Bias adjustment and understanding

[bvayling]

Ok all, with forum help I finally got my 18 trem combo fitted with the correct resistor R20 value for safe plate dissipation levels.

What I didn't do is understand how to do it. I can use the calculator on Rob's site, which uses a built in 5% allowance for the screen current. However,
I'd like to be able to calculate that manually to see exactly where things are.

I did some searching and found some direction from Zaphod Phil but I didn't quite understand it. Something is wrong with the way I
am thinking or I am measuring the wrong thing in the wrong place because my results don't add up.

Here are my assumptions and methodology,with a couple of questions where i have suspicions. Perhaps someone can point out where I went off the rails.

Bias current measured and set at idle. Volumes at 0

Cathode current=Voltage pin3/cathode resistor R20 value. Divide by 2 for value per tube.

Screen current=voltage drop across screen resistor R25/R25 value= (V pin9-voltage at capcan)/R25 value. Divide by 2 for value per tube.
This might be wrong? Should I measure directly at each end of resistor R25 with probes with circuit live?

Plate current per tube=(Cathode current per tube - Screen current per tube)

Plate dissipation per tube in W=(Plate voltage-Cathode voltage) x plate current

Is this right?

If the B+ is defined as the voltage at pin3 of the EZ81, is that where I want it to be 345V for the 18W sound? Because from the EZ81 the voltage goes to one positive terminal of the capcan through the resistor R5 to the other leg of the capcan to the board. So perhaps the voltage you want to be 345V should be measured where it comes to the board at the insertion point of R25 and R6?

Thanks all.

Bernard