[Amps] Help constructing filament choke for pair of

[Carl]

> 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.

** 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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