[AMRadio] Step Start

D. Chester k4kyv at charter.net
Wed Dec 13 16:54:04 EST 2006

... you don't have to worry about
>> the need for step-start with choke input filter using a swinging choke, 
>> until you go beyond 25 mfd of filter capacitance.
> Good.  I have eliminated it.  Not sure what the voltage will be but the 
> xformer I ordered is 2000vct @ 500 ma

If you mean the total winding is 2000 volts, with a  midtap, then the output 
voltage with choke input will be about 900 volts (transformer 1000 volts 
each side of midtap).  If you mean 2000 volts each side of midtap, then 
output voltage should be 1800 vdc.  With choke input, the nominal output 
voltage is 0.9 X Vrms of the transformer.

> I presume I can eliminate that drain if I drive with the Ranger and keep 
> it all under 500 ma?

Not sure what you mean by that.  The power supply voltage will be much more 
steady if you have a constant load on it.  If you use the same power supply 
for modulator and final, the PA will keep the supply loaded well enough to 
maintain good regulation on the class-B modulator.  That's the way most 
brodcast transmitters work.  If you use separate power supply for modulator 
and final, or if you run the rf amp as a SSB linear or  use it on CW, then 
you have to pay extra attention to voltage regulation.  That depends on your 
choice of filter capacitance, choke inductance and bleeder resistance.

My HF-300 rig uses a separate power supply for the modulator.  The modulator 
supply, and final amp supply on CW, show approximately 10% regulation on the 
meter.  I maintain good dynamic regulation by using 28 mfd of filter 
capacitance in each supply,  along with a 25,000 ohm bleeder in each.  With 
only 8 mfd of filter capacitance, the dynamic regulation will be poor for CW 
or SSB, even though the meter indicates good regulation; look at the output 
voltage on a scope and you will be surprised at the instantaneous voltage 
variations under the varying load.


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