[Milsurplus] Re: +AFs-Milsurplus+AF0- what +ACI-ARC-5+ACI- stuff is

[W7QHO at aol.com]

In a message dated 9/3/06 11:18:47 PM, neilba at clear.net.nz writes:


> ARC-5 feeds B+- to the 1625 plates in a shunt arrangement
> via an RF choke.  SCR-274N feeds B+- through the 1625
> output tank coil itself.
> In operation, there is probably no appreciable difference in
> performance.  Anyone know the reason for the change?
> 
> 
Neil,

Good question.   Answer probably related to the fact that the Navy ARC-5 
transmitters are plate modulated as opposed to the screen modulation scheme used 
in the Army SCR-274N units.   The rotors in the PA tank circuit variable 
capacitors in both units are grounded so with series feed (i.e., 
B+ through the coil) and 100% plate modulation the peak DC voltage appearing 
across the capacitor would be twice the DC plate supply value provided by the 
dynamotor.    Add a superimposed RF component and arcing across the capacitor 
plates would be a very likely occurrence at high altitude.   With shunt feed, 
on the other hand, only the peak RF voltages would appear across the capacitor 
plates.

Again, arcing would only have been a problem at altitude.   For years, hams 
have been successfully plate modulating the Army's series-fed 274N xmtrs with 
PA supply voltages of 600 or more.   Good idea, though, to replace the .01 PA 
bypass cap with a .001, 2500 volt unit before doing this. 


Dennis D. W7QHO
Glendale, CA