----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Sunday, December 04, 2011 9:47 AM
Subject: [Amps] 2 x 3-500Z filament choke + 30/60/80A fil chokes
> 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
The 33 Mix is what gets you down to 160 without having to get fancy with
using the choke as part of the input network. The network programs dont
calculate for the hammy hambone "engineering".
Carl
KM1H
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