[Amps] Lets Talk About Padder Caps

[R L Measures]

On May 28, 2006, at 1:47 PM, W7RY wrote:

> I'm trying to replace the tune and load padder caps in my home-brew
> 3CX800 amplifier. (Ten-Tec Titan clone).
>
> I need about 300 PF for the tune cap and about 1000 PF for the load.
> These are used on 80 and 160 meters.
> I have a drifting problem on these bands. Mostly on 160 though.
>
> How much current is flowing through these on 160 meters?

The rule of thumb is tank circulating current = Q x anode-I.
>
> I'm looking at the HEC site at:
> http://www.highenergycorp.com/ceramic/rftransmit.html for a suitable
> unit.
>
> I have searched the archives but no practical solutions. Just talk  
> about
> poor designs and other non descriptive stuff.
> The Alpha 89 uses strings of high voltage disk caps. (5ea 75 PF 6KV  
> for
> tune padder and 7ea 150 Pf 8KV for loading).

Not good engineering practice.  Disc ceramics are not current-rated  
for a good reason.  In my experiences, 1 ampere at 3.5MHz causes  
moderate heating in a 6kV disc cer.   Commercial amplifiers typically  
use paralleled NPØ or N750 doorknobs for tuned circuit applications.
    Since mica caps do well up to c. 5MHz, they can be used for load  
padders on 80m and 160m.  Cornell Dublier 2000v micas are suitable  
for this purpose.  Example:  the Heathkit SB-220 uses a 500pF, 2000v  
mica for its 80m load padder.

> I don't remember if the 89 has drifting issues on 160 meters. Lots of
> small value disks are cheaper than large doorknobs.
> I'm sure this is why Alpha and others use them.
>
>
> Ideas?

Use good engineering practice.
>
> Thanks and 73
> Jim W7RY
>
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R L MEASURES, AG6K. 805-386-3734
r at somis.org