[Amps] AM Downward Modulation

[harold b mandel]

In my screen-modulated days when the tetrodes in the exciter
were not neutralized, and not tuned correctly, and when
the screen supply was unstable my exciter would exhibit
downward modulation, whereby the anode current would
deflect downward on voice peaks from the quiet carrier
current.

What does it mean when a Kenwood TS-450s/AT
downward modulates in AM?

The RF Power actually decreases in both average and peak 
settings.

The waveform fills with speech, but the envelope shrinks.

Anyone ever see this or know if it is normal?

All other modes transmit normally.

Thanks,

Hal
W4HBM