What is the voltage To bias a marantz 8b?

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".

Clever suggestion, @noromance .  I hadn't thought of that technique.  Thanks!

Best regards,
-- Al

Thank you, that’s pretty much what I’ve been doing. I using some old tele’s and just wanted to make sure I don’t have too
much juice on them

Maybe this thing can help

https://www.amazon.com/Nobsound-Current-Probes-Amplifier-Cathode/dp/B0777F8DBK/ref=pd_sbs_267_3/130-...

Adjust both output tube cathode current to 50mA and you will get 0.68V across cathode resistor.