Looks good Jim. That ferrite choke definitely was too much extra C. I guess you
had a hunk of RTV to clean out.
Question: If the input is so flat, why even have variable caps there? I am
guessing you need to adjust as the tube is not going to be exactly 50 ohms, and
that number is highly dependent on the cathode current when driven or idling
and a little bit dependent on the cathode to ground rf voltage.
Are you certain the 0.01 uF ceramic disk caps (3 kV) are not above their
internal resonance and operating as lossy inductors? When I build power amps I
measure each component with an impedance meter at the RF freq of operation. The
most accurate way is to use a Hp4193A, goes to 110 MHz. They are hard to find
and the probes go for ridiculous amounts. But the older analog Hp4815A will
work too, and they are cheap if you can find one working. They are also
excellent for testing Rf chokes. I think one cap would be around 0.3 ohms of
capacitive reactance at 6 meters, if it is below it's own series resonance. If
you operate close to series resonance, then you don't need multiple caps.
Another good part to consider is the large multilayer ceramic with fat tabs,
made for significant RF current. ATC 100E is one, but they have some other
sizes and their datasheet has a plot of reactance vs freq, so you can see the
series resonance. They have fat tabs for leads so very low inductance. Once I
found that the 820 pF range was perfect short at 200 MHz, and I used one of
them across the filament of an RCA tetrode 4616, 150 KW output, grid driven.
Only one.
Did you plan on some sort of cathode bias device. Sorry if I missed that in
videos. Keep up the fun work!
73
John
K5PRO
On Sep 29, 2022, 10:00 AM, at 10:00 AM, amps-request@contesting.com wrote:
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>Today's Topics:
>
> 1. 10 kw CCS on 6M..using the 3CX-6000A7...Part 6.
> (jim.thom jim.thom@telus.net)
>
>
>----------------------------------------------------------------------
>
>Message: 1
>Date: Wed, 28 Sep 2022 14:49:19 -0700
>From: "jim.thom jim.thom@telus.net" <jim.thom@telus.net>
>To: amps@contesting.com
>Subject: [Amps] 10 kw CCS on 6M..using the 3CX-6000A7...Part 6.
>Message-ID:
> <CAB4ZMgt93AKUM3iimgreS=JrWFuvGtn5C0ogVKRe0LqudgMRoA@mail.gmail.com>
>Content-Type: text/plain; charset="UTF-8"
>
>Here the tuned input is finally installed, and tested. The tuned
>input
>consists of a 'L-PI' network. The 3CX-6000A7
>has a 50 ohm input Z, so the L-PI tuned input was designed in software
>by
>myself, using the GM3SEK PI / PI-L
>spreadsheet.
>
>The 3x6 triode has 42 pf of internal capacitance between the chassis
>grounded control grid...and the directly heated
> ('instant on') cathode. That 42 pf has to be subtracted from the
> calculated C2 value of the tuned input. Problem is, the previously
>installed fil choke assy (5 x oval beads slid over a pair of double
>insulated 4 ga, fine stranded power cables),
>measured 50 pf of stray C, between EACH fil lug..and chassis. With
>tube
>then plugged in, total stray C rises to a whopping 142 pf...which of
>course
>screws things up badly.
>
>The issue is, the fil choke assy was right up against the chassis in
>it's
>previous configuration. My fix was to remove 2 of the 5 x oval beads,
>and
>re-configure the final assy, so it hangs in mid air, between the pair
>of
>100 amp rated Super-con connectors...and the socket. That reduced the
>stray
>C from EACH fil lug to chassis...down to 22 pf. Ok, so 44 pf of stray
>C
>instead of 100 pf. I was expecting less.
>
>With the original fil assy installed, the L-PI would not resonate any
>higher than 28 mhz. With it removed, it would resonate higher, like
>40
>mhz. The 1/8" tubing coil between the pair of air variable caps was
>reduced in diameter, in small increments, and re-tested each time.
>Finally, Scott got it to resonate at 50 mhz. New fil choke assy
>re-installed, and
>L-PI tested yet again. Then again with the bottom lid on. Success at
>last. Flat SWR from 49-56 mhz. Also installed was the
>bypass caps at cold end of new fil choke assy. Also the 6 x .01uf @ 3
>kv
>rated disc caps across the cathode. 3 of those caps are paralleled..and
>ditto for the next 3 x caps. Both those assemblies are wired in
>series..and the entire mess is wired across the cathode socket
>connections.
> Drive RF from the C2 air variable, is applied to the junction point of
>the 6 x caps. This makes a better use of coupling caps, and divides
>the
>drive RF current equally 6 x ways, with 1/2 the total current being
>applied
>to each end of the cathode. This is the correct method of driving any
>GG
>/ cathode driven tube.
>
>The input RJ1A ceramic vac relay was also installed, and RG-400 teflon
>coax was used between vac input relay and the C1 air variable cap.
>More
>RG-400 used from the same vac relay..to the input SO-239 jack. The
>cone
>shaped shroud over the SO-239 input jack eliminates any Z bumps. A
>26.5
>vdc supply was temp used to power the vac relay
>for testing purposes. A temp 50 ohm resistor was wired from one
>cathode
>socket connection..to the chassis...to simulate the tube's input Z.
>
>This entire process took several hours. When you are dealing with
>miniscule amounts of uh (like .15 uh) between the 2 x
>air variables....and .015 uh between the C2 cap.... and the junction
>of
>the 6 x discs.... it becomes a slow, tedious process. The value of
>both
>those inductances greatly affects the network. If the tuned input is
>not
>done correctly, you are dead in the water. The BW is exactly where I
>wanted
>it...and jives with software. Enjoy the 6 min video.
>
>https://www.youtube.com/watch?v=Hr7BPxz0UBc
>
>
>------------------------------
>
>Subject: Digest Footer
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>------------------------------
>
>End of Amps Digest, Vol 237, Issue 22
>*************************************
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