[Amps] 10 meter amp coil (Steve Bookout)

[Steve Bookout]

GM John,

Thank you very much for the input/advice.

I was suspicious of that big old cap at 28 MHz, but just didn't know.  i 
was going to check it's capacitance/inductance with the AIM 4170 before 
installing it.  I would be more at home in a 160 meter amp, but don't 
need two of them.

The 7.5 KV caps were going to be added to the 'hot' end of the C1 cap, 
to be in series with C1's 10 - 100 pf capacitance.  They were not going 
to be installed on the DC side of the plate blocking cap.

The end of the cap is a flat disk of silver plated brass with 3 holes 
along the outside edge at 120 degree spacing.  i also have a another 
similar disk with the same size and hole spacing.  I was going to add 
the three caps, in parallel, between the end of the cap and that disk, 
using existing holes.  I was thinking it would give me about 10 to 60 pf 
of adjustment.  Just a bit easier to tweak, while only adding about an 
inch to the C1 length.

I have no ego in this build, so if it's a bad idea. let me know and I'll 
try a different approach.  I do have some various fixed vacuum caps that 
may be useful.  the 3 NPO's were small, available and workable values.

Been following your posts for years and do value your opinion.


With respect and gratitude,

Steve NR4M

On 4/16/2025 1:25 AM, John Lyles wrote:
> I will jump in with two comments that Jim probably also has. That big 
> mica ceramic 0.01 uF cap is likely not going to be a very good plate 
> blocking cap. It is likely useful down at medium wave and 160 but it 
> will probably be inductive reactance and not capacitive at 28 MHz. 
> This would become yet another L in the input to the pi net. You don't 
> even have to use NPO there, as you just want a low capacitive 
> reactance, so as to not generate much RF voltage across it. 200 pF 
> ceramic doorknobs seem skimpy to me, but 1000 pF or twice that is more 
> respectable to have a few ohms of series Xc.
>
> My second comment is not often spoken about, but ceramic doorknobs, 
> even 7.5 kV NPO, also have a characteristic that changes capacitance 
> depending on the DC voltage across them. As long as you don't vary the 
> DC significantly, then you won't need to tweak the variable for Ctune 
> optimization. For best outcome, put a fixed vacuum cap padder there.
>
> 73
>
> John, K5PRO
>
>
>> Message: 1
>> Date: Mon, 14 Apr 2025 13:43:16 -0400
>> From: Steve Bookout <steve at nr4m.com>
>> To: amps at contesting.com
>> Subject: Re: [Amps] 10 meter amp coil
>>
>> 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?
>>
>
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