[AMRadio] AM Radio

[ne1s at securespeed.us]

John, WA5BXO wrote:
> Idea 1.
> When running a Class C rig such as the DX100 there is a specified amount
> of
> grid current (drive) required to make the finals operate in Class C.  They
> will not modulate properly if this drive level is not met and the finals
> will over heat. Bad idea
>

To expand on what John wrote above (which is very true):

To plate-modulate an RF amplifier requires the RF voltage output (and also
the current output, since the load is constant) must be a linear function
of the (instantaneous) plate voltage. For example, if the plate voltage is
doubled from the DC supply voltage, the RF voltage (and current) is also
doubled - this particular example is, by definition, 100% modulation.

The above condition is only met if there is adequate grid drive so that,
WHEN THE PLATE VOLTAGE IS 2X THE DC SUPPLY VOLTAGE, ANY INCREASE IN GRID
DRIVE WILL NOT INCREASE THE RF OUTPUT. In practice you need adequate drive
to maintain the condition for positive peaks exceeding 100%. Said another
way, the RF  amplifier must remain in saturation (with respect to grid
drive) for all values of instantaneous plate voltage.

> Idea 2.
> Reducing the loading for less plate current will cause a mismatch in the
> ratio of plate voltage to screen voltage and the screen current will be
> high.  Will work to some degree but not the best idea.
>

Yep.

> Idea 3. Best idea but hard to implement
> Lower the final plate supply voltage in some way that the screen voltage
> reduces along with it but not he modulator voltage.
>

Easiest way to do this is with a variac on the primary of the plate
transformer for the class C RF amplifier, but this requires:

(1) a separate plate supply for the modulator (not the case for a DX-100).

(2) If the class C amplifier is a tetrode or pentode, the screen voltage
must be referenced to the plate voltage (for example, the use of a
dropping resistor from the (modulated) plate supply to feed the screen
grid).


73,
-Larry/NE1S