the Wells-Gardner K7500 manual actually has an absurdly detailed step-by-step how-to for white balancing in it. I didn't understand the terminology when they meant to "ground the video signal" or whatever... apparently if you hook alligator clips to the color drive transistor heatsinks on the neckboard to ground, it will create a solid white screen (or whichever corresponding primary color you have grounded ie: red and blue will yield a purple screen)
make sure your color drives are at about the halfway point too.
I personally dial all the biases and the brightness/contrast down, step the biases up to between 1/3-1/2 way, then adjust the Brightness and Screen adjustments to where your black backgrounds appear completely black (you shouldn't see the pixels), then turn your Contrast up high enough, particularly on a screen that has text, to where there's no streaking coming off the letters. you never want your Contrast turned up super high anyway, as you're just burning out your tube faster and it looks shitty anyway.
then start adjusting the biases accordingly. if it has a purple tint, turn the green up more, for instance. do this until you achieve the best white, but don't go crazy turning all the pots up, try to stay within that halfway zone. your Contrast pot should compensate for the rest.
this is my newbie way of doing it. if you want to do it the proper way, use the WG K7500 approach.
disclaimer: I haven't white balanced a monitor completely in awhile, read Wells' way and follow how their adjustment pot suggestions are supposed to be placed at. it should be worth noting that your drives/biases if turned too low will result in a less vibrant picture, but at the same time you don't want them all hopped up either.