  25,600 3-inch PMTs will be installed in Jiangmen Underground Neutrino
Observatory (JUNO) to achieve more precise energy calibration and to extend the
physics detection potential. Performances of all bare PMTs have been
characterized and these PMTs are being instrumented with the high voltage
divider, underwater front-end cable, and connector. In this paper, we present a
dedicated study on signal quality at different stages of the instrumentation.
An optimized high voltage ratio was confirmed and finalized which improved the
PMT transit time spread by 25%. The signal charge was attenuated by 22.5%
(13.0%) in the 10 m (5 m) cable and it required the addition of 45 V (23 V) to
compensate for the loss of PMT gain. There was a 1% overshoot following the PMT
signal and no sign of reflection in the connector. A group of 16 3-inch PMTs
with the full instrumentation was installed in the JUNO prototype detector
together with a few 8-inch and 20-inch PMTs, which showed good stability and
demonstrated a photon detection system with multiple types of PMTs.
