I have an 1970' Hameg HM512 scope, which is able to display 60MHz. The final goal was to try to sample those signals with a digital system. If possible to use them for some crude gamma spectroscopy, if that won't work, then just as a photon counter. I also have a plastic scintillator.
That is a pretty slow scope compared to the normal PMT output, so it is likely that the scope is filtering a good bit of your signal. If you increase the load resistor you will make the signal bigger and slower, and you should have no problem seeing it on your scope.
I kinda exposed it to some light. Oops. Luckily I have two pmt-s. I tested it by leaving it in the original cardboard box. When I open it just a little bit and flash with a led i can see the signal on the scope clearly with my jerry-rigged uhf-modded preamp.
Cracking open a cardboard box with any kind of room lights on and flashing an LED is enough to completely saturate your device. Assuming you don't blow out the PMT you will still see a signal, but gain will be dramatically reduced and you will be measuring a continuous light level, not single pulses. PMTs are even sensitive to light when off -- exposure to light will cause their dark counts to temporarily go way up, but will recover over time if you run them with a bias voltage in the dark for a while. When you shine a suitable pulse of light on a PMT you should not see a single signal pulse corresponding to your illumination level as you would expect from a photodiode. You should see a burst of single-photon pulses of roughly the same height, with the density of pulses proportional to the intensity of the light.
The thing that puzzles me is that when I leave it in the cardboard-dark I can see soma garbage coming out, and i thought that it might be light leaking, but then it goes away, and comes back. I tried staying still, making it darker, but seems that that wasn't the source. The signal looked completely random, and when I connected my headphones to the amp the sound was the same as when turning a crusty volume pot on the stereo.
PMTs have dark counts which are strongly dependent on temperature and the cathode material, and also vary with time. You could have been seeing this. To be clear, a working PMT biased correctly and exposed to no light should show random pulses of similar shape, but the pulse amplitudes will vary due to gain noise. The amount of gain noise depends on the device, but you might get a variation in pulse height of ~50%.
When bringing up a PMT for photon counting you want to first make sure it is in absolute darkness. Then look for dark counts. Try setting your scope to 1 mV/div and trigger threshold to -5 mV. Once you see dark counts, you can try *very* carefully increasing the light level and watching the pulse density go up. It won't look like much on an oscilloscope: the probability of getting two pulses in a single sweep is still rather low, but you should be able to see something. If it ever stops pulsing, lower the light levels: you are saturating it! At this point you want to implement a discriminator and hook it up to a counter. Then you will clearly be able to see the increase pulse density as you increase the illumination level.