RE: Filter-C, how much is enough? A friend of mine built a
4-1000A/8166 g-g amplifier that ran c. 6500v on the anode from a FWB
rectifier/C-filter. The only filter-C he had on hand that would handle
this much V was a 2uF unit. To get the project going, he installed the
2uF C, but he left extra space on the chassis so that he could add more
C when it became available - at the right price, of course. To
initially check out the amplifier, he fired it up with the 2uF filter
C. He fired up the amplifier on 80m. I could hear no ripple in the
SSB signal. Is this plausible? Hmmmmm. During a SSB voice
transmission, indicated anode-I is roughly 1/3 of the max anode-I
rating, so in this case, the average load would be 750mA / 3 = 250mA.
Thus, the equivalent RL on the PS was 6500V / 0.25a = 26,000-ohms. The
rule of thumb for the filter-C for a FWB is 70k / RL, or 70,000 /
26,000 = 2.7uF. This led me to conclude that this rule of thumb might
be a bit conservative. Eventually, a 4uF unit was added to the 2uF,
bringing the total to 6uF. No subsequent change in audio quality - or
change the HV meter indication - was observed.
- end
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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