I'm not sure where you are getting your information from but the manual states that at the 4V range, it has a resolution down to .001V and accuracy of .9%. At 65mV, .9% is .585mV.

I've never had any problems using the meter to measure any of my various Audio Research amps over the years. Yes, it is old and maybe I should replace it but that wasn't the problem.

The problem is my eyesight. I put the original battery back in and the low battery indicator was lit but I could not see it clearly. It is fairly faint compared to the numbers.

It seems I need to upgrade my reading glasses.

REF150 Tube Bias is increasing!?
 

Glad you’ve found the variation in your measurement system!
You asked where I got the info from. Page 29 of the users manual.

It states for the 4V range you add 2% to get an overall accuracy of 2.9%.
That’s +/- 1.9 mV.

You are also in an area of the meter’s performance where the resolution of the display matters, because .001V is the smallest increment that it will read. That adds another 1.5% error at 65 mV. This error is not speced by the manufacturer. So if you were measuring say 0.5 mV, the meter might read .001V or .000V for an error of 100%. This error decreases as the voltage increases so at full scale it would be very low.

So your .9% meter is really a 4% meter when measuring low dc voltages in the range of 65mV.

Does it matter? Probably not.

Getting a newer meter won’t make any difference because you are not looking for absolute accuracy, you are just setting the bias in the tubes to be equal.

Buy some new music with the money you’ll save by keeping your trusty meter. :)

Bias moves. 500 hours is about 1/4 the life of the output tubes. Thats like 6 months. I will rebias every 3 to 6.

I'd rather be slightly over than under. If the manual says 65mv, I'd try to get as close as possible erring closer to 66 than 63.