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-   -   How biasing mA in tubes (http://www.diyaudio.com/forums/tubes-valves/228685-how-biasing-ma-tubes.html)

 merlin el mago 26th January 2013 03:09 PM

How biasing mA in tubes

To know the biased mA in tubes I have to measure the voltage across cathode resistor, right?

 JonSnell Electronic 26th January 2013 03:15 PM

Correct.
Using Ohms Law, the current can easily be measured.

 merlin el mago 26th January 2013 03:20 PM

Thanks, I have other doubt: If I use a mu-follower like the attached schematic what is the right mA bias? 5mA across cathode resistor 470R?

http://www.valvewizard.co.uk/Mufollower2.jpg

 JonSnell Electronic 26th January 2013 03:35 PM

If, using Ohms law, you work out the current flowing through the 33K resistor, the overall value will be clear and using proportional calculations the voltages can be calculated across the cathode resistors and therefore the current flow, which will be a constant.

 wicked1 26th January 2013 03:37 PM

I = V / R

 merlin el mago 26th January 2013 03:40 PM

Quote:
 Originally Posted by Harleyjon (http://www.diyaudio.com/forums/showthread.php?p=3343180#post3343180) If, using Ohms law, you work out the current flowing through the 33K resistor, the overall value will be clear and using proportional calculations the voltages can be calculated across the cathode resistors and therefore the current flow, which will be a constant.
So I have to measure 5mA accros each cathode resistor?

Quote:
 Originally Posted by wicked1 (http://www.diyaudio.com/forums/showthread.php?p=3343182#post3343182) I = V / R
I know Ohms Law and use it.

 JonSnell Electronic 26th January 2013 03:43 PM

No. 5mA is the current flow. Therefore V = I X R
R = 470
I = 5mA
Therefore V = 2.35 Volts
If the flow is 5mA

 merlin el mago 26th January 2013 03:46 PM

Thanks I know how measure current, the question is what's the right current for my schematic? 5mA across 33K & 5mA across 560R? or are other values?

 JonSnell Electronic 26th January 2013 03:55 PM

5mA is wrong.
183.7V - 85V = 98.7V divided by 33K = 2.99mA
Current flow through the resistor not across it!

 merlin el mago 26th January 2013 04:05 PM

I'm sorry my native language isn't English, I read 5mA is the correct bias for ECC81, have I to change the resistors values of schematics accordingly to reach 5mA in each valve?

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