Hi Carl,
km1h@juno.com wrote:
> A. The 381LX capacitor has more than adequate ratings for the average
> commercially marketed ham amp. Plus, the ratings are for 105 degrees
> C, not 85 as with the CGS.
The catalog I have lists them both at the same temperature
range. Type LP has a higher temperature range, but much more leakage
current than the CGS or LX series.
The maximum temperature range is important if the capacitors in a
unventilated or high temperature location. That isn't a wise choice, but
it may be unavoidable in special situations.
> That higher temperature rating will more than
> compensate for the ESR and in a real world amp supply it will not be
> running anywhere near those ratings.
Untrue. The ESR of the LX series is over twice as high, and so power
dissipated will be more than twice as great for a given amount of ripple
current.
Over twice the heat energy must be transferred to air over a surface
area of about eight square inches in the small component, while the CGS
capacitor (with less than half the heat energy) has about 20 square
inches of surface area.
Contrary to what you seem to understand, surface area is important in
the transfer of heat. The CGS not only generates less heat, it gets rid
of it over a larger surface area.
Not only that, jamming the parts in a small area adds to thermal
problems by blocking air-flow.
> B. Why did MFJ/ Ameritron take such a cheap shot in their ad (April 97
> QST, pg 14) against modern capacitors.
I don't write or approve ads, I haven't read it, and I don't like
personal opinions or personal arguments or allegations in technical
forums.
> In the AL-811 no less! I could grant
> them a point in the AL-1500 maybe, but not in that toy amp.
The capacitors in the 811 are specifically ordered
100% tested at 525 volts for 24 hours with limits on allowable leakage
and
ESR. The capacitor manufacturer calls that "aging" the capacitor.
You probably weren't aware the very same capacitors go in the AL-1500.
> C. I'll repeat myself....the CDE 381LX, from my experience, has better
> production tolerances compared to the CGS permitting higher value
> equalizing resistors.
According to my catalog, the LX is specified with greater drift with age
and a wider leakage current tolerance from the start.
> After all, was it not you Tom, who claimed at
> the start of this subject that you had to use low value resistors?
No. I simply suggested the best idea is to use the lowest value bleeder
and
equalizer practical. The actual minimum value required is determined
through an engineering process that requires more than a wild guess.
No matter how small the equalizer required, I'd likely still use the
largest resistor pratical. I use two 200 watt 100k resistors in series
in my 7.5 kV supply. It only "need" a few meg ohms.
What's an extra few watts of heat in a PA that generates kilowatts of
heat anyway? It hurts nothing at all, unless the layout is so poor there
is no ventilation. If the layout is that bad, the layout needs changed
before worrying about the resistors.
73 Tom
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