I am in the process of putting an old Wilkinson AM-2500B AM broadcast
transmitter on 3.885Mhz. It uses a pair of 4-1000A tubes in parallel,
modulated by a pair of 4-1000A's. It came complete with a "cutback
circuit" which cuts the power for nighttime operation to 400 watts
output. It was originally tuned to 1.300khz. The tank circuit consists
of RF coupling from the tubes through a 620pf (6kv rated) cap to a
tapped 70uh inductor (Kinetronics L70-10) with a 500pf variable cap
accross it to ground. The other end of this 70uh inductor feeds a
tapped 15uh inductor (Kinetronics L15-10) that has a large motor driven
air variable cap (34pf -250pf) on its input side (which is also in
series to ground with some frequency selectable fixed caps) to ground.
Of course, the other side of the 15uh inductor goes to the 50 ohm
antenna. This is the typical pi-L circuit. Also, there is a 2nd
harmonic trap at the junction of the two inductors that consists of a 30
turn 3" dia. coil in series with a cap which goes to ground.
I have several questions for you RF gurus;
1. I used the formula LC=25,330/f squared to come up with a place to
put the inductor tap for 3885khz. I picked the middle of my known cap
tuning range (250pf) and calculated I needed 6.37uh of inductance,
re-tapped my inductor proportionally, and found a good dip with my grid
dip meter nearly right on. Question: Should I have picked a different
capacity that would have used less inductance? My method doesn't seem
too scientific to me and maybe it is just outright incorrect.
2. If I am just using these two frequencies for now (possibly 160mtrs,
1885khz later) how do I figure what I need for loading capacitance and
where should I tap my series loading coil (15uh)? Should I just set the
cap at the lowest or highest end when I strap whatever extra capacity is
needed and then do some sort of calculation based on frequency to figure
out where to tap the inductor?
3. Should I just toss the suppression trap out of the picture? I want
good harmonic suppression so is there a formula I can use to implement
its use in this circuit? Obviously I'll have to change it if I go to
160m later on.
I checked the manual and the amplifier itself, unmodulated, has a plate
voltage of 4kv under load and a 700ma current draw.
Any help I can get from you guys is very much appreciated. I want it to
sound good and operate efficiently. Thanks, Mike.
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