[Amps] 10 meter amp coil

[Steve Bookout]

Hi Jim,

I'm sending this to the list, instead of just you, as it's never too 
late for anyone to learn.

I'll stick my comments where appropriate.

On 4/14/2025 10:43 AM, jim.thom jim.thom at telus.net wrote:
> Subject: [Amps] 10 meter amp coil
>
> Hello all,
>
> I'm building a 10 meter mono band amp for my m/m contest station here in
> VA.
>
> What is the group consensus on what diameter the conductor for the plate
> tank circuit?? I tried looking in the archives, but it's not the best
> for searching (it's monthly).
>
> I'm thinking 3/8 inch should be fine.? Half inch is starting to get
> 'stupid big', IMO.
>
> Will be an 8877 @ 4kv.? Class of operation: not sure of yet.? 12 volt
> bias is probably fine.? I ONLY run CW.
>
> And, looking for a filament transformer for the 8877.? Anyone have one
> that is not in use?
>
> 73 de Steve NR4M
>
>
> Use at least   3/8" copper tubing.  That, or  5/8" to 3/4"  wide copper
> strap, wound into a coil, called a strap coil.
**** I went ahead and ordered some 3/8 in copper.  Easier to find than 
copper strap.
> Since it's a 8877, no parasitic suppressor is required.
**** I've built with an 8877 before and used a parasitic suppressor, but 
what I've read follows what you stated, that a suppressor is not 
needed.  Methods and procedures change.  Like all the caps and resistors 
in parallel with PS diodes. Used to be the norm.
> Ok, now  you can insert a small, fractional uh coil, between the plate
> block cap...and the C1 tune cap.   The tube's  anode to grid C (+ any
> stray's) and the extra 'pre-coil', will form a step down L network.    This
> will transform the plate load Z   down to a lower value, low enough that a
> practical PI network, with a lower tank Q can be used.

**** 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
>
> Jim   VE7RF
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