[en] Experimentally investigated is the operation of the signal shape discriminator (SSD) with a conical light pipe from plexiglass 60 mm length, connecting the organic liquid scintillator with the spectrometric photomultiplier EEhU-110. The scintillator diameter is 150 mm, its length equals 50 mm, the diameter of the photocathode working part is 70 mm. SSD is adjusted according to the Oven separation method, based on the saturation of the space charge from the photomultiplier last dinode. As a comparison the investigation of the SSD characteristics with the spectrometric photomultiplier FEhU-49, having photocathode with 150 mm diameter and being connected with the scintillator without the light pipe. It is shown that at the SSD usage spectrometric photomultiplier with a small photocathode gives the increase in the quality of gamma-background depression in 10 times in comparison with photomultiplier with a large square of the photocathode