> coverage. 11-12 mhz must be avoided. This is fine for most
> of us, but the L
> is not close to optimum for 160 meters, and it is
> physically much larger
> than the Harris choke. I get by with the RF Parts unit on
> 160m by using .01
> ufd @ 10 kv bypass caps at the base. This keeps most of
> the RF out of the
> power supply; not so important with oil-filled filter
> caps, but very
> important to protect electrolytic capacitor banks found in
> most amateur
> amps.
Let's look at that factually for an AL12 series amplifier:
The choke reactance is 3120 ohms.
The load impedance is 2500 ohms at 1.75kW
The RMS voltage is 2100V
The RMS choke current is .673 amperes
The net bypass capacitance on the HV, ignoring
electrolytics, totals .005 uF or more, which is 17.7 ohms
The RF voltage across the bypass capacitance is about 12
volts
The electrolytic series impedance is over 70 ohms at 1.8MHz
in sample units I tested. The RF current in the
electrolytics is under .18 amperes if we assume worse case,
but it typically measured under 75mA.
That 75mA is on top of the 2-3 amps of effective ripple
current (peak current is MUCH higher).
How much life increase should I expect if the bypass
reactance was changed from .005uF to .015uF, and ripple
current caused by RF dropped from 75mA to 25 Ma or less??
The answer is, none. The 2-3 amperes of effective ripple
current at 120 Hz dominates the problem.
This is not as bad as many Internet suggested mods are,
since the additional C at the bottom of the choke won't
hurt. Unfortunately some mods on Websites actually hurt
reliability, but this suggestion is benign. The bad ones
include resistor grid fuses, increasing power supply
capacitance, and removing caps across diodes, and increasing
resistance across electrolytics.
Food for thought...
73 Tom
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