"I have an older Centurion 422 Amp. About a 1996 build date. My Plate Voltage shows 2800 Volts no load. The manual said it should be 3100 Volts...(etc.)" It is common for HV readings to become lower as the metering resistors deteriorate from the HV being across them. Replace them with a series string of several resistors totaling the correct value. 73, Roy K6XK