Screen self-modulation - theory & practice

[KA1ZGC]

I need a better understanding of screen self-modulation in a tetrode amplification stage.

I know it can be done using a tertiary modulator winding or a choke. I'm a little vague on the choke method, but I assume it's an audio-block to allow AF energy to build potential across it and shift the screen voltage accordingly. Please correct me if I'm wrong.

Are there any other methods of getting a screen to self-modulate? What tends to work most reliably? Would this play well with a shunt-regulated screen supply?

Any info, no matter how generalized, would be appreciated.

Thanks in advance,

--Thom
Kilowatt Amplifier One Zero Grid Current

[Tom WA3KLR]

The WRL Globe King 500C uses a choke to feed the 6156/4-250A screen in AM mode.
No audio fed in, separate screen supply.

Schematic of the r.f. deck courtesy of BAMA below:

The screen modulation choke is CH-2.  No parts values on the schematics.  The parts list does not give the value, only description "Choke, screen modulation", p/n 1300-0002.

[Jim, W5JO]

The WRL Globe King 500C uses a choke to feed the 6156/4-250A screen in AM mode.
No audio fed in, separate screen supply.

Schematic of the r.f. deck courtesy of BAMA below:

The screen modulation choke is CH-2.  No parts values on the schematics.  The parts list does not give the value, only description "Choke, screen modulation", p/n 1300-0002.

That screen choke is 12 Hy @ 80-100 Ma. 2000 V. Test, 375 ohm resistance.

[AB2EZ]

Thom
et. al.

I would assume that the screen choke is chosen to be large enough to present a few thousand ohms of impedance at audio frequencies. Of note, the Globe Scout uses a resistor from the screen to the (unmodulated) B+  to accomplish the same purpose. The Globe Scout Deluxe uses a different approach: running the resistor from the screen to the modulated B+

I believe that as one modulates the plate voltage, electrons traveling from the cathode to the plate will accumulate on the screen (some of the electrons passing through the screen are captured by the screen) in larger (when the plate voltage is reduced) or smaller (when the plate voltage is increased) numbers... thus causing the screen voltage to be modulated up and down as the plate voltage is modulated up and down. For this to be effective, one needs a high enough impedance between the screen and the cathode... at least at modulation frequencies of interest.

Of interest is whether this approach results in the best audio quality v. alternative approaches of modulating the screen voltage in concert with the modulation of the plate voltage.

Stu

[Tom WA3KLR]

Look for Carl WA2UJX on 75 meters or 1885 AM and you will hear the Globe King 500 Charlie.  He may be on a rice box but he will switch over.

[Bacon, WA3WDR]

In my Viking II, I used just the series dropping resistor that had supplied screen voltage from modulated B+ before.  I simply moved the resistor to unmodulated B+.

It was a significant improvement.  With the resistor on modulated B+, the screen was overmodulated, and this caused a kink at around -85% modulation.  This kink can be seen in the classic trapezoid pictures from old ARRL handbooks.

However, full self-modulation of the screen produces slightly less than the absolute optimum screen modulation level.  WA1KNX Dino found that splitting this resistor into a larger value to unmodulated B+, and an even larger value to modulated B+, about a 40/60 ratio, resulted in the very lowest distortion modulation.

"Class C Optimization for Ultra Low Distortion", AM Press/Exchange issue #71, May 1989, Dean, WA1KNX, www.amfone.net/AMPX/71.htm

"Screen Modulation Optimization for the Heathkit DX-100", Dean, WA1KNX, http://www.amwindow.org/tech/htm/scrnmod.htm

"The Self-Modulated Screen Grid", AM Press/Exchange issue #98, Jan/Feb 1992, Bacon, WA3WDR, www.amfone.net/AMPX/98.htm

And now, AB2EZ Stu has brought up another idea: super-quality screen modulation with regular quality plate modulation.  This isn't screen self-modulation, but it is of interest in the general topic of screen and plate modulation.
http://amfone.net/Amforum/index.php?topic=12117.0