The dependence of SNR on operating voltage at low light intensities was investigated in PM’s of box-and-grid and venetian-blind structure, utilizing both the photon-counting and the dc methods of detection. SNR was found to be reasonably constant at both tube types in the tested range of operating voltages with the dc method of detection. In the photon-counting mode at a constant discriminating bias, SNR improved with rising operating voltage in the box-and-grid structure tube, but changed little in the venetian-blind structure tube.
M. Jonas and Y. Alon, "Dependence of Signal-to-Noise Ratio on Operating Voltage in Photomultipliers," Appl. Opt. 10, 2436-2438 (1971)