On Jul 15, 2006, at 8:28 AM, Gudguyham@aol.com wrote:
> In a message dated 7/15/2006 8:47:32 AM Eastern Daylight Time,
> r@somis.org writes:
> Lou == The SB-220 uses a current-limiting type filament transformer
> that allows no more than 60% of the 3-500Z's rated filament inrush
> current to flow, so step-starting the filament is unnecessary.
> However, the inrush current to the HV transformer is such that it
> will eventually erode the contacts of the ON/OFF switch, so a step-
> start is still useful.
> end
> Yes, I thought that filament transformer was designed that way, but
> if confronted with the question on a YES or NO basis, Would it be
> best to cycle the filaments or leave them on during a session on
> the air?
Lou -- Since filament inrush current is low by design in an SB-220,
there is no problem with switching the filaments off when not in use,
nor would there be a problem with leaving them on since the filament
potential is not such that it appreciably shortens the life of the
tungsten-dicarbide layer. As I see it, a standby switch in an SB-220
semi-protects the ON/OFF switch inrush-current, not the filament.
What the SB-220 can benefit from is something that does a good job of
limiting inrush current through the ON/OFF switch -- i. e., a step-
start.
> BTW the Ameritron manual states that the filament transformer is
> designed with "impedance" that essentially step starts the filament.
As I recall, that unit supposedly doesn't need a glitch R, and its
3-500Z needs to be tuned up by a method that is not supported by any
3-500 mfg., and one which makes no sense to this operator. Thus, my
advice is to measure the inrush current yo'self with a calibrated
oscilloscope. To me, the shocking thing is that people tend to take
the word of others instead of checking things out for themselves. An
example is the number of amateur radio magazine articles that were
written because of the now-laughable assumption that a SB-220 really
needs a standby switch.
end, 4:43am, 7/16/06
> Lou
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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