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PMT drift

The procedure that Seamus O'Kane mentioned in his most resent posting I 
believe is a good one to follow. Placing a piece of negative in the gate 
will reduce the amount of light reaching the PMT's and prevent heating of 
the PMT dynodes and anode. PMT's perform the function of converting 
photons into electron current flow. Light striking the photocathode 
(located just behind glass face plate) frees electrons which are drawn 
down into the PMT by an array of metal elements called dynodes. Each 
dynode has a slightly more positive charge applied to it which causes the 
free electrons from the photocathode to travel toward the final anode 
(output element). The voltages applied to the dynodes are supplied by the 
resistor ladder located in the PMT socket (separate board on 3B's). There 
are many variations of the components located on the cell sockets. I'm 
not sure what resistor values are used on the Ursa sockets but the 
resistor values must be quite low after hearing about all the drift 
problems. Low resistor values will "allow" for increased current flow 
through the PMT and therefore pose the possibility of dynode/anode 
heating/drift. When the dynodes heat they will slightly deform which 
causes a change in cell gain and will show up as differing RGB output 
levels on the telecine. By using Seamus's procedure you will not only 
reduce drifting do to PMT gain changes but also increase PMT life. 
Remember always try to limit the amount of time the telecine is "open 
gate" with full beam current. 

Larry Anderson
Allied Digital Technologies