Trigger start magnetic fluorescent ballast question

Hi, Can someone provide a link to a schematic online or reference to a good book explaining the theory of operation of a magnetic trigger start fluorescent ballast for 2 F20 lamps? I have one fixture that keeps burning out fluorescent lamps after about 500 hours of operation. It has burned out 3 lamps already; 1 made by GE and 2 Sylvanias and it's always the same socket where the failure occurs. It's the socket connected to the white wires which then connect to the yellow wires on the other lamp on the same end of the fixture. I took the fixture apart after the first lamp failed and checked all the sockets for proper connections and they were all good. The fixture always lights up reliably. I have another exactly identical fixture which I purchased at the same time from the same store in the same room and operated by the same switch as this one and it still has the original lamps in it and all the ends are still free from any end blackening. I have already replaced the ballast in the troublesome fixture but I am curious as to why it was destroying lamps.

Sounds like the lamp current is too high.
The wiring may depend on which country you are in; either 110 volt or 230 volt.
For 230 volt, there is probably one ballast choke and the two lamps are in series with each other and the ballast. A starter switch is connected across each tube through the lamp filaments. Not sure how they do it on 110 volt systems.
There may be lamp current passing through the filament due to a faulty lamp connection/connector. This would be wrong. In correct operation, no lamp current passes through the lamp filament, only at startup does current flow through the filaments and the starter switch.