I’ve never biased a hifi, but I would assume it wouldn’t be different in terms of how you calculate the mA value that you should bias your amp to.

For my guitar amp, I use the following equation:

.7(W/V) = Bmin
W/V = Bmax

W = Wattage of the tube
V = plate voltage
Bmin = starting bias value in milliAmps.
Bmax = maximum bias value in milliAmps

Plugging in the numbers for 300B specs, 61ma looks like the maximum. So you’d want to start lower (say about 40ma) when biasing.

Edit: I fudged the numbers, 60-90ma seems to be the range, not 40-60.

Oh, ok I get what you’re saying.

That if you’ve got 5.5, 5.3, 5.5, and 5.2 mA/V how do you get 61mA for each?

ma/V is the tube’s transconductance. Transconductance is important in tube matching, but it isn’t used in the calculating of the bias current, which is why the 61mA is the same throughout. Here’s a little more about the subject: http://store.electron-valve.com/potuma.html

Could you describe your biasing procedure? I looked at the consonance M100Splus manual and it didn’t say anything, and it wouldn’t let me look at the tech sheet. The one curious thing is that it said that each channel operates at 22 watts. Which seems very low for me considering the tubes used.