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All of the 'surge protector' devices use some type of component that will conduct current if the voltage reaches a specific level. Varistors and gas tubes being the most common components used in these designs. Varistors and gas tubes are limited as to how much current they can carry without failure. They also conduct to a very limited degree, before the 'on voltage' is reached, so they can and do cause some signal loss. As you have experienced, the voltage from the preamp power supply is enough to 'turn on' the varistor or gas tube in your 'protector'. When that occurs, the RF loss through the device goes way up.
In a coaxial surge protector application, the varistor or gas tube is connected to the center conductor and shield so that the voltage between the center conductor and shield will be limited to no more than the 'on voltage' rating of the 'protecting device'. If the fault current is too great or continues for too long, the varistor / gas tube will fail. If the 'protecting device' fails 'open', the 'protector device' is no longer able to limit voltage or current.
A coax grounding block makes no direct connection to the center conductor. It's intended to provide a secure and substantial connection to the shield only. When installed, the center conductor of the coax may be indirectly connected through the balun or similar component.
As I've already said, protectors that attempt to limit voltage between the center conductor and ground may be nice additions to the system, but they do not replace the two basic grounding tasks of mast and shield... outside the building.
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