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)
Amps mailing list