[Amps] 500 uh plate choke

Jim Thomson jim.thom at telus.net
Fri Jan 25 06:32:34 EST 2013

Date: Thu, 24 Jan 2013 05:34:28 -0700
From: John Lyles <jtml at losalamos.com>
To: amps at contesting.com
Subject: [Amps] 500 uh plate choke

K5UJ posted a really great idea to test an RF Choke for resonances, 
without needing even a GDO, (although every ham should still have one of 
these - in addition to an Almost All Digital L/C IIB meter!). Hams come 
up with the greatest ideas for simple methods of doing complex 
measurements with little expense, this forum remains the best!


##  W8JI has a similar procedure on his website.  Coax from xcvr to dummy load.
T connector between xcvr = dummy load.  12 vdc @ 100 ma light bulb in series with
hot side of coax coming off the T.... then to the top of the choke.  Bottom of choke bonded to

## Crank just a few watts out... just enough till the 12 vdc bulb barely starts to glow.  Then spin the
vfo from 1.8 to 30 mhz (  xcvr has to be modified for gen coverage TX).  When you hit a series resonance 
point, the bulb will get really bright.  Then sweep on either side till it starts to dim again... and note the upper
and lower freq points.   Then continue with moving the vfo higher in freq till you find the next series resonant point, etc,
till you get to 30 mhz, or the upper freq limit of your xcvr. 

##  For those folks who want to wind a single layer solenoid, like the typ  22 ga on a 1 inch diam solid teflon rod etc,
and require gaps to be installed in the choke, JI offers the following procedure.  Find the center 1st series resonant point as
described in the above paragraph.  Then run a small screwdriver up the side of the choke, with blade parallel to chassis, and
edge of screwdriver touching the choke.    start at the base of the choke and work slowly up the side. 
When the 12 vdc bulb goes from brightly lit.... to completely out, you have just found the problem area on the choke. 
Mark it with felt pen.  Repeat the same procedure for the higher  freq series resonance points.  The choke is then rewound,
but with gaps inserted at all the problem areas. 

##  That is how W8JI  designed the gapped plate choke used in the Ameritron amps.  His method of finding series resonant points
on any choke works good.  I have not had time to try the gapping procedure..yet.  The ready made Ameritron choke (also sold by RF parts)
is plenty good enough for most amps.   A few of us are planning to try the gapping procedure... to make a 22 ga version of the ameritron
gapped choke,  for use in higher plate current applications.

## as far as choke uh goes,  IMO, I don’t think it is that critical.   We tried  a 2 piece choke setup ( chokes at right angles), compromising
of a 180 uh choke ( 22 ga) in series with a 2nd smaller 45 uh choke ( 18 ga wire).  Also tried was 135 uh in series with  45 uh. 
3-5 x 1 meg @ 3 watt mof’s in series  from junction of chokes to chassis. 2 x 500 pf @ 15 kv HEC  HT-57’s  from junction point to one
side of a spst relay. other side of relay goes to chassis.   3 x parallel 4700 pf @ 15 kv disc ceramics for bypassing, at base of large choke. 
Relay just RF grounds the cold end of the 2 doorknobs on 80-10m.  The 3-5 megs is to ensure the doorknobs are actually in dc block mode.

## Both chokes used on 160m only. Small choke used on 80-10m.  Config was then changed to 135 uh + 70 uh.  The 45 uh, 18 ga winding was
removed and replaced with a 70 uh winding, with 22 ga wire, so it fit the same 1”  teflon rod just fine. 

##  This is for a 160-15m  3CX-6000A7 amp.  Funny thing is, the big choke was out of the rf deck for several days..leaving just the original 45uh
choke in place.  Other than the Tune C having to be increased, the amp ran just fine on 160m..with just the 45 uh choke ( wound with 18 ga). 

##  My theory is, although the XL of a 45-200 uh choke  is not that high on 160m, the Z is a lot higher.  One would have thought that the poor 45uh
choke would have burned up on 160m....and ditto with the pair of 500 pf HT-57 doorknobs.  Neither of them flinched..for several nights in a row,
with 12 kw pep out.  Eff was superb too, excellent.    My theory is all you are really doing is parallel resonating the plate choke, since it’s
directly in parallel with the tune cap.  Rich measures did the same sorta thing on his heath sb-220, 160m conversion.  He parallel resonated the
oem 9uh fil choke on 160m by simply paralleling another 900 pf  with the C2 cap of the new 160m Pi tuned input.  His idea was not to have to use a larger
fil choke on 160m, like other 160m conversions. 

later... Jim   VE7RF  

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