[Amps] placement of RF choke bypass cap.

[Jim Thomson]

Date: Tue, 16 Mar 2010 10:55:30 -0700
From: "Bill, W6WRT" <dezrat1242 at yahoo.com>
Subject: Re: [Amps] Placement of plate r-f choke bypass capacitor
ORIGINAL MESSAGE:

On Mon, 15 Mar 2010 21:23:05 -0400, "David J Windisch"
<davidjw at cinci.rr.com> wrote:

>The h-f amplifier designs with which I'm familiar usually have the plate r-f choke bypass capacitor placed as close as possible to the cold end of the choke.

REPLY:

You also want the cap as close as possible to the (grounded) grid of
the tube, which is the common return point for the RF being bypassed. 

##  Normally, the plate choke is next to the tube anyway.  Normal deal
is to install the bypass cap at the base of the plate choke.

##  I would install a 2nd set of bypass caps, right at the either the B+
connector, if the B+  supply is a separate box... or in the case of the
ten-tec.... 11" inchs away from the base of plate choke.    You really need bypass
caps in both places.  Reason is.. with so much RF flying about,  you could easily
get RF on the cable, well down stream  from the plate choke.  Drake uses a bypass
cap right at the Red HV millen connector om rear panel of the L4B... and a 2nd
one at the bae of the plate choke. 

## what's  really needed, is a pair of  4700 pf  discs  at the base of the plate choke,
and also  1-2  x 500-1000 pf  ceramic doorknobs..... and repeat  down stream.
I'd also put a 3rd mess of bypass caps in any outboard power supply.. that has 
Hv lytics in it.     The pair of 4700 pf  caps make a great bypass cap on the low bands,
and the 1-2 x 500-1000 pf doorknobs makes  for a superb bypass cap on the higher freqs. 






Having the ground point 11 inches away is asking for trouble. That
means that 11 inches of the chassis is "hot" for RF. Ten-Tec is
apparently getting away with it, but that's a risky business.

##  How do u know there will be  RF  flowing on the underside
of the chassis ?? ... asuming it has a ..'normal' chassis .
If the grid trip is below the chassis, and also uses rf bypass caps,
it should not be a problem. ... at least on paper. 

Jim  VE7RF




Where this might cause trouble for example, would be if the grid-trip
circuit was mounted somewhere in the 11 inches, or if the input
circuit had a ground connection there. Not good.

73, Bill W6WRT