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Old 5th February 2013, 03:37 AM   #1
ttan98 is offline ttan98  Australia
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Default 10M45 current source

Hi,

I have a single ended tube amp., the front end tube is driving the power tube(KT88) via an anode resistor. I like to replace the anode resistor with a 10M45 current source. My questions are:

1. Do I need the a heat sink(I think so) for this current source, currently it is carrying about 2-3mA?
2. What value of sink resistor(1K pot to adjust to correct current?) should I start with in order to set the current right?
3. What current do I need to drive a KT88(minimise distortion)?

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Old 5th February 2013, 04:37 AM   #2
rknize is offline rknize  United States
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1) Maybe, maybe not. It depends how much voltage it has to drop to achieve the target current in your circuit.

2) You can determine this by looking at the data sheet.

3) I'm not sure how to answer this, as there are too many variables.

This post really belongs in the Tubes/Valves forum where you may get more feedback. This forum is mainly for Tubelab products and support. George is a fan of the 10M45, but he is not the only one.
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Old 6th February 2013, 06:48 AM   #3
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moved to tubes/valves as suggested.
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Old 6th February 2013, 09:16 AM   #4
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Hi,

1. Can't help you without knowing the supply voltage you have, the tube and the chosen bias point. Post a schematic.
For a low current of 2-3mA, a bipolar transistor cascode CCS may work better than the single 10M45.
If you use the CCS, you should switch to LED bias and get rid of that nasty cathode capacitor. And you'll be able to roll a lot of different tubes in that position (bias is fixed and you don't need to change cathode and anode resistors to fit a specific tube).

2. You find the right value with a pot (don't turn it to zero ohm! or use a limiting R in series), then replace it by parallel resistors to get the right value.

3. A low drive impedance is of course better. Change the first tube to a lower Ri one if necessary, use a cascoded mosfet CCS at 8 to 10 mA and take the mu output from the CCS. Compare the sound with the classic CCS output, see which one you like best.

Search this forum on cascode CCSs and mu output.
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