I have a SB-200, 75 watts in to the amp, the grid voltage, on dead key pegs the meter and the power slowly drops from 500 Watts to about 250 watts and the grid ma drops to about 200ma Grid, ma and watts both drop within 7 seconds.

The swr remains the same, I even changed the Balun thinking the toroid might be saturating until I saw the grid ma.

I have an extra pair of 572B tubes and tried them, same problem so I purchased and installed new 572B tubes, same problem.

So when I tune the amp for max power the grid ma is way out of specs.
Can anybody help with any suggestions? I checked the grid capacitors, their fine. I have no idea what to check next.

Thank you
Eddie kj4fgi