[Amps] Doorknob Caps Needed: Background

[Radio WC6W]

Hi Dick,
  
<snip>
 
> One of the questions I have is about the Plate Coupling Cap
> value. I see values of anywhere from 100 pF to 1,000 pF used.

<snip>

  A good "rule of thumb" is 4X the tune capacitance.  The value is not at all critical.  100pF, or thereabouts, will be fine for 6 meters with your pair of 3-500's.

  There are some extra caveats on the higher frequencies where the plate output capacitance becomes a significant portion of the tune cap.  The coupling cap must now carry a considerable portion of the circulating current; an NPO ceramic part is good for this.

  The best choice would be a vacuum cap.  One from the Jennings JCSF series would be great.  There may also be some physcially larger 100pF WW2 vintage GE & Western Electric parts still extant.  These are rated 5KV but typically test way over 10KV

  Using a grossly overvalued part is proscribed. At some point enough stray inductance will accumulate to cause... problems.

73 & Good morning,
  Marv WC6W 

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