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Re: [Amps] teeny copper strap

To: Ian White GM3SEK <>,
Subject: Re: [Amps] teeny copper strap
From: Pete Smith <>
Date: Wed, 07 Jun 2006 10:09:26 -0400
List-post: <>
At 09:10 AM 6/7/2006, Ian White GM3SEK wrote:
>Pete Smith wrote:
>>Before I mail order some copper flashing or go to one of the lightning 
>>protection outfits for far more than I'll need, is there any non-radio source 
>>of appropriate stuff for grounding my SB-220 grids?  I think that stained 
>>glass makers use copper foil strips, but assume that is probably too thin for 
>>what I need.
>It's certainly thick enough to carry the RF current. Thanks to the skin 
>effect, even very thin copper foil in a 5mm (3/16in) width will carry 1500W at 
>HF without getting warm (that's 5.5A into 50 ohms).
>If it's also wide enough to provide a good low-inductance ground connection, 
>it will handle any DC current as well - up to and including the large current 
>surge from a you-know-what :-)
>As  part of the general upgrade, also consider installing:
>*  A 30-ohm 25W glitch resistor in the B+ line (choose a long, thin vitreous 
>wirewound resistor to handle any high voltages)

The length is a problem with the SB-220's layout - I had a short one in there 
but a couple of arcs later....

>* A step-start circuit, because it allows you to change the mains fuse to a 
>fast, low-rated type that will protect the whole amplifier better.

Interesting idea - I thought the CW about the SB-220 was that it had a 
transformer that was designed to provide a slow start, so that a step-start was 
really not required.  In any case, the amp has a breaker, so it would be a 
little more involved.

Thanks for the calcs, Ian - that makes my choices pretty wide open.  And thanks 
to many for their suggestions, offers to send copper sheet, etc.  I have 
several leads for local sources now, so won't need to trouble you.

73, Pete N4ZR

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