Hi Jim,
Thanks for your useful input. It's got me thinking about a thing, or two.
I think I'm making some incorrect assumptions with the spreadsheet, as
I'm not getting quite the same #'s as you are. I don't want to make
some stupid error, that'll bite me in the butt later.
I'm seeing that the larger you make the 'pre-inductor', the larger the
value of C1 becomes, and I do understand this is the whole reason for
doing the extra inductor. My question is, what is the down side of 'way
large' in order to get the C1 value much higher? Q or tuned bandwidth, etc?
I had been using a .6 uh inductor. When I changed it to 1.0 uh, the C1
value was significantly higher, which is sooooo useful. My actual mock
up that I had done a couple of years ago, that seemed like it wanted to
tune, was 4 turns, wound something like 1 3/4 dia, with 1/4 inch
tubing. To make it work better for the mechanical lash up using 2 x 200
pf doorknobs and the plate choke, it seems like I should make fit my
physical needs, as a priority. Whether it's .6, .7, .9 or 1.0, I think
an 'even turn' would work out better for me. I always find something
like 4 3/4 turns a pain to hook up and get it looking and working
correctly. Just an OCD thing; I did mechanical prototyping for decades
and the OCD thing surfaced along the way...
Waiting for the copper tubing to arrive...
Steve NR4M
On 4/16/2025 10:05 AM, jim.thom jim.thom@telus.net wrote:
Date: Mon, 14 Apr 2025 13:43:16 -0400
From: Steve Bookout <steve@nr4m.com>
To: amps@contesting.com
Subject: Re: [Amps] 10 meter amp coil
**** I've used GM4SEK's on-line tank circuit calculator in the past and
have been working with it again for this amp.
**** I don't have it all in front of me, as it's all on a computer in
the shack next door, but, as I remember, the #'s were .6 or .65 amps @
4KV and a plate load impedance of something like 3600 ohms.
**** I entered something like .6 uh for the 'stray inductance between
tube and C1' and played with the numbers a bit.? As I recall, with .6 uh
for the first 'L' coil,? I was able to get something like 18-20 pf for
C1.? That is doable!? The other inductor and C2 aren't an issue.
**** The vacuum cap I'm using for C1 is something like 10 to 100 pf @ 15
kv; a Jennings cap.? Because I need so little cap on C1, I plan on
adding 3 - 50 pf NPO 7.5 KV 'small' ceramic doorknob caps.? That should
make the tuning less 'touchy'.? Instead of a tuning range of 10 ish to
100 ish pf, it will give me ~9 to 60 pf.? All my antennas are resonant,
low SWR antennas, so the tuning C1 cap may never be touched again, once
the proper setting is found.? If I can get it tuned with a reasonable Q,
I'm not worried about frequency excursions.? On my 160 meter amp, the C1
vac variable doesn't have a turns counter, or any sort of scale, just
some really old masking tape with some felt tip tick marks.? Only been
like that for 40 years.
The pre-coil and main coil can be one coil, with the C1 cap connected to
the junction point. OR you can use 2 x separate coils. Each of the 2 x
coils can be made from tubing or strap, in any combo u want. IE:
coil-coil. Strap-coil, coil-strap, strap-strap.
****? Trying to do separate coils
I design those networks in software all the time. You need a LCR meter to
tweak the coils / taps though. Lemme know if u need help on this. I need
to know aprx loaded B+ and also loaded plate current.
**** I have an 'almost all digital' meter (AAD) which works well.? I
also have an AIM 4170, which I have used for years and have learned to
rely on, but may ask for your wisdom.
This is no place for 1/4" tubing, too small a diam, and coil will run
hot
on 10m.
The way I deal with heat is...... don't generate it in the 1st place...or
at least minimize it.
**** Amen!
Since it's only used for CW, you can reduce the idle current by a huge
amount, just more bias with a simple string of diodes. You can't reduce
the idle down to zero, or you will get key clicks. Increase bias V, till
idle current is reduced to aprx 20-40 ma. No clicks, tank eff is
improved,
and minimal idle power. I can work that out for 4 kv if u like.
**** That would be great.? My thoughts were to get it to about -20 volts
bias.? I know that -22 volts is into class C, but don't know how far
into it, it is.? I have a good stash of 1n4007 which would be fine, but
also have some others as well.
**** Do have a separate question for you.? I'm just ignorant on the
application specifics.? DC blocking cap between the plate choke and the
tank circuit.? When is capacitive reactance TOO small?? I have several
200 pf, 15 KV doorknobs.? If I put two in parallel for current,? that
gives me 400 pf for a cap reactance of about 14 ohms.? Is that good
enough at 28 Mhz??? 'X' amount of RF current thru 14 ohms gives "ZZ'
watts of heat.? Don't want heat. Then, again, I have an 'old school'
antique, boat anchor ceramic/mica HUGE transmitting cap (like 2 x 3 x 3
inch high) .01 mf @ 5000 volts.? That gives me less than an ohm of cap
reactance at 28 MHz and eliminates the need for a ceramic or fiberglass
standoff to support the 2 x 200 pf caps..? This big cap is its' own
standoff.? It's labeled for several amp of RF current at some low freq.?
My 160 amp has used an identical one for a blocking cap for 4 decades,
but there is a world of difference between 160 and 10.? Not being
familiar with any specs, or history associated with this type of
ceramic/mica cap, it may work fine,? or maybe the worst thing I could
do.? May have been from some commercial AM transmitter from 80 years ago.
?Comments?
Steve NR4M
## use .8 to 1.0 uh for the pre coil. Then it's 20/30 pf for the C1
tune cap. Then u don't have to add the 7.5 kv doorknob caps. It will tune
just fine.
## Make sure the pre coil and main coil are BOTH wound CW...... or BOTH
wound CCW
## Use something bigger, like 1N5408 or 6A10 / PM-600 for bias diodes.
Per constant current charts, with 4 kv, you need 16 vdc of bias, for 50 ma
of idle current.
That will require 22 x diodes. Leave room for more if needed, like another
3 x diodes.
## Use 2 x 200 pf @ 15 kv HT-57 doorknobs for plate block cap assy.
Those HEC/ centralab 857 caps are rated for 15 amps CCS on any freq > 5.11
mhz. So a pair is good for 30 amps, which is ample. Those are 35 kva
rated caps. Current through the block cap assy is aprx 4.2 amps. 4.2 amps
X 14 ohms xc = 59 vrms = 84 V peak. That 84 V peak AC Vdrop across the
cap plus the 4 kv B+ ( what HEC calls DC bias is what we call B+) = 4084
V peak. HEC sez the peak V across the cap plus any 'dc bias' can not
exceed the stamped HV rating of the cap.
## The tubes anode to grid C is 10 pf = 566 ohms of xc @ 28 mhz. You take
the 4 kv B+ and multiply it by .6 to obtain the RF RMS voltage ( = 2400
vrms) . 2400 vrms / 455 ohms = 4.2 amps. Think of the tubes anode to
grid C as being a major portion of the total C1 required tune C. Tube C
and C1 are in parallel. Problem is u have the plate block cap between em.
That 4.2 amps also has to flow through the block cap assy.
This is why block caps have to handle more current on 10m...vs 160m. XC
from the tube C is sky high ( and 16 times greater ) on 160m. RF current
is minimal, and minimal through the block assy. However, typ we want a
low enough XC on the block assy, such that the xc of the block assy is no
more than 5% of the plate load Z.
## The block cap has to be big enough in value to meet the 5% of plate load
Z rule.
At the high freq end, we have to worry about current. In your case,
it's a monoband 10m amp, so cap value is a non issue, esp with it's 14
ohms of xc vs the 3400-3600 ohm plate load Z of the tube.
## Don't worry about the Xc of the plate block cap assy ever being too low.
## any current through the plate block assy is watt less VA power for the
most part, but u can't exceed the RF current rating of the cap.
## strap the pair of doorknob caps such that the current is divided 50-50
between them. This implies that the distance in = distance out. IE: 2
x caps in parallel, with their main axis N-S. One strap comes from the
W side ( tube side) .....and cover's the tops of both caps. Output strap
comes from the E side..and covers the bottom of both caps.
## Don't use that giant mica / broadcast cap. They handle max rated
current at low freqs like 1 mhz....... and less on higher freqs like 10m.
Ceramic caps are the opposite.
It would probably work, but is not optimum. The HT-57 cap in the 200 pf
value is the 'go-to' cap in the HT-57 series, since it handles the most
current.
Jim VE7RF
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