I'm guessing your referring to the DC voltage bias measurement used by a lot of amps. The DC voltage is most likely the difference (drop) across a resistor tied to the tubes plate. This is an easy way to measure the plate current milliamps load on the tube. Different amps use different value resistors, so the voltage difference may not read the same from one design of an amp, to another. If they all used a *one* ohm resistor for example, a 50 milliamp (mA) load, would read 50 millivolts (mV). Or if they used a 10 ohm resistor on the tube, 50 milliamps plate load would read .5 volts. So if they use a different value resistor that wouldn't translate into the same (mA) reading.
The hard part is your question. I think it was said at one time (years back), that you could have a 5% difference between the left and right channel bias, and not notice the difference. Don't hold me to this 5%, but I don't seem to notice a 5% difference myself. Someone may come along with a different value.
If your amp has adjustable pots for the bias, you should be able to get them fairly close. On a stereo amp, changing one side (channel) will cause the other side (channel) to vary some. Also, if it has several adjustable pots, changing one, may cause any other to vary. Adjust the bias cold at first. You have to keep going back and forth, let the amp run 10 minutes, check again. Check after a half an hour, adjust if needed, than in a hour, check and adjust if necessary.
The hard part is your question. I think it was said at one time (years back), that you could have a 5% difference between the left and right channel bias, and not notice the difference. Don't hold me to this 5%, but I don't seem to notice a 5% difference myself. Someone may come along with a different value.
If your amp has adjustable pots for the bias, you should be able to get them fairly close. On a stereo amp, changing one side (channel) will cause the other side (channel) to vary some. Also, if it has several adjustable pots, changing one, may cause any other to vary. Adjust the bias cold at first. You have to keep going back and forth, let the amp run 10 minutes, check again. Check after a half an hour, adjust if needed, than in a hour, check and adjust if necessary.