Antonio Vernucci wrote:
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

There are two other ways of handling the screen voltage in a modulated
stage. The screen can be feed by a regulated voltage supply and a
separate winding on the modulation transformer is used to modulate the
screen. A variation on this is to put an audio choke in series with the
screen supply and to optionally couple the screen to the hot side of the
plate modulator winding via a capacitor.