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?
I = V / R
I know Ohms Law and use it.
This may help in your calculations
http://www.mh-audio.nl/Downloads/TubeDataSheets/Ei-ECC81.pdf
Regards
http://www.mh-audio.nl/Downloads/TubeDataSheets/Ei-ECC81.pdf
Regards
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