Date: Fri, 28 Jun 2013 15:23:58 -0700
From: Paul Baldock <paul@paulbaldock.com>
To: <kg6i@comcast.net>,<amps@contesting.com>
Subject: Re: [Amps] Help constructing filament choke for pair of
3-500Zs on 6 m
The standard ferrite rod that you use for 10-80M works fine. I've
modded quite a few amps for 6 M and I remove half the turns from the
original choke. This reduces the inductance by a factor 4 and works
great for 6M.
- Paul
#### why does half the turns result in one qtr the uh ? I can see
maybe
slightly less than half the original uh..but not a qtr. I have played
around with lots
of coils and also fil chokes..but never tried removing half the turns from
a fil choke.
Did you shift the remaining turns to the center of the rod.....or just
leave them
down at one end ? That will make a difference.
- At 10:20 AM 6/28/2013, Carl wrote:
Do the math.
A reactance of 10 times the input impedance of the tubes is
sufficient to have the choke "invisible" to the input RF.
Carl
KM1H
### are u sure about that ? The various books typ say the choke Z
should be
4 X the combined tube Z.
** The original ratios were 10X preferred. Over the years with a lot of
extra drive and modifying the input network to compensate for the choke it
went down to the 4-5X ratio
Heath only used 9 uh on the SB-220. 9uh = 198 ohms XL
@ 3.5 mhz. A pair of 3-500Z is aprx 65 ohms, so heath only uses triple.
** And its a real compromise on 80 and also known to smoke the input
coil...on the SB-200 too.
## To get 650 ohms Z from a fil choke @ 3.5 mhz, the choke would have
to be 29.57 uh,
and a whopping 58 uh on 1.8 mhz !! 2.07uh on 50 mhz.
** Yep. Ever wonder why so many home brew 4-1000 3x3, and similar amps are
such dogs on 160? Remember those 12" x 3/4" rods I used to sell? They took
care of that problem.
If you have drive to burn then use less L on the choke and more C2 on the
input. But be prepared to smoke a few marginal size choke bypasses if really
pushing the ratio...you dont want RF in the filament transformer
## I measured 38 uh for each winding, when using a 8 inch long type 43
rod..with a
one half inch diam....and using bifilar wound 10 ga magnet wire.
Plastic clamps used on each
so only seven and a half inchs of rod actually has wire wound on it.
** The rods I use are 33 mix
later... Jim VE7RF
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