How biasing mA in tubes
 

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

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

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

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.

I = V / R

So I have to measure 5mA accros each cathode resistor?

I know Ohms Law and use it.

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

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?

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

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?