[ARC5] SS Cathode modulator.

[Fuqua, Bill L]

  Oh, by the way it was an April issue. That being said, did anyone notice the 6 meter SSB transmitter design. Take a close look and tell me if it will actually produce SSB.
73
Bill wa4lav

________________________________________
From: Dennis Monticelli [dennis.monticelli at gmail.com]
Sent: Monday, October 12, 2015 11:49 PM
To: Fuqua, Bill L
Cc: kgordon2006 at frontier.com; Arc5 at mailman.qth.net
Subject: Re: [ARC5] SS Cathode modulator.

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<mailto: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<mailto:arc5-bounces at mailman.qth.net>] on behalf of Kenneth G. Gordon [kgordon2006 at frontier.com<mailto:kgordon2006 at frontier.com>]
Sent: Monday, October 12, 2015 8:56 PM
To: Arc5 at mailman.qth.net<mailto: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
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net<mailto:ARC5 at mailman.qth.net>

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net<mailto:ARC5 at mailman.qth.net>

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html