[AMRadio] Ranger driving SB220

Jim Wilhite w5jo at brightok.net
Tue Feb 12 09:15:31 EST 2008

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.


> Ron,
> Yes, the VKII uses a pair of 6146s and it has a 6AQ5 adjustable screen 
> clamp tube.
> I see no reason to wast power with an attenuator and there is a sweet 
> spot in the final current thru the mod iron that requires a less than 
> full rated final current to avoid saturation.
>  It seems to me that a fine adjust here would allow power reduction 
> for use with an amp and also enable fine tuning the final current 
> without reducing optimum drive current when used barefoot.
> Just an idea. I like the ability of the DSB transmitters like the HT 
> 37, FT101,etc to lower the output power without having to retune the 
> final.
> 73,

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