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 26th January 2013, 04:09 PM #1 merlin el mago diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe How biasing mA in tubes To know the biased mA in tubes I have to measure the voltage across cathode resistor, right?
 26th January 2013, 04:15 PM #2 JonSnell Electronic   diyAudio Member     Join Date: Dec 2012 Location: The Jurassic Coast, England. GB Correct. Using Ohms Law, the current can easily be measured. Last edited by JonSnell Electronic; 26th January 2013 at 04:17 PM. Reason: More info
 26th January 2013, 04:20 PM #3 merlin el mago diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe 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?
 26th January 2013, 04:35 PM #4 JonSnell Electronic   diyAudio Member     Join Date: Dec 2012 Location: The Jurassic Coast, England. GB 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.
 26th January 2013, 04:37 PM #5 wicked1   diyAudio Member     Join Date: Jan 2008 Location: Midwest I = V / R
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Quote:
 Originally Posted by Harleyjon 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 I = V / R
I know Ohms Law and use it.

 26th January 2013, 04:43 PM #7 JonSnell Electronic   diyAudio Member     Join Date: Dec 2012 Location: The Jurassic Coast, England. GB 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
 26th January 2013, 04:46 PM #8 merlin el mago diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe 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?
 26th January 2013, 04:55 PM #9 JonSnell Electronic   diyAudio Member     Join Date: Dec 2012 Location: The Jurassic Coast, England. GB 5mA is wrong. 183.7V - 85V = 98.7V divided by 33K = 2.99mA Current flow through the resistor not across it!
 26th January 2013, 05:05 PM #10 merlin el mago diyAudio Member     Join Date: Sep 2009 Location: Catalonia - Europe 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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