[ARC5] SS Cathode modulator.

[Dennis Monticelli]

This four transistor circuit from* 73* is very primitive. As others have
noted there is poor control of the operating point and non-linear
conversion of voice to plate current.  In addition the output transistor
would need to have excellent SOA in order to survive because the base is
looking into a relatively high resistance.  However, if built I do believe
it will function (as long as it survives).

There is a better way.  Years ago I designed a SS circuit to function as a
cathode modulator for a typical class C stage.  This circuit has a
well-defined operating point, SOA protection, and uses feedback for
linearity.  The circuit operates as a V to I converter.  Because it does
not simulate a voltage controlled resistor like the other circuits, neither
the grid voltage nor the gm of the power tube has any effect on the
linearity or tube operating point.  The V to I converter represents an
ideal current source that accurately follows the audio input regardless of
cathode voltage, while the action of the grid drive is to simply chop the
tube's conduction at the carrier rate.  The only place where non-linearity
enters the picture is the plate/screen current split, which is not
perfectly constant with cathode current level.

Dennis AE6C

On Mon, Oct 12, 2015 at 8:13 PM, Fuqua, Bill L <wlfuqu00 at uky.edu> wrote:

>   It will work but with a ceramic or crystal microphone and since it uses
> a 3 transistor darlington driving the base of the power transistor directly
> the cathode current will
> be very unstable and it will be very distorted as well with splatter. And
> that is with a modern transistors. Perhaps by connecting the bias resistor
> to the collector/cathode terminal and
> a resistor to ground from the input base to ground it will become more
> stable, and more linear.
>
>   73
> Bill wa4lav
> ________________________________________
> From: ARC5 [arc5-bounces at mailman.qth.net] on behalf of Kenneth G. Gordon [
> kgordon2006 at frontier.com]
> Sent: Monday, October 12, 2015 8:56 PM
> To: Arc5 at mailman.qth.net
> Subject: [ARC5] SS Cathode modulator.
>
> Actually, from reading the blurb which accompanies the circuit, it would
> appear that all it does is control the cathode current of the tube(s) being
> modulated.
>
> In other words, Q-4 is an audio-controlled variable resistor in the
> cathode-to-ground circuit.
>
> The 6 volts has nothing to do with it there. That simply provides operating
> voltage for the other 3 transistors. Current drain from the 6 V source is
> minimal.
>
> The other 3 transistors simply drive the base of Q-4 and are connected the
> way they are so that they present a high-impedance at the INPUT.
>
> R-1 sets the quiescent current through Q-4 at no modulation.
>
> Although I am still not sure about it, I'll bet it does work.
>
> As I said, I'll bread-board one of these up and try it out as soon as I
> can.
>
> If anyone else feels like trying it, go ahead.
>
> Ken W7EKB
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