"Antonio Vernucci" wrote in message
...
I am planning to build a suppressor-grid modulated AM transmitter.

In all the diagrams I have seen they use proper pentodes, i.e. tubes that
have a real suppressor grid (e.g. 803).

I have seen no diagram using beam power tubes (e.g. 813 or 814) which have
beam forming plates instead of the suppressor grid .

Before purchasing 803 tubes, I would like to be sure that 813s or 814s are
really unsuitable for suppressor-grid modulation.

My question is whether, with 813s or 814s, the plate current can be
actually controlled by varying the beam forming plates voltage.

Does anyone have experience on that issue?

This does not answer the question, but is offered as a bit of homebrew
lore.
After WWII, the 1625's were plentiful and cheap. Some Ham found that with
certain brands of 1625's, the suppressor grid was not tied to the cathode
inside the glass envelope. The two were tied together inside the tube base.
So, after removing the base, the two elements could be separated from each
other.
The aim was, guess what? Suppressor-grid modulation! I can remember
hearing some of these rigs on the air. I don't know just what percentage of
modulation was possible, but I remember it being pretty weak. Good strong
signal with very low modulation level.
The aim had been to use what parts one had, and it surely was an
inexpensive
way to join the AM crowd!

Old Chief Lynn, W7LTQ