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Re: [Amps] Ferrite Rod

To: "Larry Carman" <lncarman@swbell.net>, <amps@contesting.com>
Subject: Re: [Amps] Ferrite Rod
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Wed, 3 Jan 2007 19:03:45 -0500
List-post: <mailto:amps@contesting.com>
>    Maybe a combination of air wound choke on secondary and 
> a floating
> filament transformer would suffice. Use enough choke 
> between tube & filament
> transformer secondary to server for maybe 14 MHz to 30 and 
> let the floating
> transformer arrangement serve best below the 14Mhz. This 
> helps to eliminate
> some cost by eliminating the ferrite rods and the worry 
> about saturating
> them.

Hi Larry,

The reactance of the filament choke is in series with the 
stray capacitance of the transformer to ground.

Let's say you have 250 ohms inductive reactance from the 
choke on 14 MHz, and 250 ohms capacitive reactance from the 
transformer's stray capacitance on 7 MHz.  Our plan was for 
one to "take over" as we change frequency. Let's assume the 
reactances change at a linear rate with frequency (they 
never do in large components, but we will assume that 
perfect world).

 The problem is the series reactances cancel each other.  On 
14 MHz the inductive reactance would be +j250 ohms (the 
choke) in series with  -j125 ohms (the transformer). Now we 
only have +j125 on 14 MHz. We don't have our original 250 
ohms until we reach 5 MHz or 19 MHz. We've made a hole in 
impedance that reaches zero ohms at 9.9MHz. It can even get 
worse than this because of transmission line effects in 
physically large components.

It's really much easier just to wind a proper filament choke 
for broadband, or if we only operate low frequencies just 
use a low capacitance very well insulated transformer with a 
fully bypassed primary.

By the way, my first experience with this was trying to do a 
filament system in a 3CX10000A7. The peak RF voltage in the 
transformer got so high it arced over from primary to 
secondary, connecting the power line hot lead to the 
filament. After I ruined the transformer, meters, and bias 
circuit I thought about the problem and reached the 
conclusion it was a bad idea.

73 Tom 


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