Mesa Boogie Mark IIB bias install

[Xavier]

I would like to install a bias pot on my 1981 Mesa Boogie Mark IIB. I understand there are two resistors involved, 6.5k and 10k. Which one do I take out and install a pot in its place. Am I correct in this? If not, how do I go about doing this modification and what kind of pot do I use. Thanks

[GibsonLover]

I have a MKIIB woodie that I'm embarrassed to say I can't remember the last time I checked the bias...Worse yet I have two schematic of it, one showing a bias pot, and I doubt that's accurate.
Luckily, bias circuits almost always only have a few components, so you can look at other schematics to see which resistors to change. Usually you will be adding a 10K or 25K pot. One method NOT to copy would be some of the Fender schematics out there, where the bias going to the output tubes comes from the bia pot wiper. If the pot fail where the center wiper is dirty or worn and does not make contact, your tubes will see NO negative bias and therefore be set for full tilt damage. Look at a Marshall example, like the 1987 circuit, where the pot is connected as a rheostat. If the pot opens (less probable if connected as a rheostat), there is still negative voltage on your outputs keeping your output out of a damaging condition.

[stokes]

On the MkII's I've done I used a 50k or 25k cermet pot,depending on the value of the resistor in the amp.I like to use a pot of twice the value of the resistor I replace.If yours actually has the values you mention,use a 25k and replace the 10k.The cermet pots are small and can tolerate dc voltage better than the standard volume type pots.If you've ever heard a scratchy volume pot due to dc leaking onto it,that sound is the voltage slowly eating the pot.