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How does this AM modulator work?

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neazoi

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Here is a very old design of a broadcast AM LW transmitter. I wonder how the class-A modulator tube 6aq5 works?
Meaning, what is it's effect on the operation of v1b?
 

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BigBoss

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Thetrode tube modulates the anode of V1B.You see a resistance and a RFC between power supply and the anode of V1B and thetrode modulates the RF current between this resistance and the RFC. coil..
 
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neazoi

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Thetrode tube modulates the anode of V1B.You see a resistance and a RFC between power supply and the anode of V1B and thetrode modulates the RF current between this resistance and the RFC. coil..
So it is like having a variable resistance in the anode of v1b, varying the output power of the transmitter and so "AM modulate"?

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Complete article you are referring can be found here: https://www.americanradiohistory.com/Archive-Poptronics/70s/1972/Pop-1972-01.pdf

Explanation of anode (plate) AM modulation can be found in many articles on web.

So it is a Heising modulator that is used in this transmitter? https://www.w8ji.com/Heising modulation.htm

By definition Heising modulation has a 1:1 modulator impedance to PA impedance ratio. Heising modulation shares power supply current between a PA stage and a shunt modulator tube. When the modulator tube is biased "on", the modulator tube pulls current away from the PA stage. When the modulator tube is biased off, current is diverted to the PA stage. Power supply load current is nearly constant throughout the modulation cycle, hence the alternative name constant current modulation.
 

SunnySkyguy

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this is similar to a common source FET amp oscillator with v control gain with ratio of resonant impedance to RdsOn + Rs, also affected by gm in between.
 

neazoi

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Interesting note vfone!
The last grid is connected internally to the cathode, it should be displayed in the schematic, but otherwise pin numbers are correct.

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By the way, may I ask, what is the voltage guess on the positive terminal of the electrolytic C1B?

I am asking this because I would like to replace the rectifier with a tube. So I have to roughly know the expected voltage at this place.
An EZ80 with 90mA max current would be sufficient I think, because the transformer 125V winding in the schematic is rated at 50mA A center tapped transformer would be required then. Shall I make this transformer winding 2x125v or it needs to be a bit greater (tube voltage drop)?
 

vfone

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That is the Suppressor-grid, and is employed to catch electrons that have been released from the anode. Internally, this grid uses just a few wires, placed near the anode, and usually connected to the cathode potential.

The other grid (Screen-grid connected to a positive potential) the main function is to reduce the internal capacitance between control grid and anode, capacitance which might cause instability if the gain/slope of the tube is too high.
 
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