[AMRadio] Ranger driving SB220

John Coleman jc at pctechref.com
Tue Feb 12 10:42:33 EST 2008

Hi Jim,
	Yes, if you change the loading so as to draw less current in the
final then the modulation circuit will see higher impedance.  In this case,
the change to a higher Z load for the modulator is not a bad thing.  Audio
out put stages will generally produce cleaner audio when they don't need to
work so hard at delivering the current.  It is easier for them to get the
modulation audio voltage necessary for 100 percent modulation when the
impedance they see is higher and less current is on demand. 

OTOH, lowering the voltage to the final (only the final) with out changing
the loading will reduce current and voltage so that the load resistance that
the modulator sees is the same as before.  But less modulation voltage will
be required to hit 100 percent of the new DC voltage.

The thing to watch on tetrode finals is the screen current.  As you reduce
the loading on the plate the screen current will rise.  This is not to bad
(although worth looking in to) if the screen gets its DC and modulation from
the plate supply point, but if there is a separate screen supply then you
will need to adjust it down to prevent excessive screen current. 

All in all, the best way to reduce power for a driver /linear combo is to
reduce the plate / screen supply voltage for the driver final only, via a
dropping resistor or a separate power supply for that tube only. 

There are other considerations about reducing the load on a final, as the RF
voltage at the plate of the tube will increase and may cause arcing of the
plate tuning cap or some other component.  It is also easy for the modulator
to over modulate because it doesn't need to deliver as much current.

John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim Wilhite
Sent: Tuesday, February 12, 2008 8:16 AM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] Ranger driving SB220

I see this discussion every so often and wonder something.  If you lower 
the plate voltage or current the final (s) draw, what is happening to 
the impedance the modulation transformer sees?

The design of a class C final looks at the plate load impedance of the 
final.  If you alter that, the modulation transformer sees another 
impedance and may not work properly.  Some instruction manuals point 
this out in them.

The manual for my Globe Champ says to load to 320 ma. plate current @ 
1kv.  In another paragraph it says to load the plate current to no less 
than 275 ma. for the reason specified, plate load impedance.

So it seems to me that one would want to load a driver to the proper 
output designed into the rig.  If you need less driving power then 
dissipate with the attenuator like Johnson built or build an exciter for 
the power level you need.  Another approach I have seen is to feed the 
transmitter into a dummy load with a T connector and use the other T 
output to go the an amplifier input.  The HT 32/37/44 are low power and 
can be used easily to drive an amp.  They only give about 25 watts out 
which is about right for most amps.

The bad thing about sloppy modulation from an exciter is that a Class A, 
AB or B amp will just amplify it and you really don't want that going 
out over the airwaves.


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