[AMRadio] Series Modulator


John Coleman, ARS WA5BXO wa5bxo at pctechref.com
Sun Mar 6 22:12:30 EST 2005


Dennis, 
	I don't have the 1954 Handbook to look at the article but, by
definition, the series modulator is hooked up so that the entire power
supply voltage going to the final is varied in accordance with the bias
or audio on the modulator.  It is High Level Plate Modulation, and the
modulator tube must deliver 50 percent (average audio power for 100%
modulation) of the DC input to the final class C device, just as with a
XFMR coupled Plate modulator.

The modulating device (tube or transistor) may be placed in series with
the B+ lead (just as a secondary of a modulation XFMR) to the Final as a
cathode follower with less than unity gain.  This requires the audio
going to the input to be higher than the modulation voltage.  The
modulating device may also be driven by a transformer with the output
winding connected between grid and cathode of the modulator.  In this
way the modulator device has gain even though the output is still on the
cathode which is connected to the plate circuit of the final.

In the circuit that Jim (WD5JKO) has shown, the modulating device is
connected in the cathode return circuit of the final.  At first glance
it might appear that the circuit is a cathode or bias modulator (similar
to grid bias modulation).  With closer inspection however, and you will
see that the grid leak resistor of the class C final, is not returned to
ground, but is instead returned to the cathode of the final.  This
little thing makes this a high level plate modulation system because the
bias on the final (measured from cathode the grid) doesn't change
anymore than it would with standard plate modulation.  If the class C
final voltage, measured from cathode to plate is 250 Volts then the
modulator must be able to vary that voltage from 0 to 500 volts for 100%
modulation.  Typical this circuit will not be able to achieve 100%
modulation but with careful adjustments it can get close.   

John, WA5BXO

     

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of W7QHO at aol.com
Sent: Sunday, March 06, 2005 5:32 PM
To: jcandela at prodigy.net; amradio at mailman.qth.net
Subject: Re: [AMRadio] anyone for qrp am?


In a message dated 3/6/05 3:02:36 PM, jcandela at prodigy.net writes:


> Another alternative is series modulation. This has efficiency issues
using 
> a class A series modulator, but for low power AM this might be the
ticket.
> Regards,
> Jim
> WD5JKO

Essentially a form of grid modulation, see ARRL Handbook 1954 page 250
for a 
version using   a 6SL7 and a 6Y6.   I've heard these on the air sounding
very 
good.

Dennis D. W7QHO
Glendale, CA
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