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
On Sep 2, 2023, 10:00 AM, at 10:00 AM, amps-request@contesting.com wrote:
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>Today's Topics:
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> 1. 12 KW CCS ON 160-15M...USING THE 3CX-6000A7... PART 3
> (jim.thom jim.thom@telus.net)
>
>
>----------------------------------------------------------------------
>
>Message: 1
>Date: Sat, 2 Sep 2023 02:43:31 -0700
>From: "jim.thom jim.thom@telus.net" <jim.thom@telus.net>
>To: amps@contesting.com
>Subject: [Amps] 12 KW CCS ON 160-15M...USING THE 3CX-6000A7... PART 3
>Message-ID:
> <CAB4ZMgsWja5EHHmovXL4VZ6wq_Xt9en0FW0tbYR04KEHZKWutQ@mail.gmail.com>
>Content-Type: text/plain; charset="UTF-8"
>
>Here the manually tuned, tuned input is installed. Typ a pair of 4
>section broadcast caps are used for this application...and has been
>used
>several times in the past. (17-540 pf per section, and all 4 x
>sections
>strapped in parallel.....= 68- 2160 pf ). With a pair of 4 x section
>caps
>used, they only have to be padded on 160M. In this case, the 4
>section
>broadcast caps are too long.
>
>Plan B. A pair of 2 x section broadcast caps are used (load cap used
>on a
>SB-220).....and padded on 160M and 80M). Each section of the 2 section
>caps are 15-450 pf...and again, both sections are strapped in parallel.
>The progressively shorting bandswitch is from the kw tank circuit of a
>TL-922 amp.
>
>The bandswitch is used to change taps on the (aprx) 3.5 to 4 uh coil
>seen
>in the video, and also add padding to each 2 x section cap on 80m, and
>160m.
>
>Each air variable gets it's own 6:1 vernier reduction drive assy. A
>small
>2" diameter turns counter drives each 6:1 vernier. Each cap only
>rotates
>180 degs to get from min to max (1/2 rev). With a 6:1 vernier
>installed,
>it then requires 3 full revolutions to get from min to max C. With
>the
>mini turns counter installed, (0-99 increments per rev) it's then 0-300
>tiny increments to get from min to max C. This will provide for the
>ultimate repeatable.... 'dial it up by the numbers'. A simple cheat
>cheat
>is then made up for several parts of each band.
>
>In normal operation, it's just change the bandswitch to the correct
>band,
>and dial up both caps to their respective pre-sets...done. Then the
>input
>SWR is always 1:1 across each band. In the past, when using the same
>manually tuned, tuned inputs, we also have to factor in the tube's
>stray C
>(grid to cathode) as the tube C is directly in parallel with the C2 cap
>of
>the tuned input. We also have to factor in the stray C from each fil
>lug
>to chassis, as there is more stray C at the hot end of the fil choke.
>We
>also have to factor in the length of coax from IPA (intermediate power
>amp)
>to input of the tuned input. We also have to factor in the bias used
>(fully adjustable), drive level, (also adjustable).. and also B+
>used...(can be altered by varying the taps). We also have to factor
>in
>the length of coax from tuned input to the cathode. Ideally, you want
>the
>C2 cap as close to the cathode as possible, esp on upper HF, but that
>is
>not always possible. Typ eff will suffer on 10 + 6m, with 50 ohm coax
>lengths > 1' on 10m, and even less on 6M. Typ 'fixes' include using
>paralleled 50 ohm coaxes ( which effectively adds C to the cathode),
>or
>adding C to the cathode (cathode to chassis)
>
>In the past, we tested the manually tuned, tuned input by applying a
>800
>watt cxr for 10 mins on each band. Done with wattmeter's on both sides
>of
>the tuned input....then into a 50 ohm DL. The 3CX-6000A7 (and also
>the
>smaller 3CX-3000A7) have a 50 ohm input Z. With input swr tweaked for
>1:1
>we get max drive applied to the cathode...and also max grid current.
>
>Trying to design and fabricate 5-9 separate, bandswitched / relay
>switched
>PI tuned inputs....that will each handle 800 watts CCS cxr, is an
>exercise
>in futility. Arco compression trimmers are only good for 200 w
>pep...and
>also have to be padded on 80 +160m. Correct sized toroid's and magnet
>wire
>are a non issue. Been there, done that. If fixed caps are used, they
>have
>to handle the current, and you need a box of em to arrive at the final
>tweak...pita. Then either a 2 x wafer bandswitch is used, or a small
>single wafer bandswitch is used to activate a mess of relays. You end
>up
>with a convoluted, complicated mess on your hands....and even more
>complicated if the tuned input bandswitch is mechanically ganged to the
>10
>kw output bandswitch.
>
>The dead simple, manually tuned, tuned input is far simpler, and bullet
>proof...and fast to design and install. With 800 watts into 50 ohms,
>it's
>only 4 amps into 50 ohms, and only 200 V rms (283 V peak). Those
>broadcast style caps typ have .032" spacing, and rated for 1.1 kv.
>With
>older, dirty ones, toss em into the dishwasher....(with plates
>un-meshed),
>and they come out squeaky clean.....and hi-pot test to an easy 1.5 kv.
>
>We intentionally don't mechanically gang the manually tuned, tuned
>input
>bandswitch with the 10 kw output bandswitch. In normal operation, it
>does
>not take long to change bands, with the 2 x bandswitches and 4 x caps,
>using cheat sheets for both the input and output tuning. (< 1 min from
>15m
>down to 160m).
>
>https://www.youtube.com/watch?v=VqmrX1IMFSM
>
>
>------------------------------
>
>Subject: Digest Footer
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>------------------------------
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>End of Amps Digest, Vol 249, Issue 2
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