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.
73,
Rick/K5IZ
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
