Date: Fri, 2 Dec 2011 09:07:32 -0000
From: "Iain" <iain@g4dpf.co.uk>
Subject: [Amps] 2 x 3-500Z filament choke
Hi All
I am gathering the parts to build a two valve 3-500Z linear amp.
All the suggestions for the filament choke seem to be to purchase from RF
parts or surplus sales in the USA.
I haven't been able to find any info on winding a homebrew one.
Is there something really special about the core or will any ferrite rod do
?
Any info on wire size and the number of turns would be very useful
Thanks Iain G4DPF
## I bought just the 8" long x 1/2" diam rods [ type 43] from SSON. They
were
wound with 10 ga polyimide magnet wire. SSON also had the plastic, 1/2" ID
clamp down nylon ends, that slide over the ends of the rod..and pinch it tight.
The nylon clamps ate then machine screwed to a stand off. 10 ga magnet wire is
required to handle the 30A CCS of current. Watch out, even with solid
connections,
you will still get a V drop across em.
## Those rods are brittle. The 10 ga solid magnet wire is stiff, and tough to
work with.
The procedure is to BIFILAR wind both windings at the same time...onto some
1/2" OD
Aluminum tubing, with the AL tubing firmly clamped into the vise. Use a
12-16" long
AL tube. As you get 1/2 to 3/4 of a turn onto the tubing, simply grab the
starting end, and rotate
it 180 degs, then wind another 1/2 turn onto the rod, then rotate the windings
backwards
another 180 deg, etc. Leave loads of slack/pigtails at both ends !!
## Using 8" long AL tubes [1/2" OD], slide the finished windings off the
12-16" tube,
and on to the temp 8" AL tube, complete with nylon clamps + standoffs on
each end.
Then mount the standoffs into the underside of the RF deck. OK, then form
each of the
4 x wires very carefully. Then cut off all excess wire, and also allow enough
length to
install the crimped lugs, or machine screw box lugs, or solder em to the socket
at socket
end, and terminate the 'cold' ends to some type of cross connect terminal
strip. Of course,
the low V wires from the fil xfmr will also have to terminate to the same
terminal strip.
You will also have to install a .01uf @ 1 kv disc ceramic cap from each
terminal on the same strip,
to chassis. [ 2 x caps required, one per leg]. The caps will kill any
residual RF that gets past the
chokes.
## Ok, now you can test the tubes out, and check for V drops in all the
wiring. When that's done,
remove the entire mess, and remove the AL tubes....and THEN replace with the
8" long ferrite rods.
Then re-install the assy. The reason the AL temp tubes are used is so there
is NO chance of breaking
the brittle rod while forming in the stiff 10 ga wires, etc. IF using lugs,
or crimp lugs at the terminal strip,
they have to be installed such that you align them properly [rotated into
proper position]. You don't want
to have to twist the lug, in order for the hole ion the lug to line up with the
machine screw in the terminal
block. It should all just drop in.
## Mine ended up being aprx 52uh per winding. Forget how many turns but 10
ga magnet wire is aprx
.108" Those nylon clamps on each end will eat up exactly 1/2" .... so the
useable portion of the rod is
7". When I measured the SSON type 43 rods, they were exactly 7.875" long [
7 + 7/8"]
7" / .108" = 64 turns of wire required, [ 32 PER each winding] .5" x 3.14"
= 1.57" per winding.
1.57" x 64 turns = 100" wire required in total [ 50" per winding] Then also
allow 6-10" for each of the
4 x pigtails so it's really 124-140" wire required.
## You can also use 1/2" wood dowelling for the former, etc, but make damn
sure the 1/2" dowelling is
actually .5" OD..and not < .5" To be safe, measure the OD of the Ferrite
rod, and also any AL/wood
former you use. My completed BIfilar winding simply slid off the 14" AL
tube...and onto the 8" AL tube
[both from the same scrap piece of 1/2" AL tubing, some scrap ele tip off an
old wilson yagi].
The assy also slid onto the ferrite rod easily. You don't require the
expensive polyimide 10 ga magnet
wire either [15 kv rated expensive stuff], any brand of magnet wire will do.
Check for shorts between windings,
nicks, uh [ if u have a LCR meter], before you slam a 30A load on it.
## I have made several of these BIFILAR fil chokes, using the above method. We
have also done the same choke,
but using bigger 8 ga magnet wire, [ .134"] in which case the inductance will
drop down to aprx 30 uh per
winding.
## for folks who require a 60A or 80A fil choke, you parallel 2 x of the
above choke assy's. For a 3CX-3000A7
fil [ 50A], 2 of the 10 ga Bifilar choke assy's are built. Then any one
winding of choke 'A' is paralleled with
any one winding of choke 'B' ..and vice versa. This ensures opposing current
flow in each rod assy = zero rod
saturation. With opposing current flow [ just like a regular bifilar], the 60
hz fields in each winding will cancel out.
Saturation is not caused by drive RF. For a 3CX-6000A7 [79A], 2 of the 8 ga
Bifilar assy's are built. They are
paralleled in the same fashion as the 10 ga assy. When the windings are
paralleled in this fashion the uh does
not drop in half, like you would think. The 10ga assy is 38 uh....and the 8
ga assy is 22 uh. Both have been used
from 160-10m. 1 kw of drive will not put a dent on them.
## The RF parts 30A choke has real short pigtails, and is made for PC board
mount. I have no clue how long they
are, nor what type of ferrite was used, or what the uh is per winding.
Hope this all helps.
later....... Jim VE7RF
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