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Re: [Amps] teeny copper strap

Subject: Re: [Amps] teeny copper strap
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Wed, 7 Jun 2006 17:55:31 +0100
List-post: <>
Pete Smith wrote:
>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....

Most amps have enough space to mount two long resistors in a "V" 
configuration, so the two ends are a short distance apart while the 
mid-point is tucked away somewhere safe.

>>* 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 
>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.

The SB-220 has a separate heater transformer that will limit the 
switch-on surge through the tube filaments; but it can still use a 
step-start to reduce the current surge through the HV transformer and 

What people tend to forget after installing a step-start is that they 
can also change over to a more sensitive fuse/breaker that will protect 
the amplifier better. If they don't do that, they're missing half the 

>In any case, the amp has a breaker, so it would be a little more 
See your point, but there's no reason not to use a fuse as well.

>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

73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)

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