[Amps] Filter-C

[R.Measures]

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