WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I should know this but I've tied my brain in a knot. Before first power-up of a fixed bias amp do I want the voltage on the grid of the O/P tubes more or less negative? Don't want my tubes to go into meltdown.
Set to the most negative voltage available. After a suitable warm up and checkout of all the other voltages, then slowly increase the bias in the less negative direction until the desired cathode current is reached.
It's best to set on the low side and watch it for a while since some tubes, particularly those that have been sleeping for say 50 years, will drift around for a while before settling down.
I leave meters connected to any new creation for a few days, and recheck often to see if an upward creep trend is starting.
The only little tweak on the excellent advice, is to leave grid “most negative” for between 1 and 4 hours after power up, before attending to the adjustment-for-desired-quiescent current flow. Gives everything a substantial amount of time to warm up, come into balance. I then usually only adjust bias to the 75%-of-desired value. And let it sit for another few hours. For the reasons TubeLab_com spells out. That's it… GoatGuy ✓