You may have one of two different PMT amplifiers in your setup. One which has a variable gain.
The typical settings of this amplifier should be: left the switch on GND, middle switch on L or H but with the knob set to a gain of 10^4, next switch on FBW and DC for the rightmost switch.
If you have this amplifier, measure the output with an oscilloscope and making sure the channel is in high impedance mode (>1Mohm). Adjust the output with the offset screw on the left (do not confuse with the bias screw!) so that the reading is around 380mV. Then, edit the scanbox_config file and set pmt_amp_type to ‘variable’.
Alternatively, you may have a fixed gain amplifier that is a bit smaller and looks like this.
Here the gain is fixed (your version will read 5 x 10^4 V/A and 100MHz bandwidth). Follow the same instructions above to set the bias to 1.9V by means of the offset screw. Then, make sure the pmt_amp_type is set to ‘fixed’ in the configuration file.
That’s all. These settings will allow you to maximize the dynamic range of your digitizer card and the contrast of the images.
Note: the pmt_amp_type variable is only present in the latest release of the software. So you will need to update to use this feature.