On Jun 7, 2006, at 7:09 AM, 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
>> 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....
Tune-C arcs are good news compared to bandswitch arcs.
>> * 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.
The fil xfmr is current limited. The HV xfmr is not because if it
were current limited, the HV regulation would be worse than terrible.
> In any case, the amp has a breaker, so it would be a little more
> 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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