Would like to check bias and replace tubes. Service manual says bias is adjusted to 40vdc from pin 3 of any 7868. So that's measuring between pin 3 and chassis ground? And then the adjustment pot is turned to 40VDC and that sets all 4 output tubes at once? I know there is a mod to be able bias each individual but that's beyond me right now... I just need to check and maybe try some new tubes. Please advise.