A trick I used to use was to dim the light down so the amp is in the dark but you can see what you’re doing. Turn them on with test speakers connected. Have adjustment tool ready and as tubes heat up, adjust bias so that the plates start to glow slightly red. Then adjust back so the red fades away. While it’s not accurate, it will ensure they are biased. Be careful not to let the power tubes glow strongly red as they could fail (spectacularly.) You can tell from the heat generated after 10 mins or so how heavily biased they are. Adjust when warmed up for sound quality ensuring they don't "red plate".
