[Elecraft] KPA500 Problem

[Andy Durbin]

"I have a KPA500 with a low HV reading. It is on the center tap and AC voltage is 115v No load and 110v full load. The HV goes from 77v NL to 56v FL (500W output)      Is that something I should look into? "

The KPA500 high voltage (nominal 60 volt) power supply is unregulated. The mains power input to the KPA500 is probably poorly regulated.   The voltage drop you will see on the mains input and the KPA500 60 volt supply is directly dependent on the PA current and you don't say what that was.

The fact that you have 500W output at 56 V does not define the PA current.  PA current, PA dissipation, and PA efficiency, are strongly dependent on the load presented by the antenna system.

My limited experience with the KPA500 suggests that you should:
      Attempt to minimize voltage drop in the mains feed
      Present the KPA500 with a load as close to R50 j0 as possible
      Keep PA voltage above 60 volts for all operating conditions
      Avoid nuisance high voltage warnings

How close you come to meeting those goals is partly dependent on the variation in the unloaded voltage at the mains outlet that you are using. (Summer HVAC use causes several volts variation at my nominal 120 V outlet even though the street transformer is less than 50 ft from my KPA500).   

Some data for my station:
Mains voltage is now 123.0 V and my KPA PA voltage is 81.5.   A TX into a  dummy load (R 51 X 0.4) at 14.074 MHz showed mains voltage 117.8,  RF out 487 W, PA V/I 62.6 V / 14.0 A, PA Diss 389 W, efficiency 55%.

My mains drop of 5 V matches yours.  My PA voltage drop of 18.9 V  is lower than your drop of 21 V and I would expect the difference to be explained by a difference in PA current.

73,
Andy, k3wyc