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Old 26th January 2013, 03:09 PM   #1
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Default How biasing mA in tubes

To know the biased mA in tubes I have to measure the voltage across cathode resistor, right?
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Old 26th January 2013, 03:15 PM   #2
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Correct.
Using Ohms Law, the current can easily be measured.

Last edited by JonSnell Electronic; 26th January 2013 at 03:17 PM. Reason: More info
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Old 26th January 2013, 03:20 PM   #3
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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?

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Old 26th January 2013, 03:35 PM   #4
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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.
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Old 26th January 2013, 03:37 PM   #5
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I = V / R
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Old 26th January 2013, 03:40 PM   #6
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Quote:
Originally Posted by Harleyjon View Post
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?

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I = V / R
I know Ohms Law and use it.
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Old 26th January 2013, 03:43 PM   #7
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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
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Old 26th January 2013, 03:46 PM   #8
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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?
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Old 26th January 2013, 03:55 PM   #9
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5mA is wrong.
183.7V - 85V = 98.7V divided by 33K = 2.99mA
Current flow through the resistor not across it!
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Old 26th January 2013, 04:05 PM   #10
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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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