That was interesting measuring signal voltages at the grid of the power tube on different Amps, i didn't know what to expect. What I found As the amps got bigger, the bias voltage got bigger, and the AC signal got bigger too. In each case it looks like the idea is to have the AC grid signal have a voltage that can reach at max half of the bias voltage. At max the signal is opening half of the blockage caused by the negative bias voltage. If the signal voltage equaled the bias voltage, erasing all of the blockage of the bias voltage, it would probably redplate. Changing the bias voltage will affect the ratio of signal V / bias V quite a bit. If the max signal voltage can reach half the bias voltage it provides a good range. The good sounds start happening much earlier, as early as 5 or 10%, or 15 or 20 %, the mild overdrive starts. I was strictly looking at the ratio of the signal voltage divided by the bias voltage, but it could also be looked at as what is the remaining bias voltage once you subtract the signal voltage from it? Comparing across amps with different bias voltages this way would give completely different results than comparing the ratios.