On The Omnipotent Eye, Andreas Davour has this to say about PHB-only AD&D. That stirred my mind, and this is what I came up with:
The weapon tables are in the PHB, as are the HD for the Classes. It would likely look something like Tunnels & Trolls:
Roll your weapon di(c)e and add or subtract any Strength modifiers*, subtract your opponent's value (as above), and then read the AC as follows:
+3 + Fighter Level
+2 + Cleric Level
+1 + Thief Level
+0 + M-U Level
Subtract (Dex-Modified AC#) from 10 and apply the result as HP of damage stopped.
--Thus, AC02 = 08 Hits Protection, AC05 = 05 Hits Protection, AC09 = 01 Hit stopped.
[*] I goofed on the original post. I intended for only the dice of damage roll to be modified and compared.
Roll Ability Score or less on d20, reading the governing Ability Score on the Defensive Adjustment modifier, and subtracting it from the d20 roll.
--For example, an Int-based Save with a Int 15 provides an Adjustment of -1 to the d20 Roll.
I suppose one could apply the Combat Bonus above, but only to Saves thought of as Class-favoured Saves.
--I've run my game so long now that I don't think of 'level' as doing a lot of modifying, as opposed to the Character changing over time.
JB injected some Level-love to the ideas, although there is no mention of the sort in the PHB that I can recall. I've just included it to please the natives. ;)