I am building an AM transmitter using a 829B final tube (with the two tetrodes
in parallel) operating in class-C (750V on plate, 200V on screen).

The modulation transformer has a single secondary winding.

To obtain both plate and screen modulation, I am planning to adopt the
well-known circuit whereby the tube screen is fed by the 750V plate supply
through a high-power series resistor.

I have two questions at that regard.

FIRST QUESTION

According to the ARRL Handbook (1968 edition), the screen resistor must be
connected to the "hot" end of the modulation transformer secondary. This seems
quite logical to me.

On the contrary, the EIMAC "Care and feed of power grid tubes" booklet states
that the screen resistor must be connected to the "cold" end of the secondary.
Their argument is that, when plate voltage increases (due to modulation), the
screen current decreases and so does the voltage across the screen resistor,
thus resulting in a higher screen voltage. In other words the screen is
subjected to an automatic modulation effect.

Does anyone have practical exeperience on the pros and cons of the two methods?

SECOND QUESTION

To protect the final tube, I am planning to have a fixed negative bias on the
control grid that sets the the tube close to interdiction (some 10 - 20 mA of
idling plate current) in absence of RF driving voltage on the grid.

In such resting condition the screen current should presumably be very low, with
a consequent low voltage drop across the screen resistor.

The screen voltage will set at some voltage that will surely be higher than the
normal operating voltage (200V). I am not sure whether this can cause problems
to the tube.

Would you have any suggestion to avoid such voltage growth effect, other than
the classical solution of adding an extra clamp tube that, in absence of RF
drive, draws current from the screen resistor?

Thanks in advance for answers.

Tony, I0JX

Rome, Italy