for the Chop Shop and then for the Argentum, because there is a Charge Pump, you'd try and adjust it so those read 9V.

Thanks again for your help as I learn and continue to move forward with building. Maybe one day I'll post pictures of my finished products here!

 

[temol]

Set your multimeter to DC Voltage measurement. If there's "Autorange" then no further settings necessary. If you have to manually select voltage measurement range then choose the lowest one that gives you readings on the drains of the transistors or the test pads. Usually 20V range enough.
Power the pedal and short the audio input. You need to bias the transistors with no signal. Connect the black probe to circuit GND, red probe to the drain of the transistor or test pad.



Whipped Cream - no schematic, no build doc so can't help much here. Maybe no biasing required?

 

[Fuzz Wizard]

Set your multimeter to DC Voltage measurement. If there's "Autorange" then no further settings necessary. If you have to manually select voltage measurement range then choose the lowest one that gives you readings on the drains of the transistors or the test pads. Usually 20V range enough.
Power the pedal and short the audio input. You need to bias the transistors with no signal. Connect the black probe to circuit GND, red probe to the drain of the transistor or test pad.

View attachment 42270

Whipped Cream - no schematic, no build doc so can't help much here. Maybe no biasing required?

How do I short the audio input? I biased my mofeta to 4.25v without the short and there was probably some signal coming through, it doesn’t sound great.

 

[Robert]

Third, with a circuit like the Whipped Cream, why is there no trimmer pots to bias those JFETS, or does it not matter in this particular circuit?

Two are source followers, the other two form a muamp.

Neither of these configurations require bias adjustments.

 

[temol]

How do I short the audio input?

If it's already boxed and wired - just connect tip of the input jack to GND. If you have pcb only, no switches, jacks, etc - connect pcb input pad to gnd (with wire).

Any signal going throug the circuit may cause temporary shift of the bias point.

 

[Feral Feline]

Good thread, @playedincanada

 

[playedincanada]

Set your multimeter to DC Voltage measurement. If there's "Autorange" then no further settings necessary. If you have to manually select voltage measurement range then choose the lowest one that gives you readings on the drains of the transistors or the test pads. Usually 20V range enough.
Power the pedal and short the audio input. You need to bias the transistors with no signal. Connect the black probe to circuit GND, red probe to the drain of the transistor or test pad.

View attachment 42270

Whipped Cream - no schematic, no build doc so can't help much here. Maybe no biasing required?

For something like the Chop Shop, or any pedal for that matter, what should the potentiometers on the front side be set to anything in particular? Like should the sag control be all the way up?

 

[szukalski]

For something like the Chop Shop, or any pedal for that matter, what should the potentiometers on the front side be set to anything in particular? Like should the sag control be all the way up?

It will depend on the pedal. In the Chop Shop in particular, the Sag is reducing voltage to the circuit, so you will want it at minimum to set your bias.

Other pedals may have an external bias pot plus an internal bias trim pot, like the Sunface. Then you want to set the external bias at mid way and set the internal bias pot.

 

[temol]

Short input to GND, set Sag to max voltage (zero drop), gain to max.

 

[Alan W]

Short input to GND, set Sag to max voltage (zero drop), gain to max.

Damn! I didn’t know about setting gain to maximum—is this true for most pedal circuits? Paranoia sets in (maybe just a remainder from yesterday’s rash of “circle jerk paranoia”) as I picture needing to go back and check bias on many builds…