Answer
If $V_{AC}>>\phi$ every photon that encounters an electron will have sufficient energy to liberate it from the surface so the rate of electrons passing the ammeter will be constant.
$K>0$ if $eV_{AC} - hf > \phi$, and in the limit of $\phi_{small} $ this becomes $eV_{AC}-hf>(\approx 0)$
Work Step by Step
Recall $eV_{AC} = hf + \phi$, such that $eV_{AC}-hf $ is the photoelectron's kinetic energy and if that energy is greater than the work function $\phi$ the electron will escape the surface.
If $V_{AC}>>\phi$ every photon incident on the surface will liberate a photoelectron (above some threshold $f_0$) .
In the realm of $V_{AC}>> \phi, \phi$ becomes trivially small and a constant stream of electrons will pass from the surface through the experiment's ammeter, giving a constant current.
