Adjusting Drain Voltage

[minions]

Hello all,

It's been a while since I last posted. Actually quite a long time, and I'm trying to get back into building effects. I'm currently working on this bass distortion circuit: runoffgroove Ginger

I'm fairly confident, except for one thing. At the bottom of the schematic, it says, "Adjust Q1 and Q2 drain to 6V".

I think I know what this means, but I'm not too sure. Clarification would be appreciated.

What I think this means is that there is power coming from the 9V battery (9V of it!) to the drain, and if you adjust the trim pots that are connected to each transistor, it limits this power. If you take a reading of the voltage coming off of the drain, you need it to read 6V.

Is this correct?

If not, could someone please explain this to me?

Thank you!

minions

[JohnH]

Dec 15, 2015 12:38:10 GMT -5 minions said:

Hello all,

It's been a while since I last posted. Actually quite a long time, and I'm trying to get back into building effects. I'm currently working on this bass distortion circuit: runoffgroove Ginger

I'm fairly confident, except for one thing. At the bottom of the schematic, it says, "Adjust Q1 and Q2 drain to 6V".

I think I know what this means, but I'm not too sure. Clarification would be appreciated.

What I think this means is that there is power coming from the 9V battery (9V of it!) to the drain, and if you adjust the trim pots that are connected to each transistor, it limits this power. If you take a reading of the voltage coming off of the drain, you need it to read 6V.

Is this correct?

If not, could someone please explain this to me?

Thank you!

minions

Yes that's right. You measure dc voltage at the drain of Q1 and Q2 (top connections on the JFETs per the schematic, where they connects to the trim pot and the 220nF cap), and adjust the trim pot to get 6V. The idea is that it is a bit more than 1/2 the supply voltage, since when signal goes up and down, that JFET circuit is better able to let the output rise up to the full 9V than to pull it down towards 0V. The authors may also have preffered the tone set that way. For that type of stage, I usually bias to about 5V.

I like JFET circuits too, and runoffgroove got me started with them.

[minions]

Perfect! I'm glad it's that simple. I'll probably start with 6V and then adjust to see how I like it. Thanks!