Hi new to the board. MOSFET rebiasing and resistor adjustment question.

[mandarb777]

Hi I am quite new to the board and DIY stompboxes.  I've enjoyed reading on this site a lot and started with an A/B selector with level controls (pots before the DPDT switch), because I have 2 basses and am lazy about adjusting volumes all the time.
My current project includes the Analog Alchemy Obsidian Overdrive.  I am building in negative ground instead of the original positive ground with MOSFET BS170's (Thanks Aron!) instead of the BS250's.
My question is what is rebiasing in theory and how would I physically go about it.  I did try to breadboard this and got 9v on the 3.3M and 3.9K resistors. So, I am either horribly messing up on the breadboard itself, or I have to do the adjustments which I don't understand how to do.  I don't have a camera, but I'll try to get a hold of one if a picture of the breadboard would help to clarify if the breadboarding is at fault.

Here's the link to the Obsidian schematic.
http://www.diystompboxes.com/analogalchemy/sch/obsidian.html
Thanks,
mandarb777

[Gus]

Note in the linked schematic all three gain stages are setup the same.

Have your read about Ohms law?  If so the 3.3meg and 1.5meg are a voltage divider that set a voltage at the gate and with the other two resistors (3.9K and 1K) set the operating points.

Get a 9V battery and clip, 1.5meg,3.3meg,1K and 3.9K(Joe's schematic has a note to adjust the 3.9K) and a mosfet  Don't worry about the other parts yet.

wire up this circuit fragment and using a DMM set to DC volts measure the SUPPLY and DRAIN and SOURCE voltages.  Measure the resistors with a meter set to Ohms before installing them, don't hold both ends of the resistors when measuring this will affect the reading of the higher value resistors because of the conductivity of your body.  You can get an idea of how to move the drain resistor by the voltage readings.

If all the mosfets are from the same batch you might be able to use the same resistor values in each stage.

Report back with the voltages.

  Be careful with the mosfets they can be damaged from static.  You can do a fast test with a meter set to ohms and measure from gate to source or drain it should be open or a very high resistance.

[mandarb777]

Gus - I got the impression that you want me to do it for one gain stage and report back on that.  So,
Supply - 8.6 v
Drain (on MOSFET) 6.11 v
Source (on MOSFET) 0.46 v
Another noobish question - was I supposed to measure on the MOSFET or the resistors?

Thanks so much,
mandarb

[Gus]

The readings look OK meaning if all 3 stages are like this it should work(maybe not it's best(taste) but we can adjust that).

  Now if you have a 3.3K and a 4.7K try them in the drain to +9VDC for the 3.9K and note the new voltage readings at the drain and the source.  Use the one that gets you closer to the 4 to 5 V  range at the drain drain on the schematic.
Once you have a fragment like this working you can try the other mosfets to test them

I am using the note Joe has on the schematic of adjusting the drain R value.

[mandarb777]

Thanks so much Gus!  I used 6.1K (The 3..9K and 2.2K in series) resistor and got 4.7 on the drain.  the 4.7K was around 5.5 on voltage so I thought an extra K would get me down to the acceptable range.  Thanks for helping a noob out.  (I thought I was supposed to adjust the resistor itself not the value of resistance at that point  .)
David