The AM Forum

This is a continuation of a question I asked yesterday, with some more content...

Leading off, I don't know what happens when you do this...

OK, we have a grid driven, grid leak biased class C tube stage. It's plate modulated.

For a given RF drive level:  what happens if one varies the grid leak resistor? 

Intuitively I expect that the self rectification will make whatever voltage the available amount of self-rectified current would impose on the resistor -- that is a higher grid leak resistance would give a higher bias voltage, while a lower R would decrease bias voltage.  Thus one would set bias voltage by adjusting drive with a fixed resistor sized to arrive at a specific grid current and bias voltage combination -- 100v and 10mA defines it to be 10k, drive till you get there, kind of thing. 

So what happens if you somehow inversely vary the resistance with drive, such that the voltage is relatively the same while current jacks around with drive changes?

Why this may be of benefit/what I am thinking/wondering:
If the bias voltage drops, plate current increases (non-linearly/cutoff and all that considered). But if the resistance is inversely proportional, the drive decreasing will not cause the plate current to increase until it is basically gone.  There may not be sufficient drive for linear modulation, but the tube won't run away either. If the drive is too hot, the bias voltage also will not increase, thus plate current remains stable.

The diagram I posted before and here is a way to get a drive-inverse resistance:  Zeners.  A Zener acts like a variable resistor. In the typical Zener regulated circuit, a voltage is fed past the Zener with a resistor in series with the source. As the voltage climbs, the Zener conducts more/passes more current/acts like its resistance is less until enough current is going through the series source resistor to drop the voltage value to the Zener rating. If this is implemented as a grid leak, I should be able to set the bias voltage via the Zener string, and then the drive level sets the grid current.

The question:  Why not? will it work? is there a benefit? is the benefit worth it? am I chasing shadows? out of my head?

Ed

See the schematic below.

Most vintage tube transmitter builders want to let the input RF set the bias and if for some reason the bias goes below a certain value, the circuit below can be used to level it out.

Phil-AC0OB

Here is some more information from one of our contributors, W4AMV.

https://www.ermag.com/wp-content/uploads/2021/08/FIGURES_Excitation-to-the-Grid-and-Driving-Impedance-of-the-Grounded-Cathode_W4AMV_update_7_19_21.pdf