I think it may be interesting a switch for select the voltage between 9 or 18v in the overdrives, like the mod in the Catalinbread (VSw). How could it be the layout with a external simple voltage doubler? Thanks!
Use a SPDT toggle and take the 9V supply to lug 1. The 9V supply also needs to be daisy chained to pins 1 and 8 of the charge pump IC as usual, then take the 18V output from that after the diodes to lug 3 of the toggle. Then take lug 2 to wherever the main supply rail for the circuit it.
So any layout on here that is suitable for 18V supply, you would swap the 9V connection shown on the board layout for the lug 2 connection from the toggle.
I've been thinking about this too. Though I've never heard a A/B comparison of 9v and 18. Is the difference big enough to make the switch worth the trouble?