> At 10:41 AM 4/28/97 +0000, you wrote:
> >Be sure you apply that current rating correctly. On lower
> >frequencies current is determined by the I*X voltage drop
> >across the component and the voltage drop across the component. On
> >160, you are well within the rating of the component. The 1.6 amperes
> >does NOT indicate the capacitor is close to HEAT failure from
> >excessive current, al low frequencies the current rating indicates
> >the point of arcing from VOLTAGE failure.
>
> The 1.6 ampere rating @ 1 mhz is the maximum allowable current to maintain
> the N750 temperature coefficient rating. Drift goes to pot above this #.
The Heath plate padding capacitor is rated at 3.9 amperes at 1 MHz
(page 4 of HEC's catalog), that's 936 ohms and the peak voltage is
about 5100 volts.
All low value HT series caps are voltage limited, not current
limited, on low frequencies. I never knew Heath to use a HH series
cap, since the X5 drift was too high. The problem is NOT the maximum
temperature rating for the N750 drift rating. The N750 indicates
parts per million drift per degree C rise. The rating of X5 type
dielectrics is due to maximum heat rating before failure, and tied
into the poor dissipation factor of the lower grade ceramic.
By the way, you can not get a 50 or 58 series capacitor above 199 pF
in the N-750 rating, they are all 3300 ppm. The current rating of the
Heath (and Ameritron) doorknob in the loading padding cap is 3.8
amperes at 2 MHz.
I have no idea where the 1.6 amperes comes from, is it for a 500 pF
X5 TC series??
> The 1000 pf doorknob in the Alpha 77DX worked fine. In the SX, you need
> to replace it with 2-500 pf units to stop the drift problem on 160m.
I would be amazed any 1000 pF doorknob did not drift in a loading
cap. The AL-1500 and such had to use three 500's in parallel with a
170 and 800 air variable on 160, and when HEC started using ITT's old
material drift crept back in!!
The problem is traceable to ITT's (and HEC's recent material) having
a higher than normal dissipation factor, even though the TC is still
N-3300 or 4700.
> >Drift problems are a different issue. Drift problems have come and
> >gone with ceramic doorknobs. HEC's capacitors were originally quite
> >stable, while ITT Jennings caps were very poor. When HEC bought out
> >ITT, HEC's stability went to hell.
>
> I hope they get their act together soon, as they are just about the only
> game left in town! Prices, by the way, are through the roof!
Ceramic chips are the way to go now. You can get multi-layer NPO
type chips that have very low dissipation factors for about $5 each.
The voltage rating is about 3 kV, and they work very well if the are
not contaminated during manufacturing.
I never use ceramic caps of any type in large PA's (5 kW up), I
prefer large silver mica's.
> I agree totally, but show me ONE article in any Bill Orr Handbook, or
> ARRL handbook that specifies a shorting type band switch! I'll save you
> the trouble, there are none. This is a costly oversight!
>
> >On a 80 through ten meter tank, the designer might get by without
> >shorting unused turns, but he likely won't get away with it on a
> >160 through ten PA.
>
> I believe I stated that...I am here to tell everyone you will not get
> away with it in most cases.
Oh, we are promoting the same side....hi. Sorry.
> Pi-L's are very arc prone if you do not short the unused turns. If you
> short them, no problem! Build your amp from a schematic from Alpha,
> Ameritron, Command, or QSK; not Bill Orr or ARRL, and you will do fine!
Pi's are also a problem when unused turns are not shorted!! I've
arced an RSC 25 kV bandswitch with only 5 kW and a anode voltage of 5
kV in a properly loaded pi.
Letting taps hang loose is asking for switch failures no matter what
network you are using. It's really not much different than inserting
a bullet, rolling the barrel and going click.
73 Tom
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