newbie, How do you bias a J201 for 4.5v at drain

[ronnie]

Hello, I built the Thunderchief from Runoffgroves site, I don't know how to bias the J201's to 4.5v at drain, can anyone please help me with this, thankyou so much.

[petemoore]

Set DMM for 20v range.
 Clip black lead to ground point.
 Touch the red lead to the Drain.
  >>>Whoops, you Did use a trimpot for the Drain resistor?
 With the DMM reading the voltage of the Drain, turn the trimpot 'till the Fet gets sorta near the 1/2 supply level, [4.5 or 5v]use that or tune to ear.
 This is for a Jfet amplifier stage, a source follower would be different.

[ronnie]

Thankyou very much pete, I will try what you said.

[Jered]

If your going to set your bias this way please let me add: after you get the correct voltage at the jfet drain, measure the resistance on the trimmer and replace with a resistor of equal resistance. Trimmers have a tendancy to get noisy especially in dist. circuits.
Jered

[ronnie]

That sounds like a good idea, how's it done.

[petemoore]

You use the V/R [variable resistor = pot....100k range for Jfet is good], 'replacing the drain resistor.
 Set the 100k so the Jfet drain is where you want it voltage wise [4.5~v].
 Then without disturbing the pot setting [don't turn the shaft] remove the pot, measure the resistance setting that biased the Jfet, say the pot measures 49k...grab a 47k and use that as the drain resistor. Should be plenty close enough. Just use the nearest value fixed resistor, or build the exact amount...if you're like real picky, proably not necessary....remeasure the Drain Voltage after Fixed R is installed.