Diode bridge rectifier convert 13.5v ac to 17v dc
13.5 V is what's called the RMS voltage, a sort of average used with AC to make the math work out nicely. For a sinusoidal waveform, like what you get from the power mains or a transformer connected to it, the RMS voltage is equal to the peak voltage divided by the square root of two.

A rectifier with a capacitor will output the peak voltage minus the drop in the diodes, or \$13.5\ \mathrm{V} · \sqrt2 - 2 · 0.7\ \mathrm{V} = 17.69\ \mathrm{V}\$.