[Amps] Help constructing filament choke for pair of

[Jim Thomson]

Date: Fri, 28 Jun 2013 15:23:58 -0700
From: Paul Baldock <paul at paulbaldock.com>
To: <kg6i at comcast.net>,<amps at 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.   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.

##  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. 

##  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. 

later... Jim   VE7RF