[Amps] HF amp output power dropping

[jordan moser]

I have an old Henry 3000d RF heater that was converted to 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
 
Jordan WJ2W
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