Everybody KNOWS that the bias should be checked and readjusted (if necessary) whenever either the power tubes or rectifier are changed. They also know there are two different methods of measuring the idle current [Notice that I did NOT write either plate or cathode current!].

PLATE SHUNT

The most "accurate," but most possibly "lethal" method is called the PLATE SHUNT method. This method is the most accurate because it actually measures the amount of PLATE current (Ip). It does this because you actually "short" out the primary winding by connecting your Ampmeter directly between B+ and the plate of either output tube. Why is it "lethal" because you're making connections to the HIGHEST voltage (B+) in the amp, typically between +400Vdc and +500Vdc!!! And, voltages above 50Vdc can KILL you!

CATHODE RESISTOR

The "safest," but less "accurate" method is called the CATHODE RESISTOR method. Here a 1-Ohm/1-3W resistor is connected between the cathode (pin 8) and ground on each output tube (ie: Bias-Probe, etc.). This method measures the tubes' CATHODE current, NOT Plate current. The Cathode current (Ik) is the sum of the Plate current (Ip) and Screen current (Is):

Ik = Ip + Is

With the SHUNT method you're working with B+, but with the Cathode-Resistor method you're working with LESS than ONE VOLT, which is MUCH safer! Here's why:

E = I*R, so...

...assuming the typical 2 x 6L6, 40-50W amp draws about 20mA at idle, then the Cathode voltage (Vk) across the 1-Ohm resistor would only be:

Vk = (Ik)*(1-Ohm)
Vk = (0.020A)*(1-Ohm) = 0.020V, or 20mV!

And, at FULL output power (assuming 250mA), the Cathode voltage (Vk) would only be:

Vk = (0.250A)*(1-Ohm) = 0.250V, or 250mV!

Ok, we NOW understand why and how the cathode method is better from a "safety" viewpoint, but what about it being "less accurate"? Is there ANY way to make it MORE accurate? Yes! by knowing the division "ratio" of Plate current and Screen current!

Because of 'how' Beam Power Tetrodes (6L6, 6V6, etc) operate, the division of current between the screen and plate is pretty much constant. And, although the division ratio of Plate current (Ip) to Screen current (Is) isn't a normally given "spec," it is easily determined from the published Plate current (Ip) and Screen current (Is) values given when the Plate voltage (Vp) and Screen voltage (Vs) are both equal to 250Vdc:

6L6: (Ip/Is) = (0.072A/0.005A) = 14.4:1
6V6: (Ip/Is) = (0.045A/0.0045A) = 10.0:1

To get the ratio of Cathode current (Ik) to Plate current (Ip), we simply use the equation:

6L6: (Ip/Ik) = 14.4/(1+14.4) = (14.4/15.4)
6V6: (Ip/Ik) = 10.0/(1+10.0) = (10.0/11.0)

And, solving for Plate current (Ip), we get the equations:

6L6: Ip = Ik*(14.4/15.4) = Ik*0.935 ...and... Ik = Ip/0.935
6V6: Ip = Ik*(10.0/11.0) = Ik*0.909 ...and... Ik = Ip/0.909

For example, the RCA and Svetlana 6L6GC data sheets list the following specs for a 55W push-pull Class-AB1 amplifier:

Vp = 450Vdc
Vs = 400Vdc
Ip = 58mA(idle), 210mA(max)
Is = 2.8mA(idle), 22mA(max)

Let's see how accurate we can get. If the Plate current (Ip) is 58mA, then the Screen current (Is) should be 2.8mA:

Ik = (Ip/0.935) = 0.062A, or 62mA
Is = (Ik - Ip) = (62mA-58mA) = 3mA, which is close enough!

Conversely, if we MEASURE the Cathode current (Ik) to be 62mA, then we simply calculate the Plate current (Ip) to be:

Ip = (Ik*0.935) = (62mA*0.935) = 57.97mA, or ~58mA!

So-o-o-o, NOW you can use the Cathode-method and also get ACCURATE estimates of Plate current!