Tank B is more correct for a 12 volt system. This allows your voltage to drop between plates where Tank A puts 12 volts across each cell. In other words Tank A will overheat and Tank B will run forever (or at least until it runs out of water)
If you rewired Tank A to match Tank B, then you need to make sure that you have 14 volts and not 12. Or you could rewire Tank A with one cell left out. You only need 2 volts per plate gap so Tank A has one too many cells and Tank B has one too few.
Tank A with 6 cells = 2v per gap (preferred)
Tank A with 7 cells = 1.7v per gap (not enough to generate HHO)
Tank B with 5 cells = 2.4v per gap (acceptable)
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Some days I get the sinking feeling that Orwell was an optimist!