[Amps] placement of RF choke bypass cap.

[Carl]

> 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


Bypass caps are cheap and good insurance.

An interesting concept I saw in a commercial amp was each tube fed with its 
own plate choke with ends right at the anode cap. The B+ feed was on a small 
ceramic standoff attached to a roughly 3/4" diameter aluminum spacer to 
ground. The bypass cap was across the ceramic insulator. The tubes were 
4CX250's so all lengths were quite small.

Carl
KM1H