Mesa Boogie 50 Caliber+ Bias Cct. ??

[NGW]

Hi All,

I have the misfortune to have a Mesa Boogie 50 Caliber+ (2 x 6L6) on my bench, with some rather dodgy Bias mods and various other examples of shoddy work.
I reluctantly took this on to help a good friend out and feel obliged to get it working properly for him.

The Bias cct. has been modified to incorporate an adjustment trimpot, which has a faulty track, causing the Bias to go from cold to way too hot with the smallest movement.
I can appreciate the motive behind the Bias mods, but the way they have been done is terrible.
I have removed all the components of this modified Bias supply and now want to make it work properly.
The problem is; I am not up to speed with Mesa amps, due to avoiding them, whenever possible.

Are any of you familiar with the Bias cct. and can you provide an accurate schematic? 
I have downloaded a couple of schematics from the web, but I'm not sure whether they are correct.
Can you explain this unusual (to me) Bias cct. and the Bias connection to the phase inverter cathodes?
What would be a good method of installing an adjustable bias cct., so the Bias can be set for non-Mesa branded (highly expensive) valves?

Thanks for any help.

Cheers,
Noel