Topband: Plate chokes (Balun thread)

[Herb Schoenbohm]

 Greg,

Rockwell-Collins in the 80's marketed an AM transmitter called the 
"Power Rock" that used a clever grounded plate construction. (Achieving 
DC ground through plate resonant circuit so the RF would flow to the 
antenna at 50 ohms.) The PA supply voltage for a 3CX2500F3 was applied 
to the center tap of the filament transformer. (This principle I have 
not seem applied to amateur amps and can't understand why.) This was a 
negative 6KV in the 5KW version.  If I recall correctly this eliminated 
both the plate blocking cap and the choke.  This was a PDM "switch mod" 
transmitter so the modulation switch, another 3CX2500F3, was controlled 
through a fiber optic cable thus avoiding the DC coupling problem from a 
60hz pulse train.  A choke was required, however, to filter out the 60HZ 
switching pulses.  Today most of the equipment for Broadcaster uses 
blocks of power mosfet amps adding up to 10KW or more.

73

Herb Schoenbohm, KV4FZ

Greg - ZL3IX wrote:

>Back in the 80's, when I constructed my homebrew amp, I came up with the
>idea of removing the plate coupling cap and using a cathode coupling cap
>instead  (cold side of the PI network).  I coupled the output circuit using
>a simple 1:1 transformer with enough DC isolation between pri and sec to
>keep the HT off the antenna.
>
>I then fed the HT on to the cold side of the PI.   Thus no choke is
>required, so the amp can cover any frequency between 1.8 and 30 MHz.
>
>73, and all the best for 2004,
>
>Greg, ZL3IX
>
>
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