The AM Forum

Hello.

When reading through an old Radio Handbook last night, I saw the third winding method mentioned again.
Altough I (plan to) use a dropping resistor in a little AM transmitter, I thought about how a third winding would work, and I couldn't figure it out.

I know that with a terode, and using a dropping resistor, the screen voltage should be modulated about 60% for good results. So one resistor from the modulated B+ and one from the unmodulated B+.

Now back to the third winding idea.
If I run a seperate screen voltage through the third winding on a modulation tansformer, it will be modulated at the same percentage as the main high voltage, only at a lower voltage...
Only if the third winding would have the same amount of turns.. (impedance)

So in order to modulate the screen at approx 60%, the third winding needs to have 40% less windings (impedance). Fed with a lower (screen) voltage.

Is this the correct way of thinking?

73's Martin PA4WM

The amount of modulation is relative to the DC voltage.  Since the screen operates at a substantially lower voltage, fewer windings are required to produce an AC voltage that will modulate it 100%.  I suspect that the third winding would be designed for a specific, lower voltage level.  OR it is used for negative feedback to an upstream audio amplifier to reduce overall distortion.

I test with a one kHz tone at one volt P-P on the push-pull side, then check the relative voltage on the other windings.

Most of the time the tertiary winding was used to tickle the driver stage for easier 100% modulation.

73DG

The ART-13 is a good example and that transformer is still quite common in European surplus, even NIB.

Carl