Plate modulating a Ranger or other low-to-medium power transmitter

[AB2EZ]

A few years ago, I experimented with separate plate and screen modulation of the DX-20 transmitter, with various blends of plate and screen modulation. I observed (as I expected) that the fidelity of the screen modulation (in tracking the audio) and the level of the screen modulation were much more important than the fidelity of the plate modulation in achieving good fidelity modulation of the RF output envelope. Those of us who own a Ranger, a Valiant, a Viking I or a Viking II often make changes of one kind or another to the modulator (replacing the driver transformer, adding feedback, etc.) in order to achieve higher fidelity modulation. But, with a tetrode output stage, the real purpose of modulating the plate voltage is to obtain higher efficiency (reduced plate dissipation) than can be obtained using screen modulation alone and a fixed plate voltage. Even if the plate voltage (which one would continue to modulate, as usual) does not track the audio with good fidelity, one should be able to obtain excellent modulation by ensuring that the screen voltage is, separately modulated with a suitable, high fidelity modulator. Since the differential screen input impedance (i.e. the change in screen voltage/the associated change in screen current) changes over the range of screen voltages that will be used, it is important that the screen modulator's output impedance is less than about 0.25 of the nominal screen input impedance. Even with this constraint, the audio output power required for the separate screen modulator is only a few watts... so, with a modern high voltage FET, it should be relatively inexpensive and simple to build.

In another thread that was focused on how to reduce to average output power of a Ranger... and that was not focused on how to modulate a Ranger... the following idea emerged in a post by Joe, W3GMS:

Joe (W3GMS) wrote:

"I also use a series FET as a source follower to make adjustments to the screen of the 6146.  Be careful when doing that to control the output power.  With a given plate voltage the screen voltage has a sweet spot for best scope pattern.  With the FET you can mix audio with the gate bias voltage in order to  control the amount of audio on the screen for a given screen voltage and optimize a trapezoidal pattern.  On the AM Window, Dean WA1KNX gives some time concerning optimizing the amount of audio applied to the screen of a DX-100."  

Then, I added this comment after Joe's post:

"This is a neat way of adding a separate, adjustable, modulated B+ supply for the screen of the 6146, while continuing to modulate the plate voltage...

... For example, the DC input to the gate of the separate screen modulator could be obtained from a high impedance voltage divider connected to the HV B+. A total resistance (the sum of the two resistors in the voltage divider) of around 300k ohms would be a possible choice... to keep the total power dissipated by the two resistors in the voltage divider to less than 2 watts. The audio input to the gate of the separate screen modulator could be obtained from a stage in the Ranger's audio chain that is after the stock "audio" pot. The DC and the audio inputs to the gate of the separate screen modulator can be combined using a "modified Heising" approach, where the function usually performed by the Heising reactor is performed by the 2nd resistor in the 2-resistor DC voltage divider. E.g. if the two resistors in the voltage divider are approximately 200k ohms and 100k ohms, then the source impedance, looking (back) into the (3:1) voltage divider will be around 67k ohms. Since the DC input resistance, looking into the gate of the FET, is essentially infinite... one could also add an additional "modified Heising" resistor, in series, between the output of the voltage divider and the gate of the FET..."

Stu

[kb3ouk]

Here's a post from a while back that has a schematic of pretty much the exact same thing, except with tubes instead of FETs: http://amfone.net/Amforum/index.php?topic=35905.0 The 4X150A (the GRT-3 used them for a final) datasheet says to modulate the screens at approximately 55% in phase with the plate modulation in order to get 100% modulation from the tube in plate modulated service, and to not use a series resistor or to self-modulate the screen with a choke.

[AB2EZ]

Similar, but a key difference, between what is proposed here and the earlier thread you refer to, is to obtain the separate screen modulator's input from an audio source that does not include the plate modulation transformer (or any other transformer).

Stu

[N2DTS]

I do not understand that.
I have seen it in the RCA tube manual, and other places, but my 4x150a rig with self modulating screens runs very clean, and has a LOT of positive modulation.

On my 32v3, I did the mix of modulated and unmodulated screen voltage and get a nice 4X peak to average power out of it.

Here's a post from a while back that has a schematic of pretty much the exact same thing, except with tubes instead of FETs: http://amfone.net/Amforum/index.php?topic=35905.0 The 4X150A (the GRT-3 used them for a final) datasheet says to modulate the screens at approximately 55% in phase with the plate modulation in order to get 100% modulation from the tube in plate modulated service, and to not use a series resistor or to self-modulate the screen with a choke.

[kb3ouk]

My guess is that as long as you make the screen voltage swing high enough, it really doesn't matter how you modulate the screen. I'd say that maybe RCA figured that you couldn't get enough voltage swing by using a dropping resistor or a choke, so a seperate screen winding or other method like that would be the only way to go. Brett, I wonder how well your rig would work if you modulated it less than 100%, like around 90% positive. I'm guess that because of the amount of positive plate modulation you are running, the choke is causing the self-modulation of the screen to be high as well, probably somewhere close the that apparently magical 55% that RCA mentions.