Date: Mon, 04 Sep 2023 00:02:48 -0600
From: John Lyles <jtml@losalamos.com>
To: amps@contesting.com
Subject: Re: [Amps] 12 KW CCS ON 160-15M...USING THE 3CX-6000A7...
PART 3
<Beware that the black epoxied Mica cylindrical capacitors are largely
intended for broadcast band <use, found in transmitters and antenna tuning
units (ATU). They also can work in induction <heating generators. But
having swept them using vector impedance meters, at about 5-10 MHz i <found
many have their first internal resonance and become lossy inductors above
that freq. <Ceramic dielectric caps, NPO doorknobs for example, many in
parallel, are better for HF.
<73
<John
<K5PRO
Points well taken. Those huge 2000 pf Sangamo caps will only be used to
pad the 2 x section air variables on 160M only. They are mica, and have a
NPO TC. They are overkill for this application, but it just saved buying
yet more HT-50's. It's too bad the cap manufacturers don't publish
where each value and type of cap has it's 1st internal resonance.
He ordered 4 x 600 pf caps in HT-50 style. (The 500/600/700 pf versions
are all N5250 TC).
2 x 600 pf caps will be used to pad each air cap on 80m. The
progressively shorting 2 x wafer bandswitch will be used to not only change
taps on the new 4 uh coil (made from 1/8" copper tubing), it will also pad
each air cap with a pair of 600 pf caps on 80m...... and an additional
2000 pf per air cap (total of 2000+600+600 = 3200 pf) on 160m.
Those 600 pf caps are rated at 7.5 amps CCS on any freq > 1.49 mhz.
With a TC of N5250 ppm (per deg C temp rise), that's 5250 / 1,000,000.00 =
.00525
.00525 X 600 pf = 3.15 pf drop in C, for every 1 deg C rise in temp.
(negative coefficient)
With 800 watts max drive applied, that's 4 amps into 50 ohms. It's also
200 vrms.
600 pf = 144 ohms of XC @ 1.840 mhz. (72 ohms @ 3.680 mhz)
On 160m it's 200 Vrms / 144 ohms xc = 1.39 amps flowing through each of
the 600 pf caps. The 7.5 amp rating is for a 30 deg C temp rise over a max
ambient air temp of 25C.
On 80m, it's 200 vrms / 72 ohms xc = 2.78 amps.
1.39 amps / 7.5 rating = cap run at 18.53% of it's max rating on 160m...(
and 37% on 75m). In the past, these 500/600 pf caps have never been an
issue for drifting. They might rise by 5 deg C = 15 pf drop in C.
BUT the cold ends are machine screwed to an ice cold chassis. The hot ends
are all bonded with copper straps, so tops are also heat sinked. Based
on past experience, drift was never an issue.
Ideally, I try and use the biggest air cap possible, so it's only padded on
160m. In this case, the 4 x section air caps are too long to fit, so the
shorter 2 x section types will be used.
Jim VE7RF
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