  The linearity of charge response is an important feature of photomultiplier
tubes (PMT) for physics measurements, especially the newly developed 20"
MCP-PMT. In this paper, in addition to the traditional method of double light
sources, we applied another relative method of 20" PMT to 3" PMT to measure the
linearity of the 20" dynode and MCP PMTs in pulse mode with a waveform
digitizer. The measurements show a good linear response of 20" PMTs to 1,000
photoelectrons (p.e.). The correlations of the amplitude, rise-time, fall-time,
FWHM, baseline recovery, overshoot, and late-pulse to the output charge of the
20" PMTs derived from the waveform analysis, where the MCP-PMT shows very
different features compared to the dynode-PMT in particular.
