> From: "Bill, W6WRT" <dezrat1242@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@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
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