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[AMPS] Plate RFC choke design

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Subject: [AMPS] Plate RFC choke design
From: (Richard W. Ehrhorn)
Date: Fri, 27 Mar 1998 17:37:20 -0700
Hi Jon & everyone...

A big caveat on the two choke approach: contrary to what seems intuitively 
correct at a glance, two chokes in series - even if at right angles as they 
should be - do NOT exhibit independent series and parallel resonances. 
Careful investigation (easiest with an old HP analog vector impedance 
meter), as well as more thought, reveals that if one choke end-to-end is, 
for example, +j1000 ohms (inductance) at a given frequency and the other is 
-j1000 ohms (capacitive), the pair will will be series resonant with a 
total Z of R + j0, where R is ~1000/Q and can be VERY low.

OTOH, if you are switching between one choke for higher f's and the two in 
series for lower f's,  and if they are at right angles to each other, it is 
quite a lot easier to handle the resultant parasitic resonances than it is 
when just tapping a single choke. The problem of making one, or even a 
two-choke combo, without any low-Z (say <1-1.5k) spots near any HF band is 
not very easy to resolve even with instrumentation. I've found that when 
using two series chokes switched, it's very desirable to SHORT out the 
unused choke, which is nearly always the larger one used only on lower f's. 
Even then, it will exhibit a full ensemble of series and parallel internal 
resonances which can cause havoc unless it is thoroughly isolated - both 
capacitively and magnetically - from fields of the other choke, tank coils, 
and rf wiring.

Unless one is very lucky or wants to make a mini-career of it, the easiest 
approach is to copy exactly a choke system used in a well-proven design.

Good luck,    Dick   W0ID

----Original Message-----
From:   Jon Ogden []
Sent:   Thursday, March 26, 1998 9:33 PM
Subject:        Re: [AMPS] Plate RFC choke design

>Does anybody have a proven design for a plate choke?
>I'm building a 160-10 (no WARC) amp using a pair of 3CX800A7 tubes, (1.5A 
>Ip).  I've seen lots of plate RFC designs that use anywhere from 80 uH
>(Ten Tec
>Titan), and on up.
>All the homebrew designers treat winding a choke as a black art.  I've got
>various lengths of 1" and 1.25" Delrin rod to work with, and would like to 
>#20 wire.  It can't be that hard, can it?


Plate chokes are an art as I have learned.  The biggest problem is the
doggone resonances.  My PA had a Hammond 1521 in it and I couldn't figure
out why it kept blowing up on 10m.  Well, after coupling a dip meter to
it, I found the choke to be series resonant at 29 MHz!  If you have room,
the ARRL handbook (at least my 1989 version) has  a really nice example
of a choke in it in one of the chapters.   It's not in the PA chapter.
The choke is really long and has several sections to it all with
different amounts of windings.  I think it is something like a foot long.
 The different windings effectively act as inductances in series and help
to break up the resonances.

A lot of literature says to pick a resonant spot somewhere that's not in
a ham band, like 11 MHz or 17 MHz.  These are problems for WARC bands,
but you aren't doing WARC.  After it goes into resonances, the choke will
then begin to look inductive again and present a larger and larger
impedance as you go up in frequency until you reach a peak and then you
start going down again.  Go high enough and you'll reach another
resonance and so on.  The problem is that each succeeding peak after a
resonance is a smaller maximum impedance.

I wanted to avoid any kind of resonance in the ham bands.  So I wound a
choke on a ceramic form from an old Johnson Viking Invader.  It's about
3/4" in diameter and has 6" of winding space.  It's also grooved to help
winding.  I used #22 AWG wire.  I made this choke to be resonant around
34 MHz (about 105 windings).  Then I tried it in the amp.  Good
peformance on 10m, BAD on 80m.  Problem:  Impedance at 80m was too low
(about maybe 1 KOhm).  I had another identical ceramic form and so I
wound a second choke with about 115 windings or so.  I put the two chokes
in series and tried everything out.  80 meters now worked fine (about 2.5
KOhms now).  I have no resonances anywhere from 3.5 to 30 MHz.

I would have liked to put a few more windings on the second form and
boost the impedance up at low frequencies, but frankly, I ran out of room
on the form!  You probably could get by with 26 ga wire, but I wanted to
use 22 ga just to be safe.

Your biggest problem is that you want to go all the way to 1.6 MHz.  I
would definitely use 2 coils in series.  You might wish to check out Rich
Measures' (AG6K) home page (  Regardless of your
views on his parasitic theories, Rich has some very good information on
adding 160 meters to amplifiers.

Personally, I would use a two choke system and try to wind the second
choke with a large number of windings.  Who cares if it is resonant in a
ham band, the first choke (resonant about 34 or 35 MHz) will still
function properly.  Try to get a large diameter core (>1") as the
inductance is related to the square of the diameter.  You'll get much
more inductance with 3 inches of windings on a 1.5" core than with 3" of
windings on a .75" core.

I hope this helps.  People might think the two choke approach is goofy,
but let me tell you, it works.



Jon Ogden

"A life lived in fear is a life half lived."

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