[AMRadio] Smoothing Choke

Rick Brashear rickbras at airmail.net
Thu May 4 20:09:19 EDT 2006

Don and others...
    Thanks for all the information on chokes, capacitors and power 
supply schemes.  They have all helped me tremendously in deciding which 
way to go and what to use to get the best bang for the buck out of the 
supply for the ART-13 transmitter.  Just about the time I had my mind 
made up someone else would drop by with another great suggestion or real 
world advice.  This is a great list with a bunch of well read participants.

> This is called the dynamic regulation.  When keying a transmitter on 
> cw, or running a class-B modulator or SSB linear, the plate 
> (kilo)voltmeter may show little variation, indicating good 
> regulation.  But if you set up an oscilloscope to display the 
> instantaseous power supply voltage, you may be surprised how much the 
> voltage kicks around with the transients.  The voltage not only dips 
> down after the load is applied; when the load is removed the voltage 
> will kick back upwards, and the momentum may swing it well above the 
> nominal output voltage.  I have seen power supplies where the voltage 
> dropped below 70% nominal volatge and back up to 140% nominal or 
> higher - a 2 to 1 voltage variation while the sluggish mechanical 
> movement in the analogue voltmeter just barely wiggled with keying or 
> modulation. Monitoring the cw output from a transmitter using a 
> modulation monitor scope may show a horrible keyed waveform.
> The solution that has worked for me is to use as much capacitance in 
> the output side of the filter as possible, without kicking the 
> overload relay when the power supply is first turned on.  I prefer to 
> kill the HV during receive.  Don't like having the full HV turned on 
> while the transmitter is in standby - a death trap plus rf noise 
> generator, and it hastens breakdown of HV components.  You can use 
> step-start if you want to further increase the capacitance, but I 
> prefer to avoid that complication.  Of course,  increasing the 
> inductance of the choke will reduce the inrush current to the 
> capacitor by slowing down its charging rate.
> I converted my Gates BC1-T to CW capability, and was able to get a 
> reasonable looking waveform by increasing the capacitance in the 
> one-section choke input filter from 8 mfd to 25 mfd.  It is still not 
> perfect, but is satisfactory.  My homebrew transmitter uses about the 
> same capacitance each in the rf final plate supply and modulator plate 
> supply.  I have found that with a 2-2.5 kv power supply with a load 
> not exceeding 700-800 mills total, a 25 mfd 4 kv output capacitor is 
> about the best compromise between dynamic regulation and inrush current.
> With AM, the best solution is to use a common power supply for the 
> modulator and final.  The rf final acts like a heavy bleeder resistor, 
> and maintains good dynamic regulation for the class-B modulator 
> plates.  Poor dynamic regulation can result in poor sounding audio, 
> even though every component in the modulator is of the highest quality 
> and the circuit is of the best design.
> If you are having an inexplicable audio quality problem, take a look 
> at your HV power supply output voltage on an oscilloscope, to see what 
> the dynamic regulation looks like.  Connect up a series string of 
> resistors, so that the total resistance is several times the normal 
> bleeder resistance, and connect the vertical plates of the scope 
> across the bottom resistor.  Make sure the wattage and voltage rating 
> of the resistors is not exceeded.  Another way would be to temporarily 
> disconnect the bottom end of the normal bleeder resistor, and insert a 
> resistor of approximately 5% of the bleeder resistance in series with 
> the bottom side.  Place the scope probes between the connection 
> between this resistor and the bleeder, and ground, to read the voltage 
> across this resistor.  If the resistor is 5% of the total resistance, 
> 5% of the total voltage will appear across it.  Thus, with a 2kv power 
> supply, 100 volts will appear across the resistor, which is much 
> easier to measure with a scope than the full 2000.
> Don k4kyv

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