[AMRadio] Home brew AM Cathode modualtion

John Coleman, ARS WA5BXO wa5bxo at pctechref.com
Sat Oct 2 13:25:08 EDT 2004

My two cent worth for the new guys that might not understand:

	The technique that is used in the drawing of Jim's (JKO) showed
a 6146 in the final.  This circuit is simply a class A audio circuit
that has been placed in series with the finals B+ and screen common
circuit.  In this case the grid leak bias resistor has been returned to
the cathode so that the grid bias on the final stays constant with
modulation (except for the small usual grid leak self modulation
phenomenon that occurs on any high level modulated stage with grid leak
bias) The circuit could be modified placing the modulators in series
with the +1100 volt DC input and the results would be the same (see note
"1").  The plate efficiency of the Class "C" final is as good as any,
but the modulators will dissipate a lot of heat because of the class "A"
operation with less that 25% efficiency.  The overall efficiency is not
good.   The beauty of this circuit is no modulation XFMR with a trade
off for modulator efficiency. 
  It does require a separate filament source with good insulation for
the class "C" final to prevent filament to cathode breakdown and make
sure you get all the RF bypassed or filtered on the final's cathode.  
	It also requires twice as much DC supply voltage.  

(NOTE 1)
	Placing the modulator tubes in the cathode return of the final,
as shown, is a better construction technique that allows the modulator
cathodes to be grounded or biased in a normal manor (close to ground
potential, therefore allowing for normal RC coupling to the grids of the
modulator tubes).  If the circuit were modified, placing the modulator
tube in the plate circuit, then the cathode of the modulators would be
at a very high voltage. This would mean that the coupling to the grid of
the modulator would need special consideration or XFMR coupling,
defeating the one of the purposes of the circuit. Also, the filament
source for the modulators would need to be connected to the cathode of
the modulators and a special filament supply winding with good
insulation would be needed to handle the high voltage that would be on
the modulators.  None the less, theory of operation to the final is the
same.  It is still high level modulation.  The secondary of a modulation
XFMR could be place where the series modulator tubes are, and the supply
voltage dropped by the amount used by the series modulator tubes, and
there would be no difference than if the modulation transformer were
placed where you commonly see it in the plate circuit of the final.

Good Luck,
John, WA5BXO

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