It changes when I've just got it turned on and plugged into the cab, it slowly goes up. I know for the first few minutes a little bit of drift is normal, but it keeps drifting like 30 minutes after I set it. And not just like 35 - 36 - 35 again, like 35 - 45

When you say your bias changes do you mean while you are playing or when the amp is on, with a load, and nothing plugged in? If you have a probe reading the bias while you have a guitar hooked into the amp and you're jamming the bias reading will go nuts. I'm headed to bed but I'll help you out with the rest of your question more tomorrow.

Hey dude, in regards to your post on that JCM900 6L6-EL34 mod thread.

I have a JCM900 that takes 5881s, and I installed KT77s as recommended by Eurotubes, but the bias wouldn't stay put (If I set it anywhere between 32 and 40 it'd just go either up or down) as a result, I almost red plated them (Just saved them, turned the amp off as soon as it started humming). I've taken it to a tech since then, and they've got it working perfectly fine with 6L6's (I even printed them off an email with instructions of how to get the correct bias and what might be causing the bias to not stay put and they still said they couldn't get them to work).

Now my question (Sorry for the long intro). Do you know if there'd be a problem with my amp causing this, and what should I have set the bias at (Plate voltage in JCM900s can range from 475 to 500)