Biasing is simple to do and I’ll start with the formula first. There are two different voltages that you’ll be checking and your meter should be able to read both DCV (at least 500 VDC) & DCmV (200 mV DC) which are common to most digital meters. I also suggest using a clip on the ground lead so you only have to have one hand inside the chassis at a time.
There is a standard called maximum plate dissipation (MPD) and your power tubes should run somewhere between 50-70% MPD. In the case of the 6V6GT the MPD is 14 watts. So the formula goes as follows: 14 watts x 70% = 9.8 watts. Then 9.8 watts / plate voltage = bias in mV. So here’s an example of that with the plate voltage being 415 VDC. 9.8 / 415 = .0236 or 23.6 mV. You’ll find that as you adjust the bias to 23.6 mV you may see the plate voltage change as well, so you do the math again and after a couple times back & forth. Another aspect is if one tube draws more current than the other like 22mV & 25 mV, then you want to balance the bias in the middle between them.
So here is a pic of where your bias adjustment pot is located.
Then the ground location.
Next is where you measure the plate voltage.
Now where to measure the bias on each power tube, and it’ll be measured on the 200mV scale between the tube socket and the resistor going to ground.
Now measuring the plate voltage.
Now measuring the bias voltage on each power tube.
That’s all there is to it, and it shouldn’t take any more than a couple of minutes to re-bias the amp when swapping the rectifier or power tubes.