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Old 9th January 2011, 01:53 PM   #1
Green77 is offline Green77  Sweden
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Default Need help to calculate R in a CCS!

Hi all!

I'm going to try to build a TubelabSE with 300b tubes.
But i need to change the 10M45 to a DN2540 in the CCS.
How to calculate R16 (330 ohm) to run 10mA with the DN2540??

Schematic

Many Thanks //Daniel
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Old 9th January 2011, 02:15 PM   #2
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Daniel,\
due to variation in the manufacture the voltage required for the DN2540 will be somewhere between -1.5 and -3.5V (depletion mode). The stage wants 12mA of current. So lets take a guess at your device and assume for the moment that it needs 2.8V. V=IR so 2.8 = 0.012 * R. R = about 233R.

Install a 330R resistor, carefully measure the voltage drop across it in operation, and then recalculate with the known voltage to get your desired current. You can adjust it by either piggy backing a resistor to the 330R or replaceing it with the correct value.

May I strongly suggest that you cascode two DN2540's. A single isn't much better, if at all, to a 10M45. You could easily deadbug it on the pcboard.
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Last edited by SGregory; 9th January 2011 at 02:18 PM.
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Old 9th January 2011, 04:33 PM   #3
Green77 is offline Green77  Sweden
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Thanks

I will think about 2 dn2540..
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Old 9th January 2011, 05:06 PM   #4
Svein_B is offline Svein_B  Norway
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Gary Pimm has some measurements of the different CCS variants.

CCS performance measurments

(look at the graph at the bottom of page)

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Old 10th January 2011, 09:18 AM   #5
euro21 is offline euro21  Hungary
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In practice it's 180-190 ohm (cascode DN2540 CCS), at 12mA (depends on the FETs).
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Old 10th January 2011, 09:56 AM   #6
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hey-Hey!!!,
In my next CCS I will use a 10M45 on top of DN3545N3. The 10M45 has nearly twice the negative bias of the 2540/3545 and so would lower the gate capacitance of the lower FET substantially( a useful thing I think ).
cheers,
Douglas
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