Gary Schafer wrote:

Cathode modulation is very similar to grid modulation in performance.
After all audio is placed between the grid and cathode just the same
as it is with conventional grid modulation. The amount of cathode to
plate modulation is minimal.

Correct, and the distortion can be pretty nasty, too.

Screen modulation is similar in performance to grid modulation also.
Efficiency is around 35% carrier efficiency. Very tricky to tune up
properly.

It was the /only/ type of AM I used at first, as I was unable to get big mod
transformers. It worked well enough, though it was difficult to get close
to 100% mod. Lots of envelope feedback cured the non-linearity
distortions.

Low level modulation with a linear amplifier behind the modulated
driver also has an efficiency of around 35%. Excellent AM can be
generated this way as is done with some SSB transmitters in the AM
mode. If properly set up one can not tell the difference between it
and a high level plate modulated transmitter except that the plate
modulated transmitter will probably have higher distortion.

Low-level mod with a linear is very inefficient (with respect to DC input),
but is sometimes the only really practical way to proceed. Again, lots of
envelope feedback can sort out the distortions caused by less than perfect
PAs!

Bob
--
Everything gets easier with practice, except getting up in the morning!