Sorry but I am going to throw a wrench into this discussion. Those
swinging choke power supplies, although useful, have been out of favor for
years, especially when the loading is transient such as for a CW rig. Sure
if the critical inductance versus minimum load current are correct, the
output voltage will not soar under a light or no load. But what happens
when
the load is applied suddenly such as with CW? The answer is arguable of
course, but I think the answer is a sudden significant and momentary dip in
B+ which all depends on the L-C-L-C values chosen.
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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