JFET biasing

[A.Crossfield]

I am building the ROG Eighteen on a breadboard. It uses 3 J201 trans. I am setting the bias with resistors, not trim pots. I am able to set the drain bias at the recommended 4.5 volts on Q1 and 3, and 7 volts at Q2 with out anything else in the circuit...source resisitors, caps, input, etc... Once those parts are back in the circuit, my drain bias drops to about 0.25 volts on Q1 and 3. Do I bias the transistors first and then build the rest of the circuit, or bias when it's complete. Why does my voltage drop like that. Is that normal? Thank you.

[J0K3RX]

I am building the ROG Eighteen on a breadboard. It uses 3 J201 trans. I am setting the bias with resistors, not trim pots. I am able to set the drain bias at the recommended 4.5 volts on Q1 and 3, and 7 volts at Q2 with out anything else in the circuit...source resisitors, caps, input, etc... Once those parts are back in the circuit, my drain bias drops to about 0.25 volts on Q1 and 3. Do I bias the transistors first and then build the rest of the circuit, or bias when it's complete. Why does my voltage drop like that. Is that normal? Thank you.

That would be normal... You need to bias when it's complete. Why are you not using trimmers, do you not have any? You know you can use the trimmers to bias then remove them and measure them with your DMM and replace them with resistors of the same value...

[A.Crossfield]

Thank you for the reply. I do not have any 100k trimmers on hand. I was using a standard 100k pot. I am still very new at this, learning as I go..trying to learn by failing with as little help as possible. Again, thank you.

[Kesh]

Because JFETs vary so much among the same device number, trimmers are almost always used in effects type circuits.

So if you breadboard it, and set the resistance correctly with a pot, and then use fixed value resistors in the circuit, keep exactly the same J201 in the same place. Don't jumble them up or replace any.

[Gus]

A thread http://www.diystompboxes.com/smfforum/index.php?topic=93817.0

You can find a preamp circuit in a Fender solid state amp that uses jfets and a +- supply

[PRR]

> source resisitors

You NEED Source and Gate resistors. Otherwise the FET is dead. I don't know how you can trim to 4.5V that way. (Are you using a needle-meter?)

You *can* skip all the caps and jacks, and pots that don't connect to FETs except via caps.

If you only have one variable resistor, you can probably trim each stage separately and insert fixed resistors as you go along.