CCS question mA

[merlin el mago]

I want to adjust my CCS current, right tube draws 2,11mA & left tube draws 2,24mA, Have I to set both tubes to 2mA (I use two separate CCS: one per each tube)?

TIA
Felipe

 

[Rundmaus]

This is difficult to answer without a little more information.

Schematic? Where is the CCS used? Left/Right concerns two stereo channels?

Rundmaus

 

[merlin el mago]

Is the ECC81 mu-follower, attached schematic used & DN2540 schematic used without the VR tube see left up corner.

 

[SGregory]

Merlin,
I would take your current "sense" resistor value, multiply it by 2.25. Using that calculated number for resistor value, I would then parallel a fixed resistance and trim pot of same value to allow you to adjust it precisely to the 2,00 mA you desire. I used 2.25 to give some leeway +/- on the adjustment. You can then measure the combined resistor value and replace with a fixed or leave it as it is to allow you to adjust as required.

 

[SGregory]

I see you schema now. Increase the value of your trimmer.

 

[merlin el mago]

5K Trimmer & 4k75 resistor?

 

[SGregory]

Whats the present value of your trimmer?

 

[merlin el mago]

1K trimmer

 

[SGregory]

2k resistor and 5k trimmer would work better.
Did you read the thread regarding the use of a DN2540 for <10mA? There might be some better circuits for such low currents.

It doesn't matter if it's not constant as long as it's large- see DF96's analysis. Unfortunately, the DN2540's impedance is pretty mediocre at low currents. For the 2mA you're seeking, a bipolar cascode will perform much better.

that is from the CCS fixed bias thread.