[Boatanchors] PA Tank Circuit Padding Capacitors

[Bob Groh]

Ran some blindly quick (hence maybe a error here!) calculations and some
assumptions
We need to know voltage across capacitor and current through it (current is
forgotten all too often).

Assumptions and calculations:
Frequency = 4 MHz
Capacitor reactance = 800 ohms (50 pF at 4 MHz)
Power output = 9W
I used assumption of 20W at plate
IF plate Z is set so we are getting full B+ swing that implies about 225
Vrms or 300 Vpeak or 600 Vp-p (for 20W at plate)

Capacitor current is voltage across capacitor divided by reactance so
current is about 0.3 Arms (i.e. 3/10)
Voltage across cap (assuming it does NOT see B+) is about 300 Vpeak so 500V
rating would seem to be ok

The current is not huge but it is not trivial.  I would probably consider a
nice 500 V NPO ceramic disk of some size (RF current used to be on some
data sheets) or even a 500V silver mica (again of some size, i.e. not
physically a tiny one).

Hope this helps.

Bob Groh, WA2CKY





On Mon, Aug 1, 2016 at 7:15 AM, David Stinson <arc5 at ix.netcom.com> wrote:

> Given: Series-feed Triode PA tank with link-coupled output.
> 350 VDC B+, about 9 W out on 75 Meters.
> Padding across the tank coil with a 50 pFd doorknob was successful in
> lowering its resonant point, but space is severely restricted and doorknobs
> can be expensive.  *Rewinding the coil is not an option.*
>
> What other types and specifications of  available
> capacitors would you recommend for this service?
>
> TNX OM ES 73 DE Dave AB5S
>
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