[Amps] HF amp output power dropping

[Mike Willis]

    >> HAM use by someone else. It runs a 3CX3000A7 tube in
    >> grounded grid.
    >> Recently the output power has begun falling off rapidly
    >> whenever I apply constant drive such as during tune up.
    >> The tube is new and the plate voltage remains steady as
    >> always.Is there anything else that could be causing this?
    >> It reminds me of a tube that has gone soft although I am
    >> sure it's not the tube.
    >> Any help is appreciated
    >>
    >
    Last time I had this happen it was the antenna. I think something was getting hot and changing the match. I am sure you are tuning it into a dummy load, so it can't be that. However, it was possibly worth mentioning. 
    Mike